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Fundamental concepts of biology [3d ed.]
 9780471631453, 0471631450, 9780471631538, 0471631531

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Third Edition

Fundamental Concepts ofBiology

Jt^H

Fundamental Concepts of Biology

Fundamental Concepts of Biology Third Edition

GIDEON i

GERALD

G.

RICHARD Science Software Systems

John Wiley

&

Sons, Inc.

New

York



Irn

A.

/

so(/f/i

Florida

ROBINSON

Um\t'r^ii\

Illustration

NELSON

E.

'n/vers/t)

t>t

sour/) Florida

Program prepared In

BOOLOOTIAN

West Los Angeles, California

London

Sydney

Toronto

This book was set in Laurel by York Graphic Services, Inc., and printed and bound by Rand McNally, Inc. The designer was

Roy

The

Jones.

editor

was

Harriman. Joan E. Rosenberg

Sally

supervised production.

I'ront Cover:

Guns

at sunset

Photo courtesy of

R. A. Boolootiah

Buck cover: Detail

Copyright

g

1967. 1970. 1974 by John

Wiley

All rights reserved. Published simultaneous!)

No

part of this lx>ok

&

in

Sons. Inc.

Canada.

may be reproduced by any means, nor

transmitted, nor translated into a

machine language with-

out the written permission of the publisher.

Library of Congress Cataloging

Publication Data:

in

Nelson, Gideon E.

Fundamental concepts of biolog)

.

Includes bibliographies. 1.

Biology.

I.

joint author.

II.

joint author.

III.

Robinson, Gerald

in

\..

Title.

QH308.2.N45 1974 ISBN 0471-63153-1 Printed

C,

Boolootian, Richard

574

the United States of America

10 9 S 7 6 5 4 3 2

1

1

Preface

/,.

In •_n

in

1

Student

ih,

preparing

1< l« -
\

tins edition

nme

tin-

l

undamt

/

pre* ious editions. Tin-

ot these

first

ntal

shaped

objei ti\cs thai

n


assistaiu e

I

be

ing the manual ript re* lea ed b) other

thorough editing b) special in-depth

editors.

The Second that will

>»l>|«'c ti\

Oi the text

«•

is

to provide a Fund of subject matter

enable you to better interpret and evaluate the types

information (bund

news media and

in

m

d

biological

Willi* iwntiSc periodicals

m

We

world of rapid scientific and technological advances. In man) of die chapters we have pointed out bow the bask concepts might appl) to human affairs. Yon u ill undoubted!) believe that this

is

an important

skill

to possess

discover additional applications throughout the

Students otten wonder how a In this third edition

and

nutrition:

we added

we updated

a

text.

rev lsed edition differs

from preceding ones.

chapters on hehaxior. iinmunitx and cancer,

siib|ec

t

matter where

it

seemed necessarv.

rearranged the contents of some chapters; and .uhk-d man) new illustrations. The last chapter. The \uthor's Views, expresses our collective

viewpoints about some biological matters that are especialb important to the future of \

textbook

mankind

in

our opinion.

sue h as this involves the efforts of

a debt of gratitude to

all.

biology editor at |ohn

\\ ilex

.

tor

man) people and we owe

extended to Robert 1.. Hosiers. so patiently guiding the text through the

Special gratitude

is

many

steps required for publication. Sallv

for her valuable in-depth editing, as

is

Harriman

Ron Nelson

is

due special thanks numerous

for supervising

details. Without the professional guidance of such would never get published.

individuals, textbooks

Gideon

E. Nelson

Gerald G. Robinson Richard A. Roolootian

vi

Preface

1

Contents (

haptef

1

Life: Its Characteristics Biolog) .un\ li.ii.it

(

trusties

ol

I

jving limits

Methods ol Stud) 9 Basic and Applied Resean

2

\toiiis ,ine erroneous to assume- that

materials with the environment.

ol

time,

tins discussion

vestigations

another. In ever) place plants and adjusted

process

ol

particular!) intensive

is

th.it

marked change in plant and animal life. Fii tit

ones are the How

in.t|or

also

important molecule

ol

about

which these plants thrived

in

in tillli.

Man) important and evolution

transfei

gianl

id

in tins l>.isu in

the ionise ol tins book the discover) oi

s I

w

biochemical events

.ill

known

to offspring are well

is

which

have much

ate!)

a hasis foi inferences

to the available resources

beiedilar)

i>t

deoxyribonucleic

\

information

The

lor biologists

of events that could have occurred in earlier epochs

know!

.ill

has been acquired

biologists

l>\

Fundamental

controls

evolution

e.irlv

fwwmating puzzle

,i

environment

the

as this area of stud)

t\< t,

general mechanisms

iin.il

Nearl)

the newei areas oi biolog)

i>)

lli.

lists.

reprodiM tion net easaril)

beredit)

gem

oi

called

usual!)

l

sun.- the beginning ol is

possiUv three

ol.

origin and

*>

1

a longei

things ovei the surface oi the earth Studies

leads to the subjed

edge

eive

c

more The

oiganiams prov ides

distribution //
\

scientific

chemical

equipment;

apparatus,

etc

Eventual!) he considers his observations facta and proposes a statement tli.it relates them or suiniua

n/es then significance. Tins fog statement

is

hypothesis,

work

possible, additional ohser\ ations

It

or experiments are

performed

to test

the validity

il\

00 experimentation

to ver-

u hen the material is suitable to this technique. Experiments are designed to discover which ot die variables in a situation are significant. ih a In pothesis

In Other words,

llis

wav

that

onlv

D)

experiments must Ik- conducted in such one set ot tests differs from another set

one

taetor.

this

the proper controls

Ucfa

a

is

accomplished b) Using

rurthermore. the\ must be

individuals. Repeatability

is

one

experimentation

is

a

method

tor

determining the most probable cause-and-effect relationship. The experimenter is hound In onlv two

ot the

aspects ot the scientific method, tor

It

ever

the h\ pothesis tests the

is

it

most crucial is

probability that

it

supported verified D) what-

There

is

ot

science

is

it

now

COrrectr) interprets a

wax. the essence

the major

man) hypotheses.

experimenter applies, then

a le\el ot greater certainty.

In a

ot

nature that the\ ma\ he repe.ited hv other

basis tor accepting or rejecting

ol the h\ pothesis.

Scientists reh hea\

rules,

a

reaches a higher

set of tacts.

not to "prove"

or ''disprove,'' but to indicate relative certaint) or probability.

Methods of Study

11

made and new hypothesis

emerge

principle (law)

the imagination

theory

and observations

tist.

hypothesis verified

/

\



The proper

i.e.,

the creativity

interpretation of data, the design

and the formulation of useful new theories frequentlv involve a certain amount of intuition a hunch about how to proceed. This may sound quite unscientific, yet many scientists admit



hypothesis

that intuition

important in their investigatory

is

work.

t

In addition to gathering facts, hypothesizing, and

experiments and observations (facts)

someone must, tempt to summarize the research testing hypotheses,

t tentative hypothesis

at intervals, at-

made

progress being

a biological

in

field.

quently, this type of material appears as a

Fre-

mono-

and information

graph or an extensive review journal. Textbooks represent

article in a scientific still

another attempt

to bring together basic information

A flow chart demonstrating some aspects of methodology.

Figure 1.11. scientific

effort of other

biologists in order to present the current status or

t pre-existing knowledge

is

— of the scien-

of experiments,

t hypothesis rejected

change if additional facts Throughout this book, the

to

term principle is used in this sense. An important aspect of scientific investigation

t additional experiments

subject

in the future.

from various

and principles

fields of study.

Basic and Applied Research

The systematic accumulation scribed above

of observations de-

kinds of knowledge: the facts he has gathered, and

termed research. In biology, as in other sciences, one often encounters a distinction between basic and applied research. Scientific in-

the hypotheses or theories he has derived from the

vestigations that

At

facts.

this point, the investigator

has contributed two

These are important additions to the

field of

is

are not directed at immediate

practical applications are classed as pure or basic research. For example, a biochemist might invest

biology.

Theories are useful not only for synthesizing data

but also because predictions can frequentlv be

made

from them, thus leading to entirely new lines of investigation. For example, from Darwin's observation of the way in which young plants grow toward light, he constructed the hypothesis that the influence of light on the stem tip was transmitted to

many

years studving the effects of certain chemicals

on the energy-releasing reactions of interest

may

lie

insects.

His

entirely in insect phvsiologv rather

than in practical uses of his findings. The value of this

kind of activity

lies

primarily in

to man's understanding of the

Most research of

type

this

is

its

contribution

world around him.

conducted by biologists

the rest of the plant

associated with academic institutions. In fact, one

hypothesis led to

of the major attractions of a university teaching

by a chemical factor. This experiments that proved him cor-

career

rect.

A

theory that has been repeatedlv verified and

appears to have wide application in biology

may

become

a biological principle. Such principles are sometimes called biological laws, although this terminology does not change their status as statements

one's

is

the opportunity to engage in research of

own

choosing.

Applied research attempts to solve a problem of immediate concern or, in some cases, tries to find utilitarian uses for a

new

scientific discovery.

The

basic ideas for these endeavors arise mainly from

that apply with a high degree of probability to a

pure research, and

wide range of biological events. They are

are intimately related. As an example, a scientist

12

Life: Its Characteristics

still

man-

and Study

in this

way

the two activities

employed!)) an insecticide manufacture! ma) de msc 1 new bug killa baaed on me biochemist'i dbcoveriea

in

insect

hire

sdence

ea

is

batk and applied research since both are expensive

mainta in .is

and

basii

basic

in

Vet in die earl) 1900a, itud) ol die atom was a Beldofbask reaearch with no foreaeeable practica] ate. It dma aeema al.soluteU neceaaar) to continue to support and encourage the proceaa oi basfc re

(

useful'

effort

rhia

t

technolog)

tremel) important In modem Ufa ontrovera) somriniM-s ..uses over the relative importanoe oi

to

unimportant

teemingl)

.1

judgment can be made onl) inretroaped onaidef die enormous rfgnifi™™™. ,,i our preaent knowledge about the structure of the atom,

physiology.

Obvioualy, applied

l

research,

research doe* not seem

applied sdence. But no our

with certaint) the ultimate value

.is

an predfa



l

c

one rpts ol

l>lol'4\

problems encountered

intelligent decisions about

Living things are characterized b)

I

knowledge

the science ol Ufa

.1

combination

olten

in dail)

lac

ill

affain

oi structure,

me

tabolism, regulator) devices, reproduction, heredity, and an evolutional) histor)

5

in

scientists, um-

.i

variet)

ol

techniques and ap

acquiring new knowledge. Such knowledge must

verifiable

l>e

others

l>\

biological law

\

(1

degree

oi

probabilit)

principle to a \\u\r

Sdentific reaearch

7

about nature ot

and other

Biologists,

preaches

Basic

is

a

range

statement that applies with a high rk.

knowledge oi

life,

to

brain

few. Written for aonspecialists. \l

Segal,

I'lu

Game of Science.

Brooks

(

ok- Pul>-

ITS pp paper. \ thorough look at the attitudes, methodology, applications, and scope of sdence. \ touch of humor lishing ('o. Belmont, Cal.,

1989,

here and there livens up the subjects considerably.

Additional Readings

13

Wallia, C.S. editor*. Toward Century 21: Technology. Society,

Basic Books.

New

York. 1970. 318 pp.

A volume

and Human

for general readers

Values.

concern-

ing the influence of science and technology on our future. Based on 30 lectures at

Stanford University including brain research, medicine, and

population and environment. Interesting and readable.

Wolstenholme, Gordon editor Little, in

Brown and

.

Man and

His Future.

Co., Boston, 1963, 410 pp.

which well-known

biologists, including

A Ciba Foundation The

Volume.

results of a conference

Gregory Pincus, Albert Szent-

Gvorgvi. Alex Comfort, and J.B.S. Haldane. discuss the role of biology in the future of man. Topics include control of reproduction in

mammals, the

promise of medical science, longevity of man. and a look at next ten thousand vears. Nontechnical and interesting reading.

14

Life: Its Characteristics

and Study

man

in the

//ii\«

lh

microspheres of proteinUh material

wen produced

and lu\ amociatet u lull conducting experimental studies on the origin of lift lh Skint In/

Sidnt y Foi

chapter

Chemical Basis

Life: Its \s

we have

us to u lei it thai

>>

we can

mimic of

acquire

.1

.t

number

of

chancterisrica thai help

One f these includes me cbemica] activities It we look me general structure t me atom .it

tin- 1\

pes of molecules thai are

common

foundation for understanding these

vital

to

1

1

v

ing

chemical

processes,

A tomic

ami Molecules Stnu tun

Moms

are

vtttms

reactions

me

smallesl

particles of

elements thai enter into chemical

Nevertheless, atoms are not indivisible. Each

compad

atom

consists of

and lighter particles called electrons that orbit the nucleus at some distance from its center. Electrons art- virtuall) weightless ami each one carries a negative electrical charge. Vtomic nuclei arc composed of protons and neutrons, except for tin- hydroheavy,

a relativel)

central nucleus

gen nucleus, which contains onrj one proton. Each proton or neutron has an arbitrar) unit ami is about isoo times of atomic weight

oik- unit

heavier than the electron,

\b atom's weight results almost entirel) from

protons ami neutrons.

proton has a positive electrical charge, w hereas

its

a neutron

is

\

neutral.

Looking at the structure of atoms of different elements, we see that each element has a distinctive number of protons in its atomic nuclei \n element is a substance whose atoms all contain the same Figure 2.1 .

number of protons and the same number of electrons. Furthermore, since the number of protons equals the number of electrons, an atom is electrically neutral.

17

Figure

2.1.

important

A

schematic representation of four atoms

in biological systems.

sents the nucleus of the

The inner circle repreatom containing protons + and I

neutrons (N). The electrons arc represented by e~. Carbon, for example, has six neutrons and six protons in the

nucleus and six electrons in rapid motion around the hydrogen

nucleus.

nitrogen

2

C

13

C

Figure 2.2. These isotopes of carbon differ only in the number of neutrons contained in the nucleus: the numbers of protons and electrons in these isotopes are the same. 14 C is synthetically prepared and is frequently used in studying chemical reactions in living cells.

18

We

ma) have given

1 1 * *

Iki dI neutrons presenl

an element. Tins

o! ol

s

Untns

ol

carbon

even atom

in

due

nut always

is

C),

In the

atoms

example, ma) have one

foi

«

13

1.2

I

I

depending on the numbei oi theii neutrons These different kinds ol atoms ol the same element .u

Moleculai oxygen

become separated from

come

compln and and

\

a

substance dissolves

.1

E,

In/

activated

m

vital

This also serves to im rease

role of

lx)tli

\

dons occui between individual atoms,

opportunit) for them to

The

ol

t,, r

indicated

iv

compounds \ / H

the bransporl

chemistr)

solvent

water, the molecules or mi is .11

l"i

molecules, and not between large

tions ol these particles

me

basis foi

.1

the

realit)

hapten


nh R. I',nt,r Bottom, mi

1

orptneU*

of

rtructun oj

in outer membram rurrounds

nun

tin

12

thou

in

some algae

plants. In

in

number from

to fort)

most plants,

in

a single large

some

the leaf cells ot

a chloroplast

is

enclosed b\

membrane and is divided internally b\ numerous additional membranes that contain the

a unit

chlorophyll

Figures 3.13, 3.14

ChloroplastS,

amounts of words, their

DN

like \.

own

.

mitochondria, contain small

K\

\.

and ribosomes



in

other

protein-sx nthesis apparatus.

Structures in the Cell

For

">?

Figure 3.13. A portion of a chloroplast. The round gray masses (grana) contain the chlorophyll necessary for photosynthesis. The dark streaks extending through the photograph are membranes (X25,000) (Dr. L.K. Shwnway and Dr. T.E. Weier).

54

Mgtm

n

earth

oomei from

For the most part. these are green plants. accurate and meaningful summaries t photosv nthesis when the process is viewed -it the level of the whole plant. For example, the equation helps sun and

tfae

is

trapped

us to see that the

l>v

amount

ot

(

(),.

water, and tight ener^\ present

in

the

environment probabl) affect the rate of photosynthesis. However. l>e\ond the simple observation that photos\ nthesis transforms i.e.. carbon-to-carbon energ) from tighl into a universal!) usable form bonds neither the formula nor the statement re\eals what reallv occurs. plant's





When rate!)

these summaries came into common use, the) summarized accueverything that was known about photosynthesis. However, man's

knowledge of this essential phenomenon has become much more extensive, ami our discussion must reflect this new knowledge. Historical

Background

was carried out Helmont in the 1640s He weighed dried earth, placed it in a large pot. and then planted a weighted willow shoot in it. For a period of five years, only water was added to the soil. Then the willow tree and the soil were weighed. The weight

One b)

of the earliest experiments relating to photos) nthesis

the

Belgian scientist Jean-Baptiste van

63

in separate sealed glass vessels,

and

the contain-

left

the air

dark for several hours. Then he tested in each container by introducing a burning

candle.

The candle did not burn

ers in the

in

indicating an absence of oxygen.

any of the

He

trials,

then placed

the vessels in the light and after a few hours found

burned in any vessel that contained green plant parts, indicating the presence of oxygen,

that the candle

but did not burn in the other vessels

(

Figure

4.1).

Ingenhousz also showed that the brighter the light, the more rapid was the formation of oxygen by green plants. Just after 1800, Nicolas de Saussure, a Swiss scientist,

produced the

first

quantitative studies of

photosynthesis and showed that the amount of oxy-

gen produced was the same as the amount of carbon dioxide utilized by the plant. He carefullv measured the amounts of oxygen, nitrogen, and carbon dioxide in

the vessels before and after illuminating the

He also weighed the plants. By comparing the amounts of gases present before and after the experiment, he was able to show the changes that were brought about by photosynthesis. It was possible at this time to write the following plants in them.

general equation for photosynthesis: Ingenhousz found that the green parts of plants such as leaves, stems, and green seeds produced Figure

4.1.

oxygen when kept candles kept in

in light, as

evidenced

hij

carbon dioxide

+

green plants

water

the burning

bottom row of bottles. The same plant parts the dark top row did not show this activity.

living material

+

oxygen

in the

Notice that the light

is

this

equation does not indicate

why

needed, what component of the green

parts of plants functions in this reaction, or the identity of the organic product.

had changed only slightly, but the tree had gained more than 160 pounds. Van Helmont concluded that the tree had risen from the water alone. He did not realize that carbon dioxide from the air had contributed to its gain. In 1774 Joseph Priestley, an English pastor and chemist, showed that plants could utilize carbon dioxide and produce oxygen (but he did not call the substances by diese names Shortly after the of the soil

.

Priestley experiments, Jan Ingenhousz, a

Dutch

sci-

Bv 1875

the equation

6C0 2 + 6H,0 +

chloroph>

energy from light

C 6 H 12

6

"

+

>

60,

had been experimentally determined. At that time it was thought to be a simple one-step reaction. Over the next sixty years evidence slowly accumulated to the contrarv. Investigators showed that

many organisms could

incorporate carbon dioxide

into carbohydrates without light as a source of en-

demonstrated that only leaves and green stems could carry out such activity, and only when they were illuminated. Ingenhousz's experiments

ergy. This incorporation

were simple. He placed

different plant parts such

ent source of energy was used to drive the reaction.

wood, and seeds

Shortly after 1900. F.F. Blackman. an English plant

entist,

as leaves, green stems, older stems,

64

Photosynthesis: Energy Fixation

same way

seemed

to

occur in the

as in photosynthesis, except that a differ-

n

physiologist, studied the rates

occurred

thesis

Showed

undei

pmi

tin

th.tl

Ltd

ess

several different reactions ies,

from those

depend

n

which

rea
\

but

not directl)

ns

t

and

the light

oi in

is

dioxide

It

source

tin-

Although several studies had given

oxygen,

tin-

.uiposite ut

1

came from carbon

now known, howevei

i>(

1

light.

photosynthesis

is

.1

modern biochemical techniques became

Until

in

t.tlk

take place

that

ol the light

products

to

photosynthesis

ol

guish those reactions in role

to In-

Bet ause of Ins discover-

now convenient

is

it

dark resu tions

which photosynconditions and

.it

various

it

was not

I'M

until

I

Samuel Ruben, an American biochemist and coworkers confirmed it l>\ using radioisotope the stud\

in

suspensions

ol

photosynthesis. The)

celled plants

ol single

two

in

one solution the water molecules contained lieaw oxygen l80). In the other solution, "O was In

contained

analyzed |s

()

in

The

oxide

v

oxygen released

creased However, increased gen

in

mis

ai

amount

bon dioxtd*

Figure

the plant

l>\

I

ol

in

1 I

)")

I

ol

a

technique

heav) ox)

amount

the-

ponents oi the

producing the

whole- chloroplasts from other cells In

amine

com

using an ultracentrifuge.

the chloroplasts separated out.

it

was possible

\s

to ex

the processes ot photos\ nthesis without in-

It has been learned that chlorophyll and the en zymes involved in photosynthesis arc integral parts

the structure ol the chloroplasts.

layers

of

lipoprotein

between

photosMithetu reactions.


m u ut>

r

not

m


ss


v photosynthesis from the leaves to nourish Othei tissues ol the plant Water loss from the uppei and lower snilaces of llic leal is reduced hv the rpulmnn. a tissue with

organic

.in

,

outer layer

lower surface

ol

vvaw material tailed

ol the leal has small

m

move

between the the inesophvll and the atinospheie The

ttomata, which allow spaces

The

cutin.

openings (ailed

\ it. vitamins must pass through theprocess of digestion and .^sorption without altera

Man) widespread

recommended

the

is

contribute nothing worthwhile to our calcium

die) arerela-

complex molecules; die) cannot be

An- required b) animals

grams

The vitamins have

Table 8.2

usuallv

interesting aspect of mineral nutrition

the extreme divergence in the amounts of the

different mineral salts required

bv humans. These

amino

must be broken acids,

down

into simple sugars,

fatty acids, glycerol, or other small

molecular forms. Although these digestive processes are

common to all types of organisms, the site of may be inside or outside cells. In addition,

range from several grams a dav for sodium, potas-

digestion

sium, and chloride to a tenth of a milligram a day

the digestive system

mav be

a simple sac or a tube

Digestion

97

Figure 6.2. The movement of food vacuoles (yellow circles) in Paramecium during intracellular digestion (CCM: General Biological, Inc., Chicago).

of great complexity; or the digestive processes

may

main

in

Many

internal parasites, like the

one place

until

digestion

is

completed.

even occur outside the organism. These variations are adaptations that help to provide each cell of

tapeworm, absorb predigested food from the environment and also

the organism with the nutrients that

lack digestive systems.

it

requires.

Another form of external digestion occurs

in

some

Vacuoles

The

digestion that occurs inside cells (intracellular

digestion) breaks

down molecules

that have

been

synthesized within the cell and those brought into the

cell.

Food

particles picked

up by phagocytosis

are placed in food vacuoles in the cytoplasm. Digestive

enzymes secreted by the Golgi apparatus end up in the food vacuoles, where

(see p. 49)

hydrolysis of the food particles to simple molecules occurs. These small particles enter the cytoplasm

through the membrane that surrounds the vacuole. This type of digestion animals.

is

found

in

Amoeba and Paramecium

many

kinds of

(Figure 6.2) are

two examples of animals that carry on only

intra-

cellular digestion.

External Digestion

Although intracellular digestion might be considered the simplest form,

many

fungi (molds) and

bacteria have no digestive system at tually secrete digestive

all.

They

ac-

enzymes onto the food out-

side their bodies, then absorb the simple

molecules

that result from this external digestive process (Fig-

ure

6.3).

This method

are small

98

compared

restricted to organisms that

Figure

6.3.

to the food source or that re-

shown

in the illustration.

is

The Intake and Processing of Nutrients

Bread mold exhibits external digestion as

The

predators such as spiders The) injed digestive en

different functions.

gymes

gastrovascular cavit) because

Bed

into the pre)

Tins pattern

tissues

the) suck oul

latei oi

digestion

dealing with the hard outei pre)

.iikI

area

tin-

the spidei

s

lo(

i

tin-

lique-

adapted

is

overing oi

«

1 1

1

-

to

msei

alization oi hunting to

one

web.

Some

,i\
\

ol ver)

rounded

iiik

H

From the phosphate-sugar chain. Four kinds

Structure

information thai determines the activities cell

H

bases ol the nucleotides extend out

helix

of

l)\\ molecule

the

thus

aih\

confirmed

H

— CI

C

•N I

O—

H-N

H

I

' I

I

H thymine

H

/

K

»

—N— |

\-

N^

H

H

II

1

=

C—

C 1

V

H- -/

H

/

(gun

m

/>\

7.2. \

Structural details of a nucleotide

right

ganic base

\

left)

and

in

II

C

-N

H

the four organic bases that occur

nucleotide consists of a phosphate group, a sugar [deoxyribose),

thymine

H

\

\? i/

and

this instant

111

In order for

DXA

to serve as a pattern for

RXA,

two nucleotide chains of the DXA helix must separate from each other along part of the molecule. One chain then serves as a pattern for making an RXA chain. The RXA chain is made by matching the

ribose-containing nucleotides (from a supplv in the nucleus) with the appropriate bases on the

DXA

chain. In this event, cvtosine pairs with guanine

thymine

the

(in

DXA chain RXA bases so that the

will line

JP IFtgun

7.3.

if

and the

up

to read guanine, adenine,

uracil takes the place of thvmine.

DXA adenine pairs with uracil in making

RNA. To complete

RXA

with adenine. Thus

"reads" cvtosine, thymine, adenine, the

RXA.

uracil. In

DXA)

the

RXA

molecule, adjacent

nucleotides must be linked by phosphate-to-

adenine

Formation of specific base pairs

in

DNA. The

sequence of bases in the left chain, in this illustration. defines the sequence of bases in the other chain; hence the two chains are complements of each other.

Watson and Crick's

structural

0=C

model. For then-

work, Watson and Crick were awarded the Nobel prize in 1962.

DNA DN \

controls cell activities by directing the syn-

thesis of all proteins in the cell. in

DXA may

constitutes

occur

in

The organic

bases

any order: their sequence

the genetic information that controls

which proteins are made. This control is exercised indirectly, however, since most DXA is confined to the nucleus and most proteins are made in the cytoplasm. To breach the gap, the chemical code sequence of bases in DNA is transcribed into another kind of molecule called RXA ribonucleic acid This RNA then leaves the nucleus and carries its

—\

nucleotides

« 1 *

onl\

is

is

\

1

and

guanine, cytosine |{\

breaks

it

and moves through the

\

understood

\«-t

RN

completed,

is

membrane

inn leai tli.it

\

union with

mRN

l

is

\

con

mak

foi

to attach

one or more ribosomes in the cytoplasm and become the pattern on which protein can be put to

.1

togethei from

variable

tri'ini-K

in

acids.

si/c

molecule relates

tinis

amino

RN

Messengei

\

ex-

is

Presumabl) the length

to the si/r

the protein

i

of it

to synthesize.

RN

t\|M-s ot a

for

down

lias

lew

molecules

into

its

chapter in tin-

\

we

a

shorter

life

than other

Each molecule serves as the pattern l protein and then breaks

\

Figun

7.5

Formation

basis in in/i\

RN

Messengei

constituent nucleotides, later

will see that this instabilit)

control ol activit) in the

is

in tin-

/;

\

Tht

\

pattern of

tequena oj bate* molecule, and there/on it u complement i>t trntn \ it us formed which The 0} DA ni/;\ \ alsocontaint 111 coded form tin temt information lor making prott ini in

\

controlled

i\

hi,

tin

D\ \ nt

tin

11

important

cell.

Tin- specific nature ol the instructions carried by

m KN

\ has been clearh demonstrated l>\ experiments with immature red blood cells, which are main!) concerned with producing hemoglobin. I

sing an ultracentrifuge, experimenters separated

tin-

mRN

\

from

tin-

red blood cells ol a

then added to

a

young duck This

suspension containing

mRN all

\

was

essentia]

the mate-

lor

was analyzed, the experimenters found onl) one amino acid sequence in the hemoglobin. This sequence was typical of the immature red blood cells ol the duck from which the mRNA was obtained.

experiment shows that production

of

a specific

one kind

mRN

of protein

\

(on

even

111

the presence ot the apparatus for protein synthesis

from .mother species.

other components ol immature

hemoglobin production. These materials came from immature red blood cells ot a rabbit rather than from a duck. No messenger UN \ from the rabbit was included in the suspension. When the hemoglobin that was synthesized rials

This

trols the

Experiments indicate that the genetic code tranmRN \ from I)\ \ is a triplet code-, that

SCribed into is,

the bases on the

threes.

Each group

calculation

us

tells

mRNA

are read in groups of

specifies an

that

amino

acid.

A simple

the four bases yield 64

possible triplet codes.

Transfer

RNA

Transfer

H\

portions of

\.

cleotides that to transport

formed under the direction of small

D\ A. is

consists of a single strand of nu-

coiled about

amino

itself.

Its

function

is

acids from the cytoplasm to the

Control by Genes

113

were machines on an assembly line. One after another the members of a group attach to a single messenger RNA. Moving in only one direction along the

mRNA,

the ribosomes evidently plav a part in

tRNAs

attaching

to appropriate sites

As the identifying

mRNA,

pairs with the triplet of the

tRNA

that the

carries

is

mRNA. tRNA

on the

each successive

triplets of

the

amino acid

attached to those in the

growing amino acid chain that the ribosome

When Figure 7.6. Results of an experiment using messenger RNA containing only uracil U] triplets. Only tRNA [carrying the amino acid pehnylalanine) coded with three consecutive adenines (A) matches the correct triplet

on

mRNA. Hence

tin

only phenylalanine blue arrows)

leave the

mRNA

and

carries.

completed, the ribosomes

is

also liberate the protein. It

takes roughly one minute for the ribosomes to as-

semble a hemoglobin molecule

The idea of the

will be incorporated into proti in.

the protein

in this

way.

that the ribosome "reads" three bases

mRNA

at

one time has recently been experi-

mentally confirmed.

somal action

It is

possible to stop the ribo-

move

just prior to a

or just afterward

bv removing one of the various protein factors quired for ribosomal function. The ribosomes ribosomal

site of

mRNA

an

term transfer RNA. Each

molecule; hence the

tRNA

apparently

is

coded

amino acid by means of three

for a particular

or-

ganic bases at one end of the molecule. For exam-

AAA,

consecutive adenines,

ple, three

specify the

amino acid phenylalanine (Figure 7.6). A tRNA with AAA at one end will combine with and transport onlv phenylalanine. In addition, the triplet on

tRNA

match only the correct triplet of complementary bases on the mRNA (in this instance, uracil, uracil, uracil). The tRNA, then, serves two functions: specifying an amino acid and identifying a location on the mRNA. Since there are 20 amino acids and 64 triplet codes, some of the amino acids must be represented by more than one code. Perhaps this adds needed the

in

re-

one

movement while

suspension are stopped just before

those in another are stopped just after. Both suspensions are then treated with a ribonuclease,

zyme

that will digest

mRNA

the

all

an en-

except that

attached to the ribosome. Comparison of the undigested strands of

mRNA

shows that three more bases

after

such treatment

in addition to those

already read, are attached to the ribosome after the

will

flexibility to

the coding system.

Two

of the triplets

Summary of The

Protein Synthesis

making proteins

directions for

reside in

DNA

chemical code using four nucleotides. In a process involving enzymes and the expenditure of in a

energy, the code

is

copied as a template in

which transmits the code cell

Figure

7.7).

There,

mRNA.

into the cytoplasm of the

mRNA

associates with a

serves as a "start" code as well as coding for an

group of one or more ribosomes. Under the direction of enzymes, tRNAs pick up specific kinds of amino acids from a supply that is free in the cytoplasm. This tRNA-amino acid combination is a

amino

potentially reactive substance because

do not appear to represent any of the amino acids; they serve as "punctuation" codes for ending protein synthesis. In addition,

one

triplet

apparently

acid.

Ribosomes— Sites of

Protein Synthesis

For some time ribosomes have been known

to

be

the sites of protein synthesis. Visible only with an

electron microscope, they are nevertheless abun-

dant

in all cells.

of about equal

Chapter

They

RNA

composed

and protein. (See

of the

the

mRNA

protein

is

in

has been translated into a protein.

released, freeing

mRNA. tRNA.

The

and the

ribosomes to repeat their functions.

>.

Ribosomes function

114

are spherical and

amounts of

some

combining the two components remains in the molecule. The tRNAs move the amino acids to the locality of an mRNA-ribosome complex. With the aid of ribosomes, the amino acids are linked into a protein. In other words, the code in energy used

in

small groups as

Control within Cells

if

they

Deciphering the genetic code has been a major

DNA

DNA

coding specific

mRNA

.

IlflflffW

J^yuyi

,

Figun

Summary

7.7

protein molecule ni/»\

1

amino

acid,

and

messengei l\\

of

tih

tin-

mRNA,

tins

e

mid

\.

D\

i,

.1

meaning

ol

the

is

l l

l

Ui\

group began

the National

\

)(>

s\

nil.

th>

.1

triplets

in

Nirenberg and

institutes oJ

nthesizing artificial

Health

Nirenberg's

mRN

\s

Each type ribosomes coli

ol

mRN

oi artificial l>\

produced in each rase. mRN \ was thru mixed with \

is

grinding up colon bacilli [Escherichia

the presence oi

was then added

an energ)

source and the

with carbon It.

to the

then accepts m,

ilit\

do not take the tell to

ol

meets new environments the operon theory, tor which

as

activit)

I

dons

all tin- reai

l>\

it

two

the earl) experiments used

support their theorv

[acob and

l>v

involved genes

m

production

ol

that direct the

coti

/

tor the utilization ol gal

six-carbon

Under most conditions the

sugar

regulatoi gene,

tin-

the

operatoi

iuH\

\

m

is

the Form

svnthesis hv

interacts

th.it

anv of

d

changed

so that

present

is

the form oi the

coti,

it

no longer interacts with the

r

\s a result, the svnthesis ot 111HN

arenecessar) for utilization of galactose Inti repression

the-

ol

a

genes

oi

is

removed

l>v

the-

substance lor the activitv

suitable

presence ot

/ coti shows how represmav he stopped hv the removal ot a substance makes the activitv ot the ^cuc-s iinnec essarv

\nothei example From

For example, typical environments contain cient

According

s

to this theory, not all ol the l)\ \ helix

RN

directs the synthesis ol messenger

time,

'"

'J.'

some

ol

it

is

sw itched

Regulator genes

same

the

to

acid, acetylomithine.

b)

ing operator genes. In other words, the Form of this

trv

protein

prevents is.

composed

and the structural genes Repression occurs

it

when

thesis /v

ol

an

ol

activity

an operator gene

Figure

controls

7.9).

the protein produced In

the regulator gene combines

and prevents the

the

represses

a unit

\\

initiation ot

ith

the operator gene

messenger

K\

\

svn-

along the structural genes. None of the en-

mes normally produced under the direction

of the

structural genes will he synthesized. \

repressor protein

active lonn

substance,

il

which

differs

tor

is

an

orv

mes

re-

ptophan From another amino Here, the protein produced

comes From experiments with bacteria and

organisms. However, information about the regulation of clusive.

DNA

in these cells

is

impressively incon-

Recall from Chapter 3 that the cells

is

chromosomes. For biologists throught

repressors,

to

absent, the

hence procaryotic cells, indications are strong however, that the general outlines proposed by the theorv also operate in the eucarv otic cells of higher

in-

different

is

is in a repressor lonn unless ptophan is absent and acetv lornithine is present. Most ot the evidence supporting the operon the-

present. This

to

tryptophan

the regulator gene

eucarv otic

mav he changed

a specific substance

Il

bacteria are capable ol synthesizing the en/v trv

suffi-

amino acid tryptophan

the

ot

sustain bacterial growth.

direct the production ol proteins capable ol affect-

Operon, that

amounts

quired lor deriv ing

messenger RNAs,

via

act.

\ at

oil.

these

'Relies.

on or

the regulator

\

no longer repressed along the structural genes of the operon, and thus the en/v mes are produced that

that

ol

in

epressor

/

is

operators and regulators The

the pioduct

ith

structural genes in

tin-

operon However, when gahu tose

that

w

prevents

consequent])

Tins

gene.

operator genes Function like sw itches that are turned In

re-

pressor protein, Synthesized under the direction of

sion

oil

ot

V

enzymes required

Frenchmen, Francois [acob and [acques Monod. were aw.uded tlie Nobel prize in 1965 lliv

svnthesis along this portion ot l)\

B bacterium,

ol

turning on the operator gene,

other words.

111

operator gene.

refta tion tells us that

the

a'

substance induces the production

proteins b)

Some ol Monod to

is

moments

this

the env ironmenl

scant)

Operon Tbeor) \

ot

mP.\

blocked.

is

ma) be synthesized in the cytoplasm proteins, lor example 01 derived from die environment ions and amino acids, foi example in either case, the pres-

enclosed

in

DNA

structures

in

called

a considerable period of time,

that the histones. a

group of

basic proteins that are part of the structure of chro-

Operon Theory

117

gene A

structural

regulator

gene C

gene

7.9. A model of the operon theory. The regulator gene controls segments of DXA means of a repressor protein as shown above. The repressor protein combines with the operator gene to prevent the structural genes under its control from functioning. If the repressor protein is inactivated left side of diagram', the operator gene and tlie structural genes it controls begin to function. The events shown for structural gene B also occur at genes A and C.

Figure

Inj

mosomes, were the important control mechanism. Some of the evidence came from work done by

the evidence

Barth and Barth studying the effect of different ions

are available.

is

sparse and no details of the process

of regulation of

DXA

activity in higher organisms

on developing frog embryos. They found that ions could bring about differentiation of cells in the

embryo and ability to

that the various ions differed in their

induce these

effects.

More

importantly,

weie the most effective in doing this were the same ions that best prevented the bonding of histone to the DNA. It is now thought that histhe ions that

tone

is

only a nonspecific inhibitor of

and does not account directing

mRXA

A second

118

now being

he involved

activitv

segments of

DXA

synthesis in various cells.

class of proteins, called acidic

proteins, are

may

for different

DXA

in

Control within Cells

The fascinating features of the study of the regulation by genes should not blind us to other aspects of control of cell activity. Another common method of control in higher organisms involves blocking the

synthesis of protein on alreadv existing

mRXA. The

inhibitory molecules are thought to be large mole-

cules such as proteins.

The proper

stimulus causes

the removal of the inhibitor}- molecules so that the

nuclear

studied as molecules that

the control of

Other Means of Control

DXA. However,

mRXA

is

free to continue the protein synthesis

(Figure 7.10).

When

the control

is

exerted at the level of

RX

\

DNA

DNA

'WW

1

1

1

1

mRNA

mRNA

1

inhibition

large molecules (proteins)

»

enzyme

9f

enzyme

suppression of enzyme or protein synthesis on existing mRNA

feedback inhibition of specific

mRNA

by

enzyme concentration

lis >n tin it. large motecuU tut h ns }iiot< im making / protein molecules on mfiA \ In tin cytoplasm. Removal "I tin Inhibitory molecule allou the mli \ \ to ontinu* fiakingpnm Int. fn /< edback inhibition [right mi enzyme that is In ing prodtu ed at ht tuffu « ndy high oncentration to inhibit furthei ty nthesis of the mRS \ that codes for th< enzyme \s concentration of tht enzyme drops, the inhibition is remoi ed to that more m/< VA fa form*