Evolution, 4th Edition

mUTATIon AnD VARIATIon 81

Proteins are synthesized in three steps (FIGURE 4.4). A cell first transcribes the

DNA from a gene into RNA. This immature form of the gene’s message is called

pre-mRNA. That molecule is then spliced so that parts of the molecule are removed

to form a mature mRNA. Finally, the message in the mRNA is translated into the

string of amino acids that make up the protein. As a result of the splicing step, a

gene has segments of DNA that do not code for any amino acids in the final pro-

tein. The segments of the gene that do code for amino acids are called exons, while

the noncoding segments between the exons are called introns. Many eukaryotic

genes are spliced in more than one way, removing some of the exons (as well as the

introns) from the pre-mRNA. This alternative splicing allows a single gene to code

for more than one protein. Introns and exons evolve quite differently, which gives

us important cues about how selection and other factors cause genes and genomes

to evolve (see Chapters 5 and 14). In humans, a typical gene has 7 introns and 1400

bp in its exons, and on average the introns are 17 times larger than the exons.

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Evolution4e_04.02.ai Date 11-03-2016

TAA

TAG

TGA

ATG

Second letter

T C A G

T

TCT

TCC

TCA

TCG

Phenyl-

alanine

Leucine

TTT

TTC

TTA

TTG

Serine

TAT

TA C

Tyrosine

Stop codon

Stop codon

TGT

TGC

TGG

Cysteine

Stop codon

Tryptophan

C Leucine

CTT

CTC

C TA

CTG

CCT

CCC

CCA

CCG

Proline

C AT

CAC

CAA

CAG Glutamine

Histidine CGT

CGC

CGA

CGG

Arginine

A

Isoleucine

AT T

AT C

ATA

Methionine;

start codon

ACT

ACC

ACA

ACG

Threonine

A AT

AAC

AAA

AAG

Lysine

Asparagine AGT

AGC

AGA

AGG

Serine

Arginine

G Valine

GTT

GTC

G TA

GTG

GCT

GCC

GCA

GCG

Alanine

G AT

GAC

GAA

GAG

Glutamic

acid

Aspartic

acid

GGT

GGC

GGA

GGG

Glycine

First letter

FIGURE 4.2 The universal genetic code

relates the three DNA bases in a codon to

the amino acid in the protein made by the

gene. All organisms on Earth use this code

or a minor variant of it.

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Evolution4e_04.03.ai Date 01-12-2017

Normal red

blood cell

Sickled red

blood cell

DNA sequence:

CCT

Codon:

β-Hemoglobin:

5

CCT

Pro

6

GAG

Synonymous Nonsynonymous

Glu

7

GAG

Glu

Pro

GAA

Glu

GAG

Glu

CCT

Pro

GTG

Val

GAG

Glu

FIGURE 4.3 Synonymous changes to a DNA

sequence do not alter the amino acids in a protein,

but nonsynonymous changes do. At top is the DNA

sequence for three codons of the A allele of the

β-hemoglobin gene in humans. Beneath each co-

don is an abbreviation for the amino acid it codes

for. The sixth codon, GAG, codes for glutamic acid

(Glu). A change in that codon from GAG to GAA

does not alter the amino acid, so this is a synony-

mous change. A change from GAG to GTG, how-

ever, replaces the glutamic acid with valine (Val), so

this is a nonsynonymous change. The change to the

GTG codon produces the S allele, which results in

the sickle-cell condition.

04_EVOL4E_CH04.indd 81 3/23/17 8:55 AM