An introduction to the study of man

Citation preview

J.

Z.Y

AN INTRODUCTION TO THE STUDY OF

MAN

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GHuman cVariat

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OXFORD PAPERBACKS

AN INTRODUCTION TO THE STUDY OF MAN

AN INTRODUCTION TO THE STUDY OF MAN BY J.

Z.

YOUNG

M.A., F.R.S. PROFESSOR OF ANATOMY IN THE UNIVERSITY OF LONDON AT UNIVERSITY COLLEGE

OXFORD UNIVERSITY PRESS LONDON OXFORD NEW YORK

Oxford University Press OXFORD LONDON NEW YORK

GLASGOW TORONTO MELBOURNE WELLINGTON CAPE

TOWN IBADAN

DEI.HI

NAIROBI

DAR

ES

SALAAM LUSAKA ADDIS ABABA

BOMBAY CALCUTTA MADRAS KARACHI LAHORE DACCA KUALA LUMPUR SINGAPORE HONGKONG TOKYO

ISBN O 19 881333

3

© Oxford University Press 1971 First published First issued as

igyi

an Oxford University Press paperback

1974

stored in

No part

of this publication may be reproduced, a retrieval system, or transmitted, in any form or by any

All rights reserved.

means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of Oxford University Press This book

is

sold subject to the conditions that

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shall not

by way of

trade or otherwise, be lent, re-sold, hired out, or otherwise circulated

without the publisher's prior consent in any form of binding or cover other than that in which

it is

published and without a similar condition

including this condition being imposed on the subsequent purchaser

Printed in Great Britain at the University Press,

Oxford

by Vivian Ridler Printer to the University

PREFACE Th

i

s

book arose out of lectures given

beginning of their studies.

man

possibility of studying for

It is

medical and dental students

to

at the

therefore literally an introduction to the

in a scientific

manner.

To do

this is not 'natural'

most people, and indeed may seem actually repellent

to some.

Yet

doctors and dentists have found that by approaching our problems in a scientific

way they have been

true to say that

it is

increasingly able to help us. Indeed

only since medicine became scientific that

able to be of any real help in curing large areas of

study.

Many

man's

activity in

ills. It

may be

almost

has been

that there are

still

which we can be helped by more detailed

people fear that in some way this threatens an invasion of the

individual personality. all

human

it is it

important to respect this apprehension and

It is

times to emphasize that the aim of

all scientific

studies

is

at

to assist in the

improvement of the quality of human life and to enlarge its capacities. This book is the record of the search for a method by which this can be done. Indeed it can be read as a sort of detective story in which we are is man?' or, more subtly, 'What are good ways to study men?' We begin by asking some conceptually rather simple and obviously 'scientific' questions such as 'What are men made of?' This gives an opportunity to look at some of

searching for an answer to the question 'What for the

answer

to

the spectacular recent information that biochemistry has provided about the large molecules in the body, and especially about the information-

carrying properties of the nucleic acids. pletely

new ways of

origin of

life.

These serve

are interested in about is

himself. His

organized by the information that

An

more

as preludes to the

man

is

life, like

specific questions

we

that of other creatures,

received from the past, so that he

takes actions that are likely to preserve his future.

Such knowledge provides com-

talking about the old questions of the nature and

life

and that of his species

understanding of the origin and nature of

this

in the

mechanism

for

ensuring continued maintenance or homeostasis would go far to give us that basic

knowledge of the principles of

life

for

which we are looking.

Certainly

we have not achieved

recently,

and they revolutionize our knowledge of ourselves.

it

yet,

but there have been large advances

To many

people this will seem a necessarily imperfect framework because, they

would say, the essential feature of a man is his difference from other creatures he possesses a soul, in some sense valuable in itself. Anyone proposing to study man must face this problem, and we shall

in that

indeed attempt to do

so.

The answers

will

probably be found inadequate

PREFACE

vi

in

many respects. Indeed many thoughtful people perhaps feel that we do know enough to be able to solve this or indeed any other of the funda-

not

mental problems of the origin of the world or the meaning of life, and should frankly admit our ignorance. One of our recurrent themes will be that

human ing

intelligence, for all its ability,

We

fast.

shall try to look for

is still

new

very imperfect, but

is

improv-

on some of the problems that

light

have worried mankind for centuries.

am

I

very conscious that such consideration as

questions

who

sophers

devote their lives to them. Certainly there

study of the works of those

who have

investigated

and

difficulties that this

the investigation develops

of human biology, that

Does

himself.

no substitute

we

for

many

in all his

he studies the agent of study itself— the brain.

in that

bilities

is

man

one special contribution

aspects. Nevertheless the biologist has at least

make

given to these great

is

very superficial compared to that of theologians and philo-

is

The

As

brings are major themes of the book.

shall

to

possi-

meet repeatedly the particular paradox

we are trying to find methods for studying the student reasoning and if so how does it affect

this involve circular

our whole endeavour ? These are very old and deep questions of the theory

and nature of knowledge (epistemology). Although the biologist trained to deal with such problems of logic

impossible to avoid doing

ways

in

so.

This

is

is

not

this book.

The

intention,

some of the

that he shall at least be stimulated to see

it

many

perhaps the most serious of the

which the reader must be suspicious of

however,

is

and philosophy he finds

philo-

sophical problems that are involved in his ordinary scientific and everyday

ways of speech, even though the treatment

is

too superficial to provide

really satisfying answers.

Certainly no one discipline has a

men

should do.

The body

of

monopoly of the

human knowledge

now

is

right to say

so vast that

what

no one

set

of people trained in a particular discipline can provide the guidance that

we

need.

We

have to depend upon

well as philosophers, engineers

artists as

and lawyers,

much

as priests, physicists as

historians, economists, doctors,

and

politicians, to mention only a few. The course of human life today depends upon the knowledge of all of these and a host more. I hope that the facts and attitudes recorded here will be found to be useful

and

will give

some

have aimed to do

satisfaction is

and help

to provide

to different sorts of people.

enough information

to

What

spectacle of the vast range of controlled activities that constitutes the

of

men and

of the living world. With sufficient knowledge

begin to imagine

all

we can now

the tens of thousands of chemical processes that go

an orderly way as one individual

man

or

woman

sits

I

evoke the inspiring

and

thinks.

life

just

on

We

in

can

begin to see the wonderful complexity of the instrument in which the

PREFACE thought

is

taking place, calling

events, stored in the

memory.

upon

We now

vii

range of remembered past

a great

know

a little

about the origin of the

emotional urges that keep us alive and thinking and acting, almost

in spite

of ourselves and often without even considering the consummations that

sometimes

satisfy

our desires.

Knowledge about the population and its growth will help us to think of our more than three thousand million fellow humans and of all their genetic and cultural differences. Yet they are to

make

all

working together every minute

the changing, evolving patterns of man's

can give us the means to think about

all

life.

Knowledge again

men who have gone

the

before,

gradually emerging from their rough beginnings, acquiring language and laws, clothes

and

civilization.

Was

slow continuous process?

this really a

Have we in any real sense separated from the rest of creation ? Even if man is very different it is fundamentally important to recognize how deeply he is part of the one great living world. In spite of all his artifiand

ciality

he depends utterly on plants and animals and

civilization

both assisted and menaced by the bacteria and viruses. a proper future for

man depends upon

The

is

preparation of

thinking of the whole life-system as

one. Recent discoveries have indeed emphasized the unity of

All

life.

same genetic code; all are made of almost similar elements and compounds and their cells are made up of similar units. With all the deep similarities as well as endless organisms are directed by instructions written

differences the animals, plants,

world, of which

man

is

as

and bacteria

much

in the

act together to

a part as the rest,

produce one

whatever his special

features.

In particular what the neurobiologist finds out about the brain must surely be relevant to fundamental views of the nature of all this knowledge.

The

interpretations of recent findings that are given here

may be wrong

in parts

but they suggest that the whole structure of our language and

thought

is

limited by a

Our knowledge of this but

I

have tried

to

is

pre-programme in the organization of the brain. yet really too meagre to provide sure foundations,

show how

it

may

of child development but the very

influence not only our understanding

way

we speak of our own

that

thinking

selves.

Such

difficult

and fascinating questions keep on intruding and some

people would say that in trying to answer outside his

field.

Certainly

more mundane and

less

much

of

em

tl

human

the biologist

is

stepping

biology can be pursued in a

philosophical fashion.

Many

aspects of

human

such as sex differences, reproduction, growth, and ageing are greatly

minated by exact study.

Some

life

illu-

aspects of the problems of aggression can

be studied by 'biological' methods, though here as elsewhere

we have

to

PREFACE

viii

be cautious about analogies between animals and men. The growth of population is certainly another matter that concerns us all, but about which

The

few are properly informed. factors, again,

whether

in

assessment of the importance of hereditary

medicine or education, requires very careful

evaluation of evidence. In order to try to weave the facts about such matters into a coherent

science

I

have concentrated on the

way

in a characteristic

how

ignorant of

to

this is

fact that

produce language.

done, but as more

increasing understanding of the basis of

human beings use their brains Of course we are almost wholly

is

found out

human

it

should lead us to

individuality and society.

The attempt here has been to construct with what is known about early human history and our rudimentary knowledge of cerebral physiology a scheme or model that 'explains' why we behave as we do. In particular I have stressed the importance of the idea that human brain operations revolve around a cerebral model that interprets much of the input in terms of persons. Our first explanation for all happenings is that they are due to the actions of entities like people. And this is natural because the child's first lessons are about the characteristics of people and how to communicate with them. His brain may indeed be pre-programmed to operate in this way. In any case he continues to do so for

world and

There this

is

in this

as yet

programme.

human

biology.

unifies the

It

human way

interpretations.

no physiological evidence about how the brain produces may be unwise to use it so widely as the framework of

It

But

purpose

for this

life. It

Above

These may seem I

by one who

am

it

has the unique advantage that

enables us to speak systematically about

of

it

is

to

many

aspects of

shows the basis of many of our anthropomorphic

all it

enables us to say something about the most

fundamental problem of the study of between mind and body. tion

except his most sophisticated

approaches of the biologist, psychologist, and indeed of the

philosopher. the

all

way he thinks about God and the ultimate order of the way also about himself and his own precious personality.

In this

activities.

man— his

disposition to distinguish

be absurd pretensions for an unsystematic presenta-

inexpert in

only too conscious of

many

its

of the fields of study that are involved.

imperfections and shall not be surprised

if

convince. But the search for a system has been invigorating and has taught me much that I wished to know. The facts discovered on the way should, at least, be useful to some people.

it

fails to

But the search for principles has been the main aim, principles that shall organize our knowledge of ourselves and help us to organize our lives. Of course no one in their senses would suppose that such principles can provide a

primer for

life. I

hope

that this excuses the quite ridiculous imbalance

and

PREFACE omissions

the book.

in

spiritual life;

It

contains

little

ix

about aesthetics, emotions, or

nothing of humour, acting, music, or literature; nothing of

little of human joys The psychiatrist will feel the treatment to be very dry and barren, and so may many others. It is true that these things are of the essence of human life, and in fact I have at no point been unmindful of them. Any proper introduction to the study of man would include them all. But this would be quite impossible. All these features of human life presumably have their significance for survival. Here we have been concerned to discuss the survival value for some features, for instance language. Perhaps this may help to consider the

architecture or invention, economics, sociology, or law;

and sorrows, of how

to

keep well or

why we

get

ill.

place of others.

With ficial

these excuses the fact remains that this

all

book, both

and yet understand so to

and

in principles little.

in detail.

shall

I

be

We

is

know

satisfied if the

an absurdly superso

much about man

book does anything

encourage others to try to understand more.

These

are

some of the thoughts

that have occupied

me

as

inquired into

I

many factual and practical matters involved. They have taken me into many fields where I am not expert and the treatment is nowhere as thorough

the

as

I

would wish. Every

specialist will recognize the imperfections of the

information in his subject. Therefore every student should be aware that

—

no one could possibly be that in so many them serious inquiry must involve consultation of the references given and many others. I have had the advantage of checking facts and opinions with the many this

is

fields.

not the work of an expert In each of

colleagues listed at the top of page

xi. I

am

exceedingly grateful to them,

not only for their help and criticism, but for the pleasure

I

have had in

following the leads they have given. I

my

should

like to

thank the generations of students

and commented upon them. Their

lectures

who have

difficulties

listened to

have made

aware that many of the points of view are unfamiliar and not easy

me

to accept.

Perhaps they are none the worse for the novelty— goodness knows we need

new methods. I

am most

grateful to the research assistants

who have helped

paration of the book over several years, including

Wood, and

E. A. Bradley.

later stages

and

Young also

assisted.

M.

J.

M. Nixon

scurity, as

in the pre-

Altman, C. C.

has given most valuable help in the

in preparation of the index, in

which

task R.

M. and K.

F.

has played an especially large part in organiz-

ing the material and references

made many

Hobbs

J. S.

and making corrections. Moreover she has

suggestions for helping the reader where there might be ob-

by the addition of a glossary, conversion

tables,

and the

like.

PREFACE

x

am

deeply grateful to Mrs. J. Astafiev for continuous help in the diffiwork of preparing new figures and the adaptation of those of others. Mrs. N. Finney and Miss M. Dickens have patiently prepared the long I

cult

series

of revised drafts of the chapters.

It is a

great pleasure to thank

all at

the Clarendon Press for their care in

preparation of the book.

A

work of

this sort is

and hands and

I

should

only possible by the collaboration of

like finally to

thank

all

many

brains

my colleagues on the academic

and technical staff of the Anatomy Department of University College London, for their help over so many years. J.

January igyo

Z. Y.

ACKNOWLEDGEMENTS The

commenting on

following have given great assistance by reading and

parts of the

book or providing

data.

N. A. Barnicot

E. D. R. Honderich

K.

G. Belyavin

D. W. James

R. Quirk

H.

Burhop

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Oakley

A. R. Jonckheere

A. Rosenfeld

E. Clarke

H. Kalmus

J.

A. Comfort

R.

E.

S.

M. Kempson Norma McArthur

D. T. Donovan M. J. Evans

A.

M.

C.

Thanks

I.

W.

Gray W. F. Grimes E. G.

J.

Ucko

C. A. Vernon

Matus

R.

Rotblat

P. J.

P.

Mead

D. Wall

R. A. Weiss

D. R. Wilkie

R. Napier

Harrison

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life-span

CONTENTS 1.

AND DIFFICULTIES FOR

POSSIBILITIES

A

SCIENCE

OF MAN i

2.

3.

.

The

general study of

man

i

2.

Deduction, induction, and other methods of thought

3.

By what methods can

4.

Advantages and limitations of physical science

5.

The

6.

Living

7.

Reproduction

scientific

view of

activities.

the student study himself?

man

5

6 7

Homeostasis

8

as the guarantee of continuity

8.

Exosomatic inheritance. Brain and mind

9.

Human

variety. Its

2

and of change

8

9

advantages and disadvantages

10

10.

Man's current problems

n

11.

Summary

12

WHAT ARE MEN MADE

OF?

1.

Historical and non-historical sciences

2.

The

3.

What do we understand by

4.

Advantages and disadvantages of analysis

5.

Levels of discourse

19

6.

The

21

7.

Hydrogen, oxygen, and water

8.

Sulphur and phosphorus

26

9.

Monatomic

26

search for an adequate approach to

13

man

the principles of physical science?

13

14 18

elements present in the body

24

ions

10.

Calcium

28

11.

Trace elements

29

12.

The

13.

Carbon compounds

30

14.

Proteins

31

15.

Nucleic acids

34

16.

Summary

36

alphabet of bioelements and molecules

29

LIVING ORGANIZATION 1.

Molecular order

2.

Ordering by the information of the

37

DNA

3.

Replication of

4.

The

5.

Reading the information

DNA

code

45

50

unit of inherited information

51

DNA

53

in

.

CONTENTS

4.

6.

The

inheritance of order

59

7.

The

folded structure of proteins

60

8.

Enzymes

9.

Control of enzyme

64

°7

action

CELLS, ORGANS,

AND ORGANISMS

1.

Individuals

68

2.

Cells

68

74

3.

Unicells

4.

Multicellular animals

74

5.

Control of

75

6.

Organs and

cell differentiation

7°

tissues

5.

LIVING ACTIVITIES, TURNOVER

6.

THE DIRECTION OF LIVING ACTIVITY. HOMEOSTASIS

7.

8.

80

1.

Activity

88

2.

Energy

9°

3.

Living

4.

Stability of life

5.

The beginnings

6.

Homeostasis

7.

Equilibria and steady states

8.

Maintenance of a steady

91

activities

91

92

of directed action

92

state

93

by expenditure of energy

94

THE CONTROL OF LIVING ACTIVITIES 1

The

2.

Machines

3.

Information theory

4.

The

information flow through organisms

101

principles of control

103

concepts of structure and function

97

that control themselves

98 99

5.

The

6.

Living control systems

106

7.

Natural selection

107

PERSONAL ADAPTATION. IMPROVEMENT OF THE REPRESENTATION ON DIFFERENT TIME-SCALES 1.

Adaptation of the phenotype and genotype

2.

Mechanisms of phenotypic adaptation

no no

3.

Erythropoietin

m

4

Memory. Phenotypic adaptation

5.

Summary. Adaptation on

6

Living matter as a single homeostatic system

in the brain

different time-scales

112 113 1

14

.

CONTENTS 9.

10.

11.

12.

13.

xvii

THE INDIVIDUAL MAN

117

CONSCIOUSNESS 1.

Mind and

2.

Some semantic problems

124

3.

The problem

125

4.

The

5.

Can computers think?

6.

Problems of the identification of consciousness

7.

Identification of events in consciousness

132

8.

Sleep

133

9.

The body image

135

10.

Development of consciousness

137

11.

The growth

12.

Is

matter

123

of a private language

of consciousness

criteria

127

129 in

man. Divided brains

of the model in the brain

there really a problem of

130

137

mind and matter?

138

GROWTH, TURNOVER, AND THE RISKS OF DAMAGE 1

Growth

2.

Exponential growth

141

3.

Differentiation and growth control

143

as the guarantee of homeostasis in spite of

wear

140

4.

Intermittent growth

144

5.

Turnover and growth

144

6.

Turnover and the anticipation of risks

146

REPAIR OF THE INDIVIDUAL 1.

Maintenance, repair, replacement, and regeneration

149

2.

Maintenance

150

3.

Nerve repair

4.

The nerve-growth

5.

The

6.

Theories of

7.

Wound-healing

as

an example of healing factor

processes of healing

wound

152 1

56

159

healing

in the skin

1

59

160

REPLACEMENT AND REGENERATION OF PARTS AFTER LOSS and other glands

1.

Regeneration of the

2.

Functional regeneration in the nervous system

3.

Regeneration of limbs

17°

4.

Regeneration and evolutionary advance

170

liver

165

166

...

CONTENTS DEVELOPMENT AS GUARANTEES AND REPRODUCTION

xviii

14.

OF HOMEOSTASIS

15.

16.

17.

18

i.

The

2.

The importance

3.

The

significance of reproduction

of variety and

172 sources

its

reading of the information of the egg and sperm

173 175

MATING AND FERTILIZATION 1

The

2.

Sexual stimuli

186

3.

Sexual needs

190

sources of sexual appetites

180

4.

Fertilization

5.

Artificial

6.

Cultivation of embryos outside the body

192

insemination

195

196

HUMAN GROWTH 1

Factors controlling growth

2.

Limitations on the study of over-all growth

3.

Some

4.

The

5.

The growth curve

6.

The

difficulties in collecting data

initiation of

198

on growth

embryonic growth of the

embryo

post-natal curve

199 199

201

202 208

RELATIVE RATES OF GROWTH. TEMPORAL AND SPATIAL PATTERNS OF GROWTH 1

Local differences in growth rate

209

2.

The

211

3.

Relative growth rates

spatial pattern of

growth

219

LATER STAGES OF HUMAN GROWTH Patterns of growth

226

Developmental age

230

Characteristics of the changes at adolescence

231

Changes

232

in the sexual

organs and secondary sex characters

Relationship of increased growth to other changes

235

The

236

control of maturation

Factors influencing the rate of development

238

Relation of maturation to social and economic status

241

Long-term change

in

growth and adolescence

248

19.

20.

CONTENTS MATURATION OF THE BRAIN AND THE STUDY OF THINKING i.

Growth and maturation of the

2.

Person language and the logical capacities

252

3.

'Thinking'

254

4.

Adventurous and creative thinking

257

brain

251

THE MEASUREMENT OF INTELLIGENCE 1.

The

2.

Test performance depends on the relations of the tested and tester

difficulties

and dangers of measuring brain power

260

260 261

3. All tests are empirical

21.

xix

of intellectual capacity ?

261

4.

Is there a general factor

5.

Multiple factor analysis

262

6.

Correlation of test results and careers

263

7.

Types of test and examination

264

8.

Measures of intelligence

266

9.

Development of intelligence

in infancy in the schoolchild

267

10.

Changes

11.

Studies of social and emotional development

271

12.

The development

273

in adult intelligence

271

of speech in the child

THE DEVELOPMENT OF THE CHILD AS SEEN BY PIAGET

22.

1.

The schema

2.

The

stages of

3.

The

period of sensory-motor intelligence (0-2 years)

4.

The

period of pre-operational thought (2-7 years)

280

5.

The

period of concrete operations (7-1

years)

282

6.

The

period of formal operations

years onwards)

283

7.

Examples of Piaget's experiments

or action sequence

277

development

278

(1

1

1

279

284

AGEING AND SENESCENCE 1.

Suspended animation. Cryptobiosis

287

2.

Changes

288

3.

Senescence and differentiation

290

4.

Ageing

292

in homeostatic capacity with time

in plants

5.

Senescence

6.

Is

7.

Programmed

8.

General definition of senescent changes

9.

Ageing

in

29 2

Protozoa

senescence a product of selection or of cell

its

absence ?

death

in dividing cell populations

10.

Damage and

repair of instructional molecules

11.

Cell division

and

cell

death as guarantors of the instructions

293

294 295

298

300 301

.

xx 23.

CONTENTS PATTERN OF SENESCENCE THE AND LIFE TABLES i

24.

25.

.

Changes with age

in the expectation

of

life

3°3

2.

Life tables

3, promotor; 2, gene for

regulator gene;

the structure of galactosidase, y, gene for the structure of /3-galactosidepermease; ac, gene for the structure of j8-galactoside transacetylase (these are

enzymes used

in the splitting of lactose).

(From Jacob

1966.)

been estimated in suitable bacteria. The genes for four enzymes of a certain operon of E. coli are transcribed in 7 min. Translation

translation have

mRNA proceeds at 1200 nucleotides/min (i.e. 400 aminoadded each minute) (Morse, Baker, and Yanofsky 1968). clear that the nucleotide pairs are the fundamental units of heredity,

of the relevant acids are It is

strung together in functionally significant sets, the genes (or cistrons). If the

Jacob and

Monod scheme

is

more

strictly

correct these are of two sorts,

READING THE INFORMATION

3.5

IN

DNA

59

and operator genes near by, which the whole constituting an operon. Each operon is controlled

structural genes, coding for proteins,

control these,

by one or more regulator genes, acting by producing an extra-chromosomal product.

The

now been

DNA

substance acting as the repressor of the lactose cistron has

isolated

and proved

to

be a protein that binds specifically to the

molecules of the lactose operon and

released from

it by the galacand Miiller-Hill 1967, the regulation of gene action by

is

tosidase inducers as described on p. 53 (Gilbert

Bretscher 1968).

The

explanation for

the removal of suppression has therefore been well substantiated for bacteria.

It

has never been proved, however, for truly cellular (eukaryote)

organisms.

An

alternative hypothesis to explain the regulation of gene actions for

higher organisms has been put forward by Harris (1968), as a result of fusion of cells of different types, which can be

made

to take place if cultures

of them are treated with a virus. These experiments show that the regulation of transcription

Thus is

is

by some influence of the cytoplasm on the nucleus.

in the nucleated red cells of birds only a small fraction

active

and only very small amounts of

RNA

human

nuclei are introduced into actively synthesizing

become

active

and much

RNA

is

to incorporate tritiated thymidine,

DNA,

which they never do

of the

But

are produced. cells

DNA

if their

many genes

produced. Moreover, these nuclei begin

showing that they are now synthesizing

in the adult bird,

being 'end-cells',

doomed

to

be removed from the circulation. Harris summarizes these results as follows. 'The regulation of nucleic acid synthesis in the heterokaryon a cell

which synthesizes

does not, the active

thus essentially unilateral: whenever

is

a particular nucleic acid

cell initiates this

no case does the inactive

cell

is

fused with one which

synthesis in the inactive partner. In

suppress synthesis in the active partner

.' .

.

(Harris 1968).

These experiments suggest

that regulation of

a matter only for regulator genes, but

course in any case

it

is

is

DNA

transcription

clear that the specific information

differentiation of an animal cell into

is

controlled from the cytoplasm.

not

Of

upon which

one type rather than another

is

based

must come from outside the nucleus. Evidently the relations of nucleus and cytoplasm are complex and not yet properly understood. 6.

The The

inheritance of order information about the primary source of living order, though

incomplete, forms a central theme of modern biology. a long time that the order exists, that in

some way

it

differentiates living

it is

a

We

still

have known for

fundamental feature, and that

from non-living systems. Biologists

;

LIVING ORGANIZATION

60

3.6

have often emphasized the theme that their science

is

rather than purely 'chemical'. Also, of course, that there

organism than

chemistry'.

'just

We

can

now

is

'morphological'

more

in a living

bring these vague ideas to-

gether by showing at the centre of living things molecules that carry an

Moreover, the order

inherited order.

is

form of

in the

of symbols whose significance appears only

a code, that

when they

is,

a set

are passed through

an appropriate communication channel and decoded. All of this

is

based

on firm knowledge of the chemical composition of DNA. In this sense the discoveries have truly revolutionized our approach to general biological problems.

They have

not, however, as yet led to par-

advances that have great practical importance, though undoubtedly

ticular

they will do

The

so.

relevance of such a system of ideas to a

human problem

can be illustrated, however, by the classic case of sickle-cell disease, which is

inherited as a simple recessive. Pauling and his colleagues (1949)

that the red blood corpuscles of persons suffering tain is

an abnormal haemoglobin.

It

has

from

now been shown

showed

this condition

con-

that the difference

the substitution of the amino-acid valine for glutamic acid at one point

in the protein chain. liable to precipitate.

would seem

to

This makes the molecule Homozygotes usually die

be highly disadvantageous. Yet

in populations that are subject to malaria.

less soluble

and therefore

as children,

and the gene

it is

widely spread, especially

Apparently the heterozygotes

have an increased resistance, perhaps because the parasite suffers from the abnormal haemoglobin, which

is

present in a heterozygote, though in

reduced amounts (pp. 551 and 595). In populations in which the malaria risk

dying out.

It is

the inhabitants malaria. ciple

of the

The

To

reduced the gene seems to be

selection take time. In prin-

possible in future to accelerate

them through knowledge

DNA code.

The

7.

Such changes of gene frequency by

may be

it

is

now in Curacao, whereas it remains common in Surinam of both are from Ghana but only the latter still suffer from

rare

folded structure of proteins

DNA

thus controls the sequence of amino-acids along the proteins.

understand

how

this results in the control

of living activities

we must

examine how the protein composition regulates the various structures and cell. The first aspect to be considered is the configurations adopted by the protein molecules themselves. The protein chains are mostly not simply long loose threads, but are often elaborately folded, with

functions of the

interactions

between their side chains. As a result, irregular globular strucwhich not all parts of the chain are presented equally

tures are formed, in

to the surroundings.

This

tertiary structure

determines their reactions as

THE FOLDED STRUCTURE OF PROTEINS

3-7

Fig.

A

model of the myoglobin molecule showing the location of all the atoms. The cord and its two ends are indicated N-terminal and C-terminal. The larger sphere in the molecule indicates the atom of iron, while the small sphere marks the position of the molecule of water. This would be replaced by oxygen when the myoglobin is oxygenated. (From Kendrew 1966.) 3.15.

indicates the polypeptide chain

enzymes (Figs. 3.15 and 3.16). The details of the folding are determined by the amino-acid sequence and this is therefore the next sense in which we must consider how the DNA controls the organization of the cell. Clearly

it

will dictate

which groups are presented

It is this that largely specifies

to the outside (Fig. 3.17).

the activity of the protein as an enzyme.

So even at this level we see how the DNA code controls the life of the cell by deciding the form of its parts and hence their activity. The bonds between the peptide groups of the protein chains allow considerable flexibility, and in

many

proteins this results in the chain being

rather tightly folded in a regular way.

A common

form

is

a helix in

which

there are about eleven residues for every three turns. This structure

maintained by the

affinities

is

of each positively charged hydrogen atom along

Fig. 3.16.

A

diagram showing part of the myoglobin molecule of Fig. 3.17, much some of the amino-acid side chains round and within the haem group. (From Haggis 1964.)

enlarged to show the positions of the atoms in

Fig. 3.17.

A model

of the myoglobin mole-

cule at a low resolution.

The haem group

is

the black area at the top of the molecule.

(From Bodo

et al.

1959.)

QUATERNARY STRUCTURE OF PROTEINS the backbone for

hydrogen bonding such as heating to

known

63

in the next turn. Such weak and can be broken by mild treatments about 70-100 C. This results in a change in the protein its is

neighbouring oxygen atom

relatively

as denaturatwn, depriving

properties. Interactions

it

of

characteristic

its

enzymic and other

between the neighbouring amino-acid residues of

the chain serve to strengthen the coils of

some

degree of such

vary in the extent to which they

show

effects, different proteins

helical coiling.

Haemoglobin molecules

proteins.

According

to the

are 60-80 per cent coiled,

t%^ albumin 30-45 per cent, pepsin 20-30 per cent, and casein hardly at

all.

The cule

is

protein structure

made up of

is

further complicated by the fact that each mole-

several subunits, that

is,

peptide chains with differing

sequences and hence different coiling. Perutz and his colleagues (i960)

have shown that in the haemoglobin molecule there are four subunits, two each of a and

Fig

3.18

A model

chains, differing slightly (Fig. 3.18). This association of

of the haemoglobin molecule discs.

(From

Two

of the four

Cullis et

al.

iqf>2.)

haem groups

are indicated by grey

LIVING ORGANIZATION

64

may be

parts

3.7

called the quarternary structure of the protein

a further stage

and

shows us

it

by which the cell structure is controlled through the sequences

of the peptide chains.

A

particularly important feature of the structure of proteins

may be

readily

is

changed by combination with small molecules such

tose or amino-acids.

Such

may, according

'allosteric' proteins

to

that

it

as lac-

Monod,

Changeux, and Jacob (1963), be the basis of regulation of gene action. Regulation involves the transmission of signals from the cytoplasm to the

DNA

and

this

may be

allosteric proteins,

both be active

at once.

normally empty but metabolite

on it

p. 59)

to the

known

achieved by specific repressor molecules that are

with at least two distinct receptor

One

if it

sites,

of these binds to the operon.

which cannot

The

other site

becomes activated by the presence of

as the co-repressor or inducer (say lactose in the

then the repressor molecule changes

DNA becomes inactive, the operon

is

its

is

a specific

example

structure, the site joining

de-repressed, and synthesis,

say of j3-galactosidase, begins.

8.

Enzymes

As has already become apparent, many of the characteristic and 'improbable' actions of living systems are the work of enzymes. These may be defined as catalysts, and this at once helps us to think more clearly about some of the more puzzling problems that life presents. A catalyst is

a substance that increases the rate at

which

a

chemical system achieves

equilibrium, without itself undergoing any ultimate chemical change.

Nevertheless, the catalyst

may

play a part in deciding which of several

possible reactions takes place. Thus, depending

used, alcohol can be

decomposed

to

make

upon which

catalyst

is

either acetaldehyde or ethylene

or ether. Catalysts

make

reactions proceed as they

given temperature and pressure. free energy

may do

A

would not otherwise do

at a

reaction that proceeds with a drop in

so only very slowly because too few of the molecules

reach the activation energy necessary to react. This can be increased in various ways; for example, the chemist usually does

mixture, so that the internal energy of the molecules are

more

likely to collide

and

react.

A

catalyst

is

it

by heating the

increased and they

produces the same

allowing a greater proportion of the molecules to interact.

It

effect

by

usually does

by forming complexes with the substrates, which then separate off to form the product. Catalysts produce 'improbable' results, namely actions

this

would not happen in free solutions. Often, this is simply because of some particular feature of the structure of the catalyst such as the large extent of surface presented, at which reagents can meet. Thus with many that

ENZYMES

3.8

metallic catalysts the reagents

become

65

physically adsorbed, raising their

and increasing the chances of encounters. In other

local concentration

catalysts the reagents are chemically attached to the surface, involving

quite profound changes in the bonding within their molecules.

Such

altered

molecules or molecule fragments undergo reactions very different from

When

those that take place in molecules in simple gas or liquid phases.

they are at such a surface they react at lower activation energies.

such mechanisms are imperfectly understood even

details of

inorganic catalysts.

It is

we cannot

not surprising that

in

The

simple

give precise physico-

chemical explanations for the reactions catalysed by the immense folded protein molecules. However, as physical chemistry develops, no doubt

come to understand living as well as inorganic The mechanism of action of the enzyme lysozyme is shall

we

catalytic reactions.

already quite well

understood (Dingle and Fell 1969).

The surface activity may be compared with

of metals such as platinum in inorganic catalysis the secondary, tertiary, and quarternary structures

of proteins. These provide surfaces, the active

sites, at

which the molecules

of the material to be acted upon (substrate) become attached. While there,

some way

they are in

activated and the electrons redistributed to produce

changes that would not otherwise take place at that temperature.

Knowledge of enzymes grew tion of sugar first

by yeast

showed

to

originally

from the study of the fermentaliterally 'in yeast'). Buchner

form alcohol (en-zyme,

in 1897 tnat

**

*

s

possible to prepare from yeast a cell-free

extract that will ferment sugar; therefore the reaction

chemical one, not something inseparable from

life.

is

essentially a

In fact, there are

more

than ten separate enzymes in a yeast extract, each responsible for one stage in the breaking

A

down

of sugar to alcohol.

very familiar example of an

of the

saliva.

Starch (C6

H

I0

enzyme

5 )x

is

can be

the amylase split into

(=

'starch splitter')

two molecules of the

sugar maltose by a change that proceeds with a drop in free energy— but only very slowly at 37

°C (body temperature). In

however, the reaction goes so

mouth soon

[apparatus

fast that a

after a piece of starch

to tell us that in the

is

chewed.

mouth

the presence of amylase,

sweet taste can be detected in the

We need no elaborate chemical

there

is

an enzyme able

to allow

starch to turn to sugar.

With

slightly

more trouble

acting in acid solution, break

—the enzyme being pepsin.

it can be shown that extracts of the stomach, up proteins into shorter chains, the peptides

In the

duodenum and

intestine further

enzymes,

derived from the pancreas, continue the process and break up other molecules too.

These

extra-cellular digestive

enzymes of the mouth, stomach,

LIVING ORGANIZATION

66

than those within the

There may be many hundreds, of the

cell.

perhaps only a few molecules of each.

down

of materials submitted to the

reactions by which materials are

ponents.

Many enzymes

3.8

They

latter

accelerate not only the break-

cell but,

combined

much more

important, the

to produce the cellular

com-

can work in either direction, producing splitting

or synthesis according to the concentration of reagents and other conditions.

However,

employ

ensure adequate regulation the most important pathways

to

different

mechanisms

Many enzymes

Others act upon

ticular type of molecule.

nately,

little is

for synthesis

are extremely specific

known

and breakdown.

and all

will act upon only one parmembers of a class. Unfortu-

that activity tertiary

is

it is

known

often lost by the relatively mild treatments that destroy the

and quarternary structure (denaturation by heat or

This suggests that the particular side-chains presented the molecule are significant.

It is

now known

of the enzyme (Blake

The number

et al.

of enzymes

example, in Escherichia

coli

is

pH

change).

at the outside

of

that binding of a substrate

or inhibitor causes considerable conformational changes site

means

either about the basis of the specificity or of the

by which the electron redistributions are produced. However,

round the active

1967).

very large, even in a simple bacterium

there

may be between 1000 and 2000

;

for

present

(Luria i960). Furthermore, the possible variations in the combinations of side-chains presented sre

enormous and

are

many enzyme molecules the iron-containing haem

incorporation into groups, such as

further increased by the

still

of other atoms, called prosthetic radical of haemoglobin.

groups usually confer some specific property. In

this case the

Such

formation

of particular complexes with the electronic shell of the transition metal allows that oxygen

is taken up under one set of conditions and then given up again elsewhere. Many enzymes function only with the co-operation of other molecules, the co-enzymes, which are not specific but remarkably uniform throughout bacteria, animals, and plants. Thus nicotinamide adenine dinucleotide (NAD) serves as a hydrogen carrier in oxidation reactions of many organisms, by accepting hydrogen atoms presented by one enzyme, and then giving them up subsequently on the operation of another enzyme.

+ 2H

NAD

»

',

NADH

2

-2H Other co-enzymes are thiamine (vitamin

enzyme A (pantothenate). Like that

is

to say,

cannot be synthesized by

must therefore be supplied

B,),

riboflavin (B 2 ),

and co-

NAD (nicotinate), all of these are 'vitamins',

in the diet.

man

(or

by most mammals) and

CONTROL OF ENZYME ACTION

3Q

67

Control of enzyme action

9.

Enzymes

often operate in chains, each using as substrate the product

of the one before. Obviously, anything that changes the rate of one reaction

may

alter that

So

of the whole sequence.

The

velocity of any reaction

is

con-

both by the amount of enzyme and by the concentration of substrate.

trolled

if in a series

A the reaction velocity

Va

^B

B-^C->

increases then so does

V\,

to the

maximum

extent

that the system allows. Regulation of the rates of enzymatic reactions

is

one

of the fundamental methods of control of the complicated actions that constitute living. trolling factors.

The

concentration of the substrates

Another

is

This 'product inhibition' may operate directly on

their further operation.

the preceding reaction, or by a feedback action on

member critical

one of the con-

is

the effect of the products of reaction in inhibiting

some more

distant

of the chain of enzymes, or upon the operon that produces a

enzyme. Thus an excess of the amino-acid L-isoleucine added

a culture

of Escherichia

deaminase, which

is

colt

the

enzyme Not only

first

synthesized from threonine.

to

immediately inhibits the action of L-threonine in the this,

pathway by which leucine

but the leucine

is

also inhibits the

operon that induces the formation of L-threonine deaminase, by activating its

repressor gene.

A

third

means of regulation

the formation of the enzyme.

is

that the presence of a substrate induces

The

genetic constitution of the cell

must of

course be such as to allow this; in other words, selection during the previous history of the race has ensured that the appropriate codons occur in

DNA.

the

above,

We all

is

Repression of synthesis of enzyme by the product, as mentioned

a fourth

means of control,

also acting

can thus begin to understand

how

through the operon.

in these various

the reactions in the cell are adjusted. But the total

ways the

rates of

number of them

very large and the complexity of interaction almost unimaginable.

is

It is a

great tribute to the ingenuity of microbiologists that the outlines of the plan

have been revealed

and

in bacteria.

living reactions

same

sort.

we

shall

Even

for these

we know

only some features,

organisms almost nothing. In order to control need a great deal more detailed information of the

for the cells of higher

CELLS, ORGANS, 1.

AND ORGANISMS

Individuals

No

single enzymatic protein

As our inquiry proceeds this business

is,

and

it

able to carry on the business of

is

become

will

it

requires the co-operation

how

have next, therefore, to try to see the ordered interaction of

hardly yet begun by biology. within which

all

the instructions of the

these parts. This

all

To

life

alone.

how very complex of many, many parts. We

increasingly clear

is

DNA regulate

in itself a gigantic task,

study we note that the units

initiate the

the parts necessary for the continuation of one type of

They

are found are the integrated individual organisms.

life

vary from a bac-

2X io -12 g to a blue whale of 1-3 X io 7 g or a giant red63 x io 8 g. They remain as coherent units for times varying

terium weighing

wood from

min

tree of a

few minutes (say 10 for bacteria) to several thousand years (say io9

redwood

for a

tree).

biological action, but is

we

The

'individual'

is

certainly a genuine unit of

shall increasingly learn that the life of the species

not wholly epitomized in any one individual and tha*\ moreover, the lives

of all the various species are related.

However,

it is

enough

clear

an individual that

is

a unit

that

from bacteria

of function.

roundings, integrated within

itself in its

It is

to

man we

can recognize

bounded-off from the sur-

homeostatic reactions, provided

with mechanisms for defence and repair of parts and of the whole (within limits), able to replicate itself,

exceptions).

We

and destined

shall later return to

to discuss the individual as the unit

Here we

many

to die as a

whole (again with

of these features and especially

of replication and hence of selection.

are discussing living organization,

and

it is

important to emphasize

the obvious but fundamental fact that living matter

is

always found as one

of a myriad types of individual bacteria, plants, or animals.

2.

Cells

organism are composed of cells, though the cells of bacteria from the rest. Plant and animal cells have a nucleus containing most of the together with other materials, separated by All types of

are rather different

DNA

a nuclear is

membrane from

bounded

externally

by

the cytoplasm (Figs. 4.1 and 4.2). a cell

membrane

(see Fig. 4.4),

The cytoplasm which may be

ORGAXELLES

4-

69

granular endoplasmic reticulum, with ribosomes attached

junction

between reticulum

envelope

c\

toplasm

ipproa scak

membrane

surface

agranular reticulum from ribosomes)

(plasmalemma)

(free

F

cell

1

G. 4.

1

.

Diagram of a

(Professor E. G.

reconstructed from electron micrographs.

Gray kindly made

supported by further structures, the

cell

the drawing.)

walls, especially

prominent

in

plants (see Fig. 3.2). Plant cells also often contain a very large wateryspace, the vacuole, the cytoplasm being restricted to a narrow film around this.

The cytoplasm

is

highly heterogeneous. Within a general watery phase

are suspended particles of all sizes from ribosomes of 20

nm

to

mitochondria

up to 400 fj.m long. These cell organelles will be discussed later (p Here we are concerned to notice simply the fact of the complexity of

cell

synthesis.

The ribosomes and microsomes are concerned with protein The mitochondria earn the respiratory enzymes, and are hence

called the

power packs of the

organization.

reticulum acting as the

culum D

sites

carries ribosomes

cell.

There

is

of synthesis.

a

whole system of membranous

The

and perhaps passes

granular endoplasmic

their products to the

reti-

smooth

AND ORGANISMS

CELLS, ORGANS,

7o

4.2

pore

perinuclear space

mitochondria

nuclear

membrane

pore

endoplasmic reticulum

nucleus junction of

endoplasmic "*

reticulum and nuclear envelope

mitochondrion

Fig.

—

4.2.

The

nuclear

membrane of a

Golgi membranes. In plant plasts, see Figs. 4.3

of sunlight

is

made

This crude

list

and

of the plant Oxalis. (From Marinos i960.)

of course, there are the plastids (chloro-

4.4), carriers

of the chlorophyll by which the energy

to synthesize organic

gives

electron microscope

many

cells,

cell

compounds from carbon

dioxide.

no idea of the highly complex structure that the

shows

in

any

cell (Fig. 4.5).

special organelles, such as the centrioles,

In addition, there are

concerned

Many

or the basal granules of the motile flagella.

cells

in cell divisions,

contain reserve

Each particular type of cell inclusion. Thus, muscle fibres con-

materials, granules of fat or carbohydrate.

usually has

its

own

characteristic cell

tain contractile myofibrils; connective tissue cells

produce the strong

fibres

of collagen and elastin; the various glands produce their characteristic secretions.

Each of these

cellular materials

must be formed

in the right quantity,

at the

proper time and place, basically under the control of the

There

is

elles is

achieved.

very

little

information as to

There

is

no reason

to

how

DNA.

the synthesis of cell organ-

doubt

that, as

we understand

the

SIMILARITY OF RIBOSOMES

4-2

7i

flagella

contractile

vacuole

cellulose envelope

volutin

nucleus

granule

mitochondrion

thylakoid (chloroplast lamella)

chloroplast

DNA

zone of chloroplast Golgi body

pyrenoid

starch grain

Fig.

4.3.

A diagram

pectin capsule

of a green flagellate Chlamydomunas reconstructed from electron micrographs. (After

Vickerman and Cox 1967.)

higher-order foldings of the proteins better, the control of cell

larity

and

have rings of nine

flagella

organization

is

fibres, usually

of these

out nature are becoming apparent.

components sedimenting

at

1-3

at the centre.

to

similarities of the tissues

Thus

the ribosomes of

all

This

rRNA

is

the

through-

bacteria have

23s and 16s, with molecular weights of

million. In higher plants the

weights of

with two

mammals. It depends upon produce the movements.

found from protozoa

arrangement of the proteins that As information accumulates, further

056

many

become apparent. Some of them have astonishing reguand constancy of structure. Thus the basal granules of motile cilia

organelles will

1*1

and

25s and 18s with molecular

and 07 million. In animals the smaller component of the

CELLS, ORGANS,

AND ORGANISMS

4.2

RNA is 1 8s (07 million) as in plants, but the larger component ('28s') has changed during evolution from 1 40 million molecular weight in sea urchins and Drosophila to 158 in chick and 175 in mammals. It is suggested that these differences are related to the fact that whereas plant cells remain totipotent those of animals can become differentiated to produce each a narrow range of proteins (Loening 1968, Noll 1970). But the similarities remain striking and further data within classes of animals should be most interesting.

A

curious point

is

that the

rRNA

of

chloroplasts resembles that of bacteria, suggesting that they are symbionts,

and

this

may

Even more

also

be true of mitochondria (see Dawid 1970). is the fact that competitive hybridization experiments

striking

by which nucleic acids are compared show that considerable base sequences same in all eukaryote organisms. Presumably the corresponding

are the

mitochondrion

cell

membrane

yv^ndoplasmic reticulum contractile vacuole

Fig.

The

4.4.

Electron micrograph of a

from the colonial green alga Eudorma tllinotensis. is characteristic of the young cell and probably The pyrenoid complex is sectioned tangentially, thus only the tubular cell

intense staining of the Golgi bodies

reflects

high activity.

chloroplast thylakoids penetrating the spaces between the starch plates are to be seen.

(By permission of Dr. M.

J.

Hobbs.)

SIMILARITY OF ORGANELLES

4-2

DNA

genes responsible for the 28s and 18s

Dawid, and Reeder

many

enormously

— indeed

able for study. Yet

to

be about 450

of Xenopus and they are clus-

it

of the organelles in in

(Brown,

is

is

such

a process, avail-

knowledge and methods of

that our

no one can yet truly say which features of these functions,

its

can look, of course, only

mens, but the problem

animals (and often also

all

demonstrates that there

we must admit

analysis are so feeble that

We

set

our task of obtaining a unified view of the

various parts of the cell are significant for are regulated.

73

known

1969).

similarity of so

in plants) helps

living process

RNA

are

on one of the 16 chromosomes of the haploid

tered together

The

There

stretches are also homologous.

we

not only that what

still less

how

see are artefacts.

It is

mitochondrion •

of cyst

-A cyst

wall

-

•

•^7"

endoplasmic reticulum

***^

£3T

I mitochondria nuclear ^-"" envelope .

nucleolus

*

i

endoplasm*

,

\

—

Fig.

4.5.

The

s^5^

fine structure of a soil

*

.

*

-

*

•

•

they

and dehydrated speci-

at fixed

-s

ectoplasm

amoeba, Acanthamoeba. (From Vickerman 1962.)

that

:

CELLS, ORGANS,

74

AND ORGANISMS

4.2

we have no proper ways of thinking about the organization at all. It is Nor can we generally see any

usually not highly geometric or regular.

comprehensible plan, with supply the cell

must have such

features,

lines or

and

in

communication channels. Yet

our electron microscope prepara-

tions perhaps they are already staring us in the

face— but we have not

learned to see them.

emphasize

it is

need

for imagination in

such

We have been describing the various components of the cell,

what

It is difficult to

matters.

'made of. But

sufficiently the

in life they are continually

next chapter will show. Moreover,

many of

undergoing change, as the

the changes are interrelated

and the whole system must be an intense whirl of controlled particularly unfortunate that

activity. It

is

some of our most powerful techniques de-

prive us of the possibility of seeing this organization while

it is

at work.

The

show the protein and other molecules, but only when they have been dried. What we see with it is at

electron microscope has sufficient resolution to

best like a single frame of a cine film. fractions that isolated

We

end

which

to

emphasize that

within a single 3.

centrifuge can separate for us

from the others.

shall try to build

sizing the to

The

perform particular chemical activities— but then they are

up

a picture of

all

this cellular activity

directed. In this chapter

it is all

a great variety

of different subsystems

by empha-

we

are concerned

to

be found even

is

cell.

Unicells

Some organisms

are

composed only of

a single cell

the differentiation of parts especially clearly (Fig. 4.3). flagellate

we

and

in

Thus

them we

see

within a green

find the usual nucleus, mitochondria, Golgi vesicles,

endo-

plasmic reticulum, ribosomes, and chloroplasts (Figs. 4.3-4.5); also a flagel-

lum

for

movement, with an elaborate basal apparatus and

a light-sensitive

spot, for orientation. Ciliates

such as Paramecium have an equally or more complex organi-

zation and this

is

found also

specialized for somatic

4.

life

in the sporozoa, in

only,

which some parts are

and are not passed on

in reproduction.

Multicellular animals All higher animals

they consist of

many

and plants possess

a

still

different types of cell.

higher level of organization

We

obviously cannot consider

here the various stages of this organization in lower animals and plants.

But

it is

it introduces a new level in the hierarchy human body there are about 10 15 cells (3X io 13 of them are cells) and we may estimate that these belong to at least 1000

impressive to realize that

of order. In a red blood

CONTROL OF CELL DIFFERENTIATION

44

Of course what

different cell types.

skin has

common

constitutes a cell type

characteristics but the skin of the face

is

is

75

arguable. All

not the same as

that of the chest or of the sole of the foot. Transplanted skin retains

must be

original character, so this

by function.

We might say there is a

(face skin, hair skin,

and there

trary

concerned

made of

is

neck skin,

what

a

Here again, divisions would be arbimaking enumerations. But we are

in

man is made of. We now we find that

very complex cells and

numerous

We

different sorts.

of these differences.

The

proper relation to others.

have found that he these cells

for the origin

may be

cell

to control

must appear

ulti-

systems that produce

in the correct

numbers and

The studies of embryologists have shown some-

thing of the factors by which the formation of particular cell types

We

'induced'.

something of

shall see

is

of

and significance

of the nuclei, once again, must be the

must serve

it

Each type of

differentiation.

have to seek

DNA

mate source of the order; in

etc.).

no special point

to discover

its

and not determined locally or genus 'skin', with some hundred species specific,

this

from time

to

is

time in our study,

both during embryonic development and in the adult. 5.

Control of cell differentiation

The all

different sorts of cells are

the instructions in the

produced by mechanisms that

from

of the salivary glands produce the enzyme salivary amylase, which

cells

splits

up

starch; cells of the skin produce keratin, the protective, insoluble

protein of skin and hair; and so on for

enzymes. is

select

DNA only certain ones for each type of cell. Thus

The

produced

all

the

at the right place, in the right

many thousands

of different

must ensure

that each protein

amounts, and

at the right time.

over-all regulatory processes

As yet we know only little about the control of such differentiations. When we understand them we should be much better able to control life in ourand other organisms

selves

The

(see Britten

principles of the regulation

and Davidson 1969). that most of the operons of the

may be

DNA of any cell are repressed, only certain act.

relevant ones being allowed to

In bacteria the active ones can be changed to suit the environmental

conditions. In multicellular animals

adult

is

'differentiated'

it

seems that often each

cell in

the

only certain operons are active, and most of the

;

others are firmly repressed perhaps irreversibly. There are, however,

some

which can become active under certain some range of action, for example to repair The question of how and when the operons become suppressed durlightly repressed operons,

conditions, allowing the cell itself.

ing development

swimming

is

still

unsolved. Nuclei from intestinal cells from the

tadpoles of frogs can be put into enucleated eggs and cause them

to develop into

mature

fertile

adults

(Gurdon

1962).

So the genes

at that

AND ORGANISMS

CELLS, ORGANS,

76

stage are

still

intact.

On

seem

to

some circumstances nuclear

the other hand, under

transplantation experiments

show

4.5

that the nuclei of differentiated cells

have undergone a stable change.

When

put into enucleated eggs

they produce embryos with special features, which will be reproduced

DNA

used for a further transplant. Their

their nuclei are

if

has been per-

manently changed. The problem of the control of the read-out of the infor-

DNA during development

mation in

One mechanism of part of the

for transcription

DNA

Thus

itself.

is

is

further discussed on pp. 59, 177.

to

produce

number of copies

a large

oocytes of the toad Xenopus contain 1000

and 18s RNAs, which make ribosomes, as These extra copies are synthesized during a that is to say, long short period soon after the tadpole metamorphoses before they will be needed. Only about o- 1 -02 per cent of the DNA is copied and it is not known how it is selected. There may be specific DNA polymerases that recognize the rRNA sites (Brown, Dawid, and Reeder 1969).

times as

many genes

for the 28s

are found in a somatic cell.

—

Moreover, something during cleavage. At

RNA;

nuclear

late stage

none

then transfer

is

RNA;

initiate

and

it

The cytoplasm

is

that very small pieces

of a plant, say a carrot root, can be

(sea-squirts). It

is

a

may

new blood

cells.

There

is

repair.

Understanding of how

new

plant (Steward

The

mammals

is

common

there are

many

continual controlled production of various let

alone in regeneration and

to control this production should provide

some of the most powerful new techniques

Organs and

retain their

non-growing region

perhaps, for example, those that give rise

types of new cells during normal replacement,

6.

from

to give rise to a

possible that even in adult

cells that are 'undifferentiated',

to

made

later

1970).

tissues

In some animals regeneration from small fragments

1970).

this

start again

can thus both repress synthesis and

(Gurdon and Woodland 1969, Gurdon

information content

synthesis

rRNA. Nuclei from

finally

Other evidence that the nuclei of differentiated full

RNA

the initiation of

produced; then some heterogeneous

put back into enucleated eggs stop synthesis and then

at the right time.

on

known about

is

first

for the

medicine of the future.

tissues

question of the supracellular organization of the body

is less

simple

than merely defining the various types of cell. Most parts of the body are not

made

of one single sort of

the skin

is

cell,

but of

many

sorts

combined. For example,

particularly complex. Besides the layers that

produce the outer

covering of the body there are blood vessels, lymph vessels, muscles, nerves,

sweat glands, sebaceous glands, pigment

Even

a relatively

cells,

simple tissue such as the liver

and other is

not

special features.

all liver cells. It

has

the arteries, veins, and lymphatics with their muscles, connective tissue,

AMOUNTS OF DNA

4-6

and nerves. Moreover, the

liver cells are

transformation, others to take

up waste

not

77

some

alike;

all

are for chemical

and several different

particles,

sorts

synthesize various substances.

The

production of each of these types of

and controlled

to

make each

must be

cell

strictly regulated

organ. Here are further tasks for the

DNA,

we how this is achieved. One suggestion is that various may be oscillatory and that neighbours may influence

operating, of course, through the cells and their interactions, though

have

little

evidence

cellular activities

each other (Goodwin 1964). Complicated

duced and perhaps control the

The level of the complexity of the organization is obviously

interacting cells.

vastly greater than in a bacterium or protozoan.

of the tissues the lower. to

we commonly

greater in what

is

could thus be pro-

field effects

differentiation of the various mutually

There

underestimate the complexity of the tissues

mammal. According

the higher animals than

measurements of such

are few actual

tainly already formidable,

Moreover, the complexity

call

differences. It

is

easy

which

is

cer-

of, say, a fish,

but perhaps marginally

less so

than that of a

our thesis the increase that has occurred during

to

evolution has been in the variety of devices used by the species to keep alive in face of

an unhelpful environment (The

been continually invading

less

and

life

of vertebrates). Life has

less propitious situations, for

which

more complex machinery is required. The instructions have become correspondingly more complex and the total amount of DNA has increased. Table 4.1 and Fig. 4.6 show that the amounts of DNA per cell are about a ever

.•Mammal Reptile*^

Amphibian*. Bony Fish*

/

/•Shark /•Lamprey

Amphioxus* Sea Squirt •

,

/ Coelenterate