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238U
NIT 4 G
ENETIC
S
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
HE R
OLE O
F DN
A IN H
ERED
ITY
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.
240U
NIT 4 G
ENETIC
S
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
HE R
OLE O
F DN
A IN H
ERED
ITY
CH
APTER 12: T
HE C
OD
E OF L
IFE
How
protein synthesis works
242U
NIT 4 G
ENETIC
S
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
HE R
OLE O
F DN
A IN H
ERED
ITY
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
NIT 4 G
ENETIC
S
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
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