Lewin Benjamin (ed.) Genes IX

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pairs

which A is

complementary

to T

and G

is complementary

The

strands

separate

and

use complementary

base

pairing

assem-

ble daughter

strands in

semiconservative

repli-

cation. Complementary

base

pairing

also used

to transcribe

an RNA representing

one strand

of a

DNA

duplex.

A stretch

of DNA may

code for

protein.

The

genetic

code

describes

the relationship

between

the sequence

of DNA

and the

sequence of the

protein.

Only

one of the

two

strands of DNA

codes for

protein.

A codon

con-

sists

of three nucleotides

that represent

a sin-

gle

amino acid. A

coding

sequence of DNA

consists

of a series of

codons, which

are read

from a fixed

starting

point.

Usually

only one

of the three

possible

reading

frames

can

translated into

protein.

A mutation

consists

of a change in

the

sequence

A-T

and

G-C base

pairs

DNA. A

mutation in

a coding sequence

may change

the

sequence of amino

acids in the

corresponding

protein.

A frameshift

mutation

alters the

sub-

sequent reading frame

by inserting

deleting

a base; this causes an

entirely new

series of

amino acids to

be coded after

the site of muta-

tion. A

point

mutation

changes

only the amino

acid represented

by the

codon in which

the

mutation

occurs. Point

mutations may

reverted by back mutation

of the

original muta-

tion. Insertions

may revert

loss

of the inserted

material, but deletions

cannot revert.

Muta-

tions

may

also be suppressed

indirectly

when a

mutation in a different

gene

counters

the orig-

inal defect.

The natural

incidence

of mutations is

increased

by mutagens.

Mutations may

concentrated

hotspots.

type of hotspot

responsible for some

point

mutations

is caused

by deamination of the modified

base

methylcytosine.

Forward mutations

occur at

rate

l0-6

per

locus

per generation;

back mutations are

rarer.

Not

all mutations

have

an effect on the

phenotype.

Although all

genetic

information in

cells is

carried by DNA, viruses

have

genomes

of dou-

ble-stranded

or single-stranded DNA

or RNA.

Viroids

are subviral

pathogens

that consist solely

of small circular molecules

of RNA, with no

pro-

tective

packaging.

The RNA

does not code for

protein

and its mode

perpetuation

and

pathogenesis

is unknown.

Scrapie consists

of a

proteinaceous

infectious

agent.

References

Introduction

Reviews

Cairns,

J., Stent, G., and

Watson, J. D.

(1966).

Phage

and the Origins

of Molecular Biology.

Cold Spring Harbor Symp

Quant.

Biol.

Judson, H.

(

I978) . The Eighth

Day

of Creation.

I(nopf,

New

York.

Olby, R.

ll974l.

The

Path to the Double Helix.

MacMillan, London.

DNA Is the Genetic

Materia[ of

Bacteria

Resea rcl-r

Avery, O. T., Macleod, C.

M., and McCarty, M.

(1944).

Studies

on the chemical

nature of the

substance

inducing transformation

pneu-

mococcal

types.

J. Exp.

Med 98, 451-460.

Griffith,

(1928).

The significance of

pneumococ-

caltypes. J.Hyg

27, II3-I59.

DNA Is the Genetic

Material of

Viruses

Resea rc h

Hershey, A.

D, and Chase,

(1952).Independent

functions

viral

protein

and

nucleic acid in

growth

of bacteriophage

J. Gen.

Physiol.36,

]9-56.

DNA Is the Genetic

Material of

Animal

Cet[s

Resea

rc l-r

Pellicer, A., Wigler,

M., Axel, R., and Silverstein,

(197

8) . The transfer and

stable

integration of

the HSV thymidine

kinase

gene

into mouse

cells. Cell 14, l)3-141.

DNA Is a Double

He[ix

Resea rch

Watson,

D., and Crick,

F. H. C.

(1953).

A struc-

ture for DNA. Nature

17 |, 7)7-738.

Watson,

J.D.,

andCrick,

F.H.C.

(1953).

Genetic

implications of the structure

of DNA. Nature

t7t,964-967.

Wilkins,

M. F. H.,

Stokes,

A. R., and Wilson,

H. R.

(

195

Molecular structure

DNA. Nature

t7r,738-740.

DNA Reptication

Semiconservative

Review

Holmes,

(2001).

Meselson, Stahl,

and

the Replica-

tion of DNA:

A History of

the Most Beautiful

Exper-

iment in

Biology Yale University

Press, New

Haven, CT.

References

Resea rch

Meselson, M.

and Stahl,

F.w.

(1958).

The replica-

tion of DNA in E. coli. Proc. Natl. Acad. Sci. USA

44.67t-682.

Mutations

Change the Sequence of

DNA

KCVICWS

Drake, J. W., Charlesworth, B., Charlesworth, D.,

and Crow, J.F.

(1998).

Rates of spontaneous

mutation.

Genetics

148, |

667

-l

686.

Drake,

J. W. and

Balz, R. H.

(197

6\ . The biochem-

istry of mutagenesis. Annu. Rev Biochem.45,

tt-37.

Resea rc h

Drake,

J. W.

(

I 99 t

A constant rate of sponta-

neous

mutation

DNA-based microbes. Proc.

Natl. Acad.

Sci.

USA 88,7160-7164.

Grogan, D. W.,

Carver, G.

T.,

and Drake, J. W.

(2001).

Genetic

fidelity

under harsh condi-

tions:

analysis of spontaneous mutation in the

thermoacidophilic

archaeon Sulfulobus acido-

caldarius Proc

Natl

Acad.

Sci. USA 98.

7928-79)3.

Mutations May Affect

Sing[e Base Pairs

or Longer Sequences

Maki, H.

(20021.

Origins of spontaneous muta-

tions: specificity and directionality

base-substitution, frameshift,

and sequence-

substitution mutageneses.

Annu. Rev. Genet.

36.279-30J.

Review

Many Hotspots Result from Modified

Bases

Research

Coulondre, C.

etal.

(1978).

Molecularbasis of base

substitution

hotspots in E

coli. Nature 274,

775-780.

Millar, C. B., Guy, J., Sansom, O. J., Selfridge, J.,

MacDougall, E.,

Hendrich, B.,

Keightley, P. D.,

Bishop, S. M., Clarke, A. R., and Bird, A.

(2002).

Enhanced CpG mutability and

tumorigene sis

in MBD4

ficient

mice. S cience

297

403-405.

Some

Hereditary Agents Are Extremety

Smat[

Reviews

Diener, T. O.

(1986).

Viroid

processing:

a model

involving the central conserved region and

hairpin. Proc. Natl. Acad.

Sci. USA 83, 58-62.

Diener, T.

(

1999). Viroids and

the

nature

viroid

disease s. Arch. Virol.

Suppl

20)-220.

Prusiner,

(1998).

Prions. Proc.

Natl. Acad. Sci.

usA95, r))6j-r3)8).

Resea rch

Bueler, H. et al.

(1993\

. Mice devoid of PrP

are

resistant to scrapie. Cell 73, I339-l)47.

Mclfinley, M. P., Bolton, D. C., and Prusiner,

S. B.

(1983).

protease-resistant protein

is a struc-

tural component of the scrapie

prion.

Cell )5,

57-62.

CHAPTER 1

Genes Are DNA

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3NI-IINO

USIdVHJ

Introduction

The

gene

the

functional unit of heredity. Each

gene

is a sequence within the

genome

that func-

tions by

giving

rise

to a discrete

product

(which

may be a

protein

or an RNA). The basic behav-

ior

of the

gene

was defined by

Mendel more

than a century ago. Summarized in his two

laws,

the

gene

was

recognized as a

"particulate

fac-

tor" that

passes

unchanged

from

parent

prog-

eny. A

gene

may exist in alternative

forms.

These forms are called alleles.

In diploid organisms, which

have

two sets

of chromosomes, one copy of each chromo-

some

inherited

from

each

parent.

This is the

same behavior that is displayed by

genes.

One

of the two

copies

of each

gene

is the

paternal

allele

(inherited

from

the

father),

the other

the maternal allele

(inherited

from the mother).

The

equivalence

led

to the discovery that chro-

mosomes in

fact carry the

genes.

Each chromosome consists

a linear array

genes.

Each

gene

resides

at a

particular

loca-

tion

on the chromosome. The location is

formally called a

genetic

locus. The alleles of a

F!.{'ij$li

;i":t Each chromosome has

a singte [ong molecute of DNA

within which

are the sequences of individuaI

genes.

CHAPTER 2 Genes

Code for Proteins

gene

are the different forms that are

found at

its locus.

The key to understanding the

organization

genes

into chromosomes was the discovery

genetic

linkage-the tendency

for

genes

the same chromosome

to remain together in

the

progeny

instead of assorting independently

predicted

Mendel's laws. Once the

unit

recombination

(reassortment)

was

introduced

as the

measure of linkage, the construction of

genetic

maps

became

possible.

The resolution of the

recombination map

a higher eukaryote

is restricted by the small

number of

progeny

that can be obtained from

each

mating. Recombination occurs so infre-

quently

between nearby

points

that

it is rarely

observed

between different mutations in the

same

gene.

result, classical linkage maps

eukaryotes

can

place

the

genes

in order, but

cannot determine

relationships

within

gene.

By moving to a

microbial

system

which a

very

large number of

progeny

can be obtained

from each

genetic

cross, researchers

could

demonstrate that recombination occurs within

genes.

It follows the same

rules

that were

pre-

viously deduced for recombination between

genes.

Mutations within a

gene

can be arranged

into a linear order, showing that the

gene

itself

has

the same

linear construction

as the array

genes

on a chromosome. So the

genetic

map

is linear within as well as between

loci: it

consists of an unbroken

sequence

within which

the

genes

reside. This

conclusion leads natu-

rally into the modern view summarized in

+'I{*#it[

:J.1

that

the

genetic

material

of a chromo-

some consists of an unintenupted length

DNA representing many

genes.

Gene Codes

for

a Single

PoLypeptide

The one

gene:

one enzyme hypothesis

summarizes

the basis of modern

genetics:

that a

gene

stretch of

DNA

coding

for

a singte

potypeptide

chain.

Most mutations

damage

gene

function.

The first systematic attempt

to associate

genes

with enzymes

showed

that

each stage in a meta-

bolic

pathway

catalyzed by a single enzyme

and can be blocked by mutation in a

different

gene.

This led to the lne

gene

: one

enzyme hypoth-

esrs.

Each

metabolic step is catalyzed

by a

par-

ticular

enzyme, whose

production

is the

chromosome is a very long molecule

DNA

contains

many

genes

Each

gene

part

of a continuous

responsibility

of a

single

gene.

A mutation

the

gene

alters

the activity

the

protein

for

which it is responsible.

A modification

the hypothesis

is needed

accommodate

proteins

that consist

of more

than

one subunit. ff

the subunits

are

all the

same,

the

protein

a homomultimer,

repre-

sented

by a single

gene.

the

subunits

are dif-

ferent,

the

protein

a heteromultimer.

Stated

as a more

general

rule

applicable

to any het-

eromultimeric

protein,

the

one

gene

: one

enzyme hypothesis

becomes

precisely

expressed

as lne

gene

one

polypeptide

chain.

Identifying

which

protein

represents

par-

ticular

gene

can be a

protracted

task. The muta-

tion responsible

for

creating

Mendel's

wrinkled-pea mutant

was identified

only in

I990

as an alteration

that inactivates

the

gene

for a

starch branching

enzyme!

It is important

to remember

that a

gene

does

not

directly

generate

protein.

As shown

pre-

viously in Figure

1.2, agene

codes for

an RNA,

which may in

turn code for

protein.

Most

genes

code for

proteins,

but some

genes

code for

RNAs

that do not

give

rise

proteins.

These

RNAs

may be

structural

components

of the

apparatus responsible

for

synthesizing

proteins

or may have roles

in regulating

gene

expres-

sion.

The

basic

principle

is that

the

gene

sequence of DNA

that specifies

the sequence

an independent

product.

The

process

gene

FISURg ?"2

Genes code for

proteins;

dominance is

exptained by the

properties

mutant

proteins.

A reces-

sive at[ele does

not

contribute

to the

phenotype

because

produces

protein

(or protein

that

nonfunctionat).

expression may terminate in a

product

that

either RNA

protein.

mutation is a random event with

regard

to the structure

of the

gene,

so the

greatest prob-

ability is that it will damage or even abolish

gene

function. Most mutations that affect

gene

function

are recessive:

they represent an absence

function,

because

the mutant

gene

has been

pre-

vented

from

producing

its usual

protein.

StSLjftfi t"fi

illustrates

the relationship between

recessive

and wild-t1pe

alleles.

When a heterozygote con-

tains one wild-t1pe allele and

one mutant allele,

the wild-type

allele

is able to direct

production

of the

enzyme.

The

wild-type

allele is therefore

dominant.

(This

assumes that

an adequate

amlunt

protein

is made by the single

wild-

type allele. When this is not true, the smalier

amount made

by one

allele as compared to

two

alleles results

in the

intermediate

phenotype

partially

dominant allele

in a heterozygote.)

Mutations

the Same

Gene Cannot

Complement

mutation in a

gene

affects only

the

protein

coded by the mutant copy of the

gene

and does

not aftect the

protein

coded by any

other a[[ele.

Failure

of two

mutations to comptement

(produce

witd

phenotype

when

they are

present

in trans

configuration in

heterozygote

means that they

are

part

of the same

gene.

How

do we determine

whether

two muta-

tions that cause a similar

phenotype

lie in the

same

gene?

If they map close

together, they

may

be alleles.

However, they could

also rep-

resent

mutations

two

different

genes

whose

proteins

are

involved in the same

function.

The complementation

test is used to deter-

mine

whether

two mutations

lie in the same

gene

different

genes.

The test consists

of making aheterozygote

for the two

muta-

tions

(by

mating

parents

homozygous

for each

mutation).

If the mutations

lie in the same

gene,

the

parental genotypes

can be

represented as:

,m,

rano-

The first

parent provides

arrttT\rr-urarrt

allele

and the se cond

parent

provides

an m2 allele, so

that the heterozygote

has the constitution:

2.3

Mutations

in the Same Gene

Cannot Complement

Wild-type

homozygote

Mutant

homozygote

Both alleles One

(dominant)

Neither

allele

produce

allele

produces

produces protein

active

protein

active

protein

wib type

wild type

mutant

Wild Wild

Mutant

phenotype phenotype

phenotype

wild-type

gene

present,

so the

het-

erozygote

has mutant

phenotype.

the mutations

lie in

different

genes,

the

parental genotypes

can be

represented as:

m,+

+m,

dflCl

[, * *[,

Each chromosome has a wild-type copy of

one

gene (represented

by the

plus

sign)

and a

mutant

copy of the other.

Then

the

heterozy-

gote

has the constitution:

flr*

*[,

in which

the two

parents

between them

have

provided

a wild-type copy of each

gene.

The

heterozygote has wild

phenotype,

and thus the

two

genes

are said to complement.

The complementation test is sho\ rn in more

detail

:,,,-:!ii

The

basic test consists of the

comparison shown

the top

part

of the

figure.

two mutations lie in the same

gene,

we see a

difference in

the

phenotypes

of lhe

trans

con-

figuration and the crs configuration. The trans

configuration is mutant, because each allele has

(different)

mutation. However,

the as config-

uration is wild-type, because one allele has two

mutations and the other allele has no muta-

tions. The lower

part

of the figure shows that

the two mutations lie in different

genes,

jr;:iii,ritl

The cistron is

defined by the complementation

test.

Genes are represented

by spira[s; red stars identifo

sites of

mutation.

CHAPTER 2 Genes

Code

for

Proteins

always see a

wild

phenotype.

There

always

one wild-type and

one mutant allele of each

gene,

and the configuration

is irrelevant. Fail-

ure

to complement

means that

two

mutations

are

part

the same

genetic

unit.

Mutations

that

do not complement one

another are said to

comprise

part

the same complementation

group.

Another term

that is used to describe

the unit defined

by the complementation test

is the cistron.

This is the same as the

gene.

Basi-

cally these three terms

all describe a

stretch

DNA that

functions as a unit to

give

rise to an

RNA or

protein product.

The

properties

of the

gene

with regard to complementation are

explained by the

fact that this

product

is a sin-

gle

molecule that behaves as a

functional

unit.

Mutations

May

Cause

Loss-of-Function

or Gain-of-Function

Recessive

mutations are due to loss-of-function by

the

protein product.

Dominant mutations result from a

gain-of-

function.

Testing whether

gene

is essential requires

nutl

mutation

(one

that comptetety eliminates

its

function).

Sitent mutations have no effect. either because

the base change does

not

change the sequence or

amount of

protein,

or because the change in

protein

sequence

has no effect.

Leaky mutations do affect the

function

of the

gene

product,

but are

not revealed in

the

phenotype

because sufficient activity remains.

The various

possible

effects of mutation in

gene

are

summarized

;r'Ji.rt"iiii-

:.i,+.

When

gene

has

been

identified,

insight

into its function

principle

can be

gained

generating

mutant

organism that entirely lacks

the

gene.

A mutation that completely

elimi-

nates

gene

function-usually because

the

gene

has been deleted-is called a null rnutation.

gene

essential, a

null mutation

is lethal.

To determine what effect a

gene

has

upon

the

phenotype,

it is essential to characterize

null mutant. When a mutation fails

to affect

the

phenotype,

it is always

possible

that this is

because it is a leaky mutation-enough

active

product

is made

fulfill its

function,

even

though the activity is

quantitatively

reduced or

qualitatively

different from the wild type.

How-

ever, if

null

mutant fails to affect a

pheno-

type, we may safely

conclude that the

gene

function is

not necessary.

MUTATIONS IN

SAME GENE

mutant 1

double mutant

No complementation

,/ ,/ ,f ,/ ,,{

*/ r/

One

gene

Mutant

phenotype

,f ,f

wild{ype

ild

phenotype

mutant

wild type

MUTATIONS

lN DIFFERENT

GENES

(trans

configuration)

mutant 1

Complementatio

(wild

phenotype)

./\f

./ ,

One copy

of each ./

'/ '/.

gene

is wild-type

wild type

Wild-type

gene

codes

Silent

mutation

does not affect

protein

Null

mutation

makes no

protein

ii,:r::iri i:',.r, Mutations

that

do not affect

protein

sequence

function

are sitent. Mutations

that abo[ish

at[ orotein

activity are nu[t. Point

mutations

that cause

[oss-of-

functjon

are

recessive;

those

that

cause

qain-of-function

are dominant.

Null mutations,

or other

mutations

that

impede

gene

function (but

do not necessarily

abolish it

entirely), are

called loss-of-function

mutations.

A loss-of-function

mutation

is reces-

sive

(as

the example

of Figure 2.2).

Some-

times a mutation

has the

opposite

effect and

causes a

protein

to acquire

a new function;

such

a change is

called a

gain-of-function

muta-

tion. A

gain-of

-function

mutation

is dominant.

Not

all mutations

in DNA

lead to

a detectable

change in the

phenotype.

Mutations

without

apparent

effect are called

silent mutations.

They

fall

into two

q,?es:

One tlpe involves

base changes

in DNA that

not

cause any

change in

the amino

acid

present

the corresponding protein.

The

second type changes

the amino

acid, but

the

replacement in

the

protein

does not

affect its

activity; these are

called neutral

substitutions.

A Locus May

Have Many

Different

Mutant

ALLeles

The existence

muttipte

a[[e[es

allows

heterozygotes to occur representing

any

pairwise

combination of a[tetes.

If a recessive mutation is

produced

by every

change

in a

gene

that

prevents

the

production

of an active

protein,

there should be

a large

number

of such mutations in any one

gene.

Many amino

acid

replacements may change the

structure

of the

protein

sufficiently to

impede

its

function.

Different

variants of the same

gene

are

called

multiple

alleles, and their existence

makes it

possible

to create a

heterozygote between mutant

alleles. The relationship between these

multi-

ple

alleles takes various

forms.

In the simplest case, a wild-type

gene

codes for

protein product

that is functional.

Mutant

allele(s) code

for

proteins

that are

nonfunctional.

But there are often cases in which a series

of mutant

alleles

have

different

phenotypes.

For example,

wild-type

function of. the

white

locus

Drosophila

melanogaster

is required for

development of the normal

red color of the eye.

The

locus is named for the effect of

extreme

(null)

mutations, which cause

the fly to have a

white eye in mutant homozygotes.

To describe wild-type and

mutant alleles,

wild

genotype

is indicated by a

plus

superscript

after

the name of the

locus

(r,rz+

is the wild-type

allele for

[red]

eye color

in D. melanogaster).

Sometimes

is used by

itself to describe the

wild-t1pe

allele, and

only the

mutant alleles are

indicated

by the

name of the locus.

entirely defective

form of the

gene (or

absence of

phenotype)

may be indicated by a

minus

superscript.

To distinguish among

a vari-

ety of mutant alleles with different

effects, other

superscripts may

introduced, such

as w'

wa.

The rar'

allele

is dominant over any

other

allele in heterozygotes.

There are many differ-

ent mutant alleles.

i:it'iii'i1r

,:,:; shows a

(small)

sample.

Although

some

alleles

have no eye

color, many alleles

produce

some

color. Each

of these mutant alleles

must therefore

repre-

sent a different mutation

of the

gene,

which

does not

eliminate

its function

entirely, but

leaves

a residual activity

that

produces

a char-

acteristic

phenotype.

These alleles are

named

for the

color of the

eye in a

homozygote.

(Most

alleles affect the

quantity

pigment

in the

eye. The examples in the

figure are arranged

[roughly]

declining amount

of color, but oth-

ers,

such as

wsP, affect the

pattern

in which

it is

deposited.

)

When multiple alleles

exist, an animal

may

be a heterozygote that

carries two

different

mutant alleles. The

phenotype

of such

a het-

erozygote depends on

the nature

of the resid-

ual activity

each allele.

The relationship

2.5

A Locus

May Have Many

Different Mutant Alleles

Allele Phenotype

homozygote

w* red eye

(wild

type)

wor blood

wth cherry

wot buff

wh honey

w" apricot

we eosin

wl ivory

w' zeste

(lemon-yellow)

w"P mottled, color varies

white

(no

color)

l:iir-:.iiilr

,: r

The r,v

locus

has

an extensive

series of at[e-

les

whose

phenotypes

extend

from witd-type

(red)

cotor

to comptete lack of

pigment.

between two mutant alleles is in

principle

different from that between wild-type and

mutant

alleles: one allele

may

dominant,

there may

partial

dominance, or there

may

be codominance.

Locus

May Have More

than One Wil.d-type

A[LeLe

A [ocus may have a

potymorphic

distribution of

atletes with

no individual

altete that can be

considered to be the sole witd-type.

There

is not necessarily a unique wild-type allele

at any

particular

locus. Control of the human

blood

group

system

provides

example.

Lack

,,:il1';:;H:i:,T:tJ::,i"'":iili:i;'"x';

provide

activities that are codominant with

one another and dominant over O

group.

The

basis for

this relationship is illustrated in

i:::i.il

r::.i:.

The

(or

antigen

generated

in all indi-

;*:lii:1,::1H:?:xifi

:"x'H:*:',1Ti:

codes for a

galactosyltransferase

enzyme that

ldds a further sugar

group

the O antigen.

Ihe specificity

this

enzyme determines the

blood

group.

The A allele

produces

an enzyme

that uses

the cofactor UDP-N-acetylgalactose,

creating the A antigen. The B

allele

produces

an enzyme that uses

the cofactor UDP-galac-

tose, creating the B antigen. The

A and B ver-

sions of the transferase

protein

differ in four

amino

acids that

presumably

affect its recogni-

tion

of the type of cofactor. The

O allele

has

mutation

small deletion) that

eliminates activ-

ity, so no modification

of the O antigen occurs.

CHAPTER 2

Genes

Code

for Proteins

i I{"litlRt: l.{i

The AB0 btood

group

[ocus codes for a

ga[ac-

tosyltransferase

whose specificity determines the btood

gr0up.

This

explains

why,4 and B alleles

are dom-

inant

the

and

BO heterozygotes:

the cor-

responding transferase activity creates the A or

B antigen. The A and B alleles are codominant

in AB heterozygotes, because both transferase

activities are expressed.

The

OO homozygote is

a null that has neither activity and therefore

Iacks both antigens.

Neither

A nor B can be regarded as uniquely

wild type, because they represent alternative

activities

rather than loss or

gain

function.

situation such as this, in which there

are

mul-

tiple functional alleles

population,

described as a

polymorphism (see

Section 4.3,

Individual Genomes Show

Extensive

Variation).

Recombination 0ccurs

Physical Exchange

DNA

Recombination

the

resutt

of crossing-over that

occurs at chiasmata and involves

two of the four

chromatids.

Recombination occurs by

breakage

and reunjon

that

proceeds

via an

intermediate

of hybrid DNA.

chromatids, 2 from

each

parent

a D

Chiasma

is caused by

crossing-over between

2 ot the chromatids

Two

chromosomes remain

parental (ABand

ab).

Recombinant

chromosomes

contain material from each

parent,

and

have new

genetic

combinations

(Ab

and aB).

!::.Sijiti.

li..'

Chiasma

formation

is responsibte

for

gen-

erating recombinants.

Genetic

recombination

describes

the

generation

of new combinations

of alleles

that occurs at

each

generation

in diploid

organisms. The

two

copies of each chromosome

may have

different

alleles at

some

loci.

By exchanging

correspon-

ding

parts

between the chromosomes,

recombi-

nant chromosomes

can be

generated

that are

different from the

parental

chromosomes.

Recombination

results from

physical

exchange of chromosomal material.

This is vis-

ible in the form of the crossing-over

that occurs

during meiosis

(the

specialized

division that

produces

haploid

germ

cells).

Meiosis starts

with

cell that

has

duplicated its chromosomes,

that it has four copies

of each chromosome.

Early in meiosis, all four

copies are closely asso-

ciated

(synapsed)

in a structure

called a biva-

lent. Each individual chromosomal

unit is called

chromatid at this stage. Pairwise

exchanges

of material occur

between the chromatids.

The visible result of a crossing-over

event

is called a chiasma and is illustrated

diagram-

matically in

Fili*fi{

;"1.1. A

chiasma represents a

site at which two of the chromatids in a biva-

lent have been broken at corresponding

points.

The broken ends have

been

rejoined

crosswise,

generating

new

chromatids. Each new

chro-

matid consists of material

derived from one

chromatid on one

side of the

junction

point,

with material from the other chromatid

on the

opposite

side. The two recombinant

chromatids

have reciprocal structures. The

event

described

as a breakage and reunion. Its nature

explains

why a single recombination event

can

produce

only 50% recombinants: each individual recom-

bination event

involves

only two of the four

associated chromatids.

Parental DNA molecules

Recombination

intermediate

Recombinants

l:i*ijSi:

il"ji Recombinatjon

invotves

pairing

between

complementary strands of

the two

parentaI

duplex

DNAs.

The complementarity

the two strands

DNA is essential for the

recombination

process.

Each

of the chromatids

shown

Figure 2.7

consists of a very

long duplex

of DNA.

For them

to be broken and

reconnected

without any

loss

material requires a

mechanism

to recognize

exactly corresponding

positions. This is

pro-

vided by

complementary

base

pairing.

Recombination

involves

process

in which

the single strands

in the region

of the crossover

exchange their

partners. li:iriiF-h, t.li shows

that

this creates

stretch

hybrid

DNA, in which

the single strand of one

duplex

paired

with its

complement

from the other

duplex.

The mech-

anism,

course,

involves other

stages

(strands

must

be broken

and

resealed),

which we

dis-

cuss

in more detail

in Chapter

20, Repair Sys-

tems, but the crucial

feature

that

makes

precise

recombination

possible is the complementar-

ity

DNA

strands.

The figure

shows only

some

stages of the

reaction, but

we see

that a stretch

hybrid DNA forms

in the

recombination

inter-

mediate when a single

strand

crosses over

from

one duplex to the

other.

Each

recombinant

con-

sists of one

parental

duplex

DNA

at the left,

which is connected

a stretch of

hybrid

DNA

to the other

parental

duplex

the right.

Each

duplex DNA

corresponds

one of the

chro-

matids involved

in recombination

in Fig:e2.7.

The formation

hybrid

DNA requires

the

sequences of

the two

recombining

duplexes

2.7

Recombination

Occurs

Physical

Exchange of

DNA 29

be close

enough to allow

pairing

between the

complementary

strands. If there are no differ-

ences

between the two

parental genomes

this

region,

formation of hybrid DNA

will be

per-

fect. However,

the

reaction

can be tolerated

even

when there are small differences. In

this

case, the hybrid

DNA has

points

of mismatch,

at which

a base in one strand faces a

base

in the

other strand that is not complementary

it.

The

correction of

such

mismatches

is another

feature

genetic

recombination

(see

Chapter

Repair Systems).

The

Genetic

Code

Is Triplet

The

genetic

code is read in tripLet nucteotides

catled

codons.

The

triplets are nonoverlapping

and are

read from

a fixed

starting

point.

Mutations

that insert

or delete individuaI bases

cause a

shift

the triotet sets after the

site

mutation.

Combinations

mutations

that

together

insert

detete three

bases

(or

multiples of three) insert

delete amino

acids, but do

not

change the reading

of the tripl.ets

beyond the [ast site of mutation.

Each gene

represents

particular protein

chain.

The concept

that each

protein

consists of a

par-

ticular

series of amino acids

dates from Sanger's

characterization

insulin

in the 1950s. The

discovery

that a

gene

consists

of DNA

presents

with the issue

of how a

sequence of

nucleotides

in DNA

represents a

sequence of

amino

acids in

protein.

crucial feature

of the

general

structure of

DNA

is that it is

independent of the

particular

sequence

of its clmplnentnucleotides.

The

sequence

nucleotides

DNA is important

not because

of its structure

per

se, but

because it codes for

tne

sequence

of amino

acids that constitutes

the

corresponding polypeptide.

The relationship

between

a sequence

of DNA and

the sequence

of the corresponding protein

is called

the

genetic

code.

The

structure

and/or enzymatic

activity

each

protein

follows from

its

primary

sequence

of amino

acids. By

determining

the sequence

of amino acids in

each

protein,

the

gene

able

to carry

all the information

needed

to specify an

active

polypeptide

chain. In

this way,

a single

tlpe of structure-the gene-is

able to represent

itself

in innumerable

polypeptide

forms.

CHAPTER 2

Genes

Code

for

Proteins

Together the various

protein

products

of a

cell undertake the catalytic and

structural activ-

ities that are responsible for establishing its

phe-

notype.

Of course,

addition to sequences that

code for

proteins,

DNA also contains

certain

sequences whose

function is

to be recognized

by regulator molecules, usually

proteins.

Here

the function

the DNA is

determined by its

sequence directly,

not

via any intermediary

code. Both types of region-genes expressed

proteins

and

sequences

recognized

as such-

constitute

genetic

information.

The

genetic

code

deciphered by a complex

apparatus

that

interprets

the nucleic

acid

sequence. This apparatus is essential if

the

infor-

mation carried in DNA is to have

meaning. In any

given

region,

only one of the two strands

DNA

codes for

protein,

so we write the

genetic

code

a sequence of bases

(rather

than

base

pairs).

The

genetic

code is read in

groups

of three

nucleotides, each

group

representing

one

amino

acid.

Each

trinucleotide sequence is

called a codon. A

gene

includes

a series of

codons that is read sequentially from

a start-

ing

point

one end to a termination

point

the other end.

Written

the conventional

3'direction, the nucleotide

sequence of the

DNA strand that codes for

protein

corresponds

to the amino

acid sequence of the

protein

writ-

ten in

the direction from

N-terminus to

C-terminus.

The

genetic

code

read in nonoverlapping

triplets

from

fixed

starting

point:

Nonoverlapping implies

that each

codon

consists of three nucleotides

and that

successive

codons are represented

successive

trinucleotides.

The use

fixed

starting

point

lrreans

that assembly

of a

protein

must

start at

one end and work to

the other,

so that

different

parts

of the coding

sequence

cannot be read independently.

The nature

of the code

predicts

that two

types

mutations

will have

different effects.

If a

particular

sequence is read

sequentially,

such

as:

UUU AAA

GGG CCC

(codons)

aal aa2 aa3

aa4

(amino

acids)

then

point

mutation

will affect only

one amino

acid. For

example, the

substitution

of an A

some other

base

(X)

causes

aa2 to

be replaced

by aa5:

UUU

AAX GGG CCC

aal

aa5 aa7 aa4