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Fig

ure 12.1: A

DN

A m

olecule looks

like a twisted

ladd

er.

DN

A is in the new

s almost every

day. Find an article in a new

spaper, magazine, or on the

Internet that mentions D

NA

. Write

a reflection about the article. How

does the article m

ake you feel about studying D

NA

?

12.1 The Role of D

NA

in Heredity

On

ly in th

e last 50 years have scien

tists un

derstood the role of D

NA

in h

eredity. T

hat u

nderstan

ding began

with

the discovery of D

NA

’s structu

re. In 1952, R

osalind

Fran

klin (1920–1958) u

sed a techn

ique called x-ray crystallograph

y, to capture th

e first im

age of a DN

A m

olecule. W

ith th

e help of F

ranklin

’s photo, Jam

es Watson

(1928–presen

t) and F

rancis C

rick (1916–2004) were able to piece togeth

er the first

accurate m

odel of DN

A. In

this section

, you w

ill learn h

ow th

e structu

re of DN

A is

related to its fun

ction as th

e hereditary m

olecule.

DN

A structure

The DNA m

oleculeA

DN

A m

olecule looks like a tw

isted ladder (Figu

re 12.1). Its sh

ape is called a dou

ble helix. A

helix is a sh

ape that tw

ists. Th

e tw

o sides of the D

NA

ladder are made of su

gar molecu

les altern

ating w

ith ph

osphate m

olecules. T

he ru

ngs of th

e DN

A

molecu

le are made of ch

emical bu

ilding blocks called bases. T

he

four bases fou

nd in

DN

A are aden

ine (A

), thym

ine (T

), cytosine

(C), an

d guan

ine (G

).

Base pairs E

ach ru

ng of th

e DN

A ladder con

sists of a base pair. Th

e base on

one side of th

e molecu

le always m

atches u

p with

a certain base on

th

e other side. T

he base A

only pairs w

ith T

and C

only pairs w

ith

G. T

his base pairin

g is very importan

t to the fu

nction

of DN

A.

23912.1 T

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CH

APTER 12: T

HE C

OD

E OF L

IFE

DN

A replication

What is DNA

replication?W

e learned in

Ch

apter 10 that before m

itosis, the am

oun

t of DN

A

doubles. D

NA

replication is th

e process of a DN

A m

olecule m

aking

a copy of itself. DN

A replication

occurs before m

itosis begins an

d before th

e first division of m

eiosis. It ensu

res that each

daugh

ter cell h

as an exact copy of th

e genetic m

aterial from th

e parent cell.

DN

A rep

lication

ensu

res tha

t each

d

au

gh

ter cell ha

s an

exact cop

y of th

e DN

A from

the p

aren

t cell.

The process ofDNA replication

DN

A replication

results in

one D

NA

molecu

le becom

ing tw

o dau

ghter m

olecules—

each an

exact copy of th

e original m

olecule. T

he steps of th

e process are ou

tlined below

.

1.D

NA

replication begin

s with

the partial

un

win

ding of th

e double h

elix. Th

e base pairs separate.

2.A

special molecu

le moves alon

g each origin

al stran

d of DN

A an

d “reads” the bases.

3.A

new

strand is assem

bled along each

original

strand. T

he pieces are assem

bled from m

olecules

in th

e cytoplasm.

4.W

hen

the process is com

plete, two dau

ghter

molecu

les will h

ave been produ

ced. Each

daugh

ter m

olecule is iden

tical to the origin

al molecu

le.

5.B

oth stran

ds of the origin

al DN

A m

olecule h

ave rem

ained in

tact. Each

daugh

ter molecu

le is made

of one origin

al strand an

d one n

ew stran

d.

DN

A replication

- the process of a D

NA

molecule m

aking a copy of itself.

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Fig

ure 12.2: T

he relation

ship

between

chrom

osomes, gen

es, and

DN

A.

base sequence - the order of base pairs along a gene.

protein synthesis - the production of proteins in the cell.

Protein synthesisChrom

osomes,

genes, and DNAW

ith th

e exception of red blood cells, w

hich

have n

o nu

cleus or

nu

clear DN

A, each

one of you

r body cells contain

s a complete

(diploid) set of chrom

osomes. E

ach ch

romosom

e is made u

p of th

ousan

ds of genes. E

ach gen

e consists of a sequ

ence of D

NA

base pairs (F

igure

12.2). In total, th

e DN

A in

one of you

r cells contain

s abou

t 3 billion base pairs! T

he order of base pairs alon

g a gene is

called its base sequence.

Genes andproteins

Gen

es con

trol th

e pro

du

ction

of p

rotein

s. You

r body stru

ctures are m

ade of proteins. A

s a result, th

ose proteins

help determ

ine you

r traits. For exam

ple, the color of you

r eyes is determ

ined by a protein

. Protein

s are made of lon

g chain

s of sm

aller molecu

les called amin

o acids. T

he produ

ction of protein

s in

the cell is called protein synthesis.

Amino acids

make up proteins

Th

e ord

er of b

ase p

airs a

lon

g a

gen

e form

s a co

de th

at

tells a cell w

hich

pro

tein to

ma

ke. S

ets of three bases alon

g a stran

d of DN

A form

three-letter codes th

at tell the cell w

hich

am

ino acids m

ake up th

e protein. T

here are 20 differen

t amin

o acids. T

hose am

ino acids can

be put togeth

er in m

any w

ays to m

ake million

s of different protein

s. Du

ring protein

synth

esis, the

cell reads the th

ree-letter codes along th

e DN

A m

olecule an

d uses

that in

formation

to build a protein

from differen

t amin

o acids.

The role of RNAP

rotein syn

thesis takes place in

ribosomes ou

tside the n

ucleu

s. It in

volves anoth

er nu

cleic acid called RN

A. R

NA

is different from

D

NA

because it con

sists of a single stran

d. Also, in

stead of the

base thym

ine (T

), RN

A h

as the base u

racil (U). In

RN

A, A

pairs w

ith U

instead of w

ith T

. Messen

ger RN

A carries th

e three-letter

codes from th

e DN

A in

the n

ucleu

s to the ribosom

e. Tran

sfer RN

A

decodes the base sequ

ence an

d carries the correct am

ino acids to

the ribosom

e.

24112.1 T

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CH

APTER 12: T

HE C

OD

E OF L

IFE

How

protein synthesis works

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Fig

ure 12.3: A

mu

tation in

the

nu

mber of ch

romosom

es.

mutation

- a change in the hereditary m

aterial of an organism.

Mutations

What are

mutations?

Usu

ally, the processes of D

NA

replication an

d meiosis h

appen

with

out m

istakes. How

ever, mistakes do h

appen. T

hose m

istakes are called m

utation

s. A m

utation is a ch

ange in

the h

ereditary m

aterial of an organ

ism. M

utation

s can h

appen in

any cell an

d in

any gen

e. Th

ey are sometim

es caused by exposu

re to chem

icals or oth

er environ

men

tal condition

s.

Gene mutations

A gen

e mu

tation in

volves a chan

ge in on

e of the bases in

the

sequen

ce along a gen

e. A ch

ange in

the base sequ

ence ch

anges on

e of th

e three-letter codes for an

amin

o acid. Th

is may cau

se the cell

to produce th

e wron

g protein. In

the exam

ple below, on

e of the

bases in th

e sequen

ce is substitu

ted for anoth

er. Th

e mu

tation

causes sickle cell an

emia, a blood disorder.

Chromosom

e m

utationsO

ther m

utation

s involve a ch

ange in

the stru

cture or n

um

ber of ch

romosom

es. For in

stance, du

ring m

eiosis one or m

ore pairs of ch

romosom

es may fail to separate. S

ex cells with

extra sets of ch

romosom

es may be produ

ced ( Figu

re 12.3). In plan

ts, extra sets of ch

romosom

es may cau

se desirable traits. Som

e varieties of straw

berries have extra sets of ch

romosom

es. Gen

erally, these

varieties produce larger berries.

24312.1 T

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A IN H

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CH

APTER 12: T

HE C

OD

E OF L

IFE

Genetic disorders

What are genetic

disorders?S

ome m

utation

s are helpfu

l wh

ile others are h

armfu

l. You

’ll learn

how

mu

tations can

help a popu

lation of organ

isms in

Ch

apter 13. G

enetic disorders are a h

armfu

l effect of mu

tations. A

genetic disorder is an

abnorm

al condition

that an

organism

inh

erits from

its parents. G

enetic disorders can

result from

mu

tation of a sin

gle gen

e or mu

tation of th

e chrom

osomes. In

order to be passed on to

offspring, th

e mu

tation m

ust be presen

t in th

e sex cells.

Mutations of a single gene

So

me g

enetic d

isord

ers result w

hen

a m

uta

tion

cau

ses the

pro

du

ct of a

sing

le gen

e to b

e altered

or m

issing

. An

example

of this kin

d of disorder is cystic fibrosis. Cystic fibrosis affects abou

t 30,000 ch

ildren an

d adults in

the U

nited S

tates. A recessive allele

causes th

e body to produce an

abnorm

ally thick, sticky m

ucu

s that

clogs the lu

ngs an

d leads to life-threaten

ing lu

ng in

fections. T

hick

mu

cus also clogs th

e organs of th

e digestive system an

d often leads

to digestive problems an

d liver damage. O

ther exam

ples of this

type of genetic disorder in

clude h

emoph

ilia and sickle cell

anem

ia—both

diseases of the blood.

Too many or too

few chrom

osomes

So

me g

enetic d

isord

ers result fro

m to

o m

an

y o

r too

few

chro

mo

som

es. Dow

n’s syn

drom

e is a genetic disorder in

wh

ich a

person’s cells h

ave an extra copy of ch

romosom

e 21 (Figu

re 12.4). P

eople with

Dow

n’s syn

drome h

ave some m

ental an

d physical

limitation

s. How

ever, they can

lead norm

al, productive lives.

Determining

genetic disordersD

octors use a procedu

re called amn

iocentesis to fin

d out if a baby

will h

ave a genetic disorder. In

amn

iocentesis, th

e doctor removes a

small am

oun

t of the flu

id that su

rroun

ds the developin

g baby. Th

at flu

id contain

s cells from th

e baby. Next, th

e chrom

osomes from

the

cells are analyzed to look for abn

ormal gen

es or chrom

osome

nu

mbers ( F

igure 12.5).

Fig

ure 12.4: D

own

’s synd

rome is

caused

by an extra copy of ch

romosom

e n

um

ber 21.

Fig

ure 12.5: A

mn

iotic fluid

.

genetic disorder - an abnormal

condition that an organism

inherits.

244U

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How

many am

ino acids are present in a protein that requires 1,500 bases in its code?

A haploid cell has one

chromosom

e from each

homologous pair and is

symbolized by N

. A diploid cell

has chromosom

es in homologous

pairs and is symbolized by 2N

. P

olyploidy is a mutation w

here the cells of an organism

have chrom

osomes that occur in groups

of three (3N), four (4N

), or more

(xN). U

se this information to

answer the follow

ing questions:

1.A

corn plant has 80 chrom

osomes and is 4N

. W

hat is the normal num

ber of chrom

osomes in a corn

plant’s body cells?2.

In peas, 2N = 14. H

ow m

any chrom

osomes are in a pea egg

cell?

12.1 Section Review

1.B

elow is a sequ

ence of bases alon

g one side of a D

NA

molecu

le. W

rite out th

e sequen

ce of DN

A bases th

at wou

ld pair with

the

ones sh

own

.

2.W

hat is D

NA

replication an

d wh

y is it importan

t?3.

Nam

e the fu

nction

of each in

protein syn

thesis: m

essenger

RN

A, tran

sfer RN

A, D

NA

, and ribosom

e.4.

Write ou

t the m

essenger R

NA

bases that w

ould pair w

ith th

e D

NA

strand sh

own

in qu

estion 1.

5.A

certain species of squ

irrel is usu

ally gray. Occasion

ally a w

hite squ

irrel, called an albin

o, is born. A

n albin

o squirrel

happen

s because:

a.D

NA

replication does n

ot occur

b.m

itosis produces too m

any w

hite fu

r cellsc.

a mu

tation in

the gen

e for fur color occu

rsd.

both of th

e parents h

ave wh

ite fur

6.N

ame tw

o genetic disorders an

d explain th

e type of mu

tation

that cau

ses each.

7.W

hich

process could resu

lt in th

e type of mu

tation th

at causes

Dow

n’s syn

drome?

a.cellu

lar respirationb.

meiosis

c.m

itosisd.

amn

iocentesis