HUMAN BIOLOGY

410 Chapter 21

What is the geNetic coDe?

• The genetic code is a set of sixty-four different groups of
  three mRNA bases called codons.


• A cell’s protein-making machinery “reads” codons, which
  specify different amino acids.


• Accordingly, mRNA provides the instructions for assembling
  amino acids into the polypeptide chains of proteins.

taKe-home messaGe

codon A set of three nucle-

otide bases in mRNA; they

are the three-letter “words”

of the genetic code.

genetic code The array of

codons that provide instruc-

tions for making proteins.

start codon A codon that

marks the start of a new

amino acid chain during

protein synthesis.

stop codon A codon that

marks the end of a new

polypeptide chain.

A

T

C

G

U

A

C

G

C

G

U

A

G

C

G

C

G

C

G

C

G

C

A

T

A

T

A

T

A

A C U C C U G A G G A G A A G

T

mRNA

DNA

mRNA

codons

amino

acids

reonine Proline Glutamate Glutamate Lysine

B

the Genetic Code

second base

U

U

U C A G U C A G U C A G U C A G

C

A

G

CAG

third

base

first

base

UUU

UUC

UUA

UUG

CUU

CUC

CUA

CUG

phe

ser

leu

leu

UAU

UAC

UAA

UAG

tyr

UGU

UGC

UGA

UGG

cys

trp

UCU

UCC

UCA

UCG

asn

lys

GUU

GUC

GUA

GUG

val

GCU

GCC

GCA

GCG

ala

AUU

AUC

AUA

AUG

ile

met

ACU

ACC

ACA

ACG

thr

CCU

CCC

CCA

CCG

CAU

CAC

CAA

CAG

pro

CGU

CGC

CGA

CGG

arg

GGU

GGC

GGA

GGG

gly

AAU

AAC

AAA

AAG

his

gln

asp

glu

GAU

GAC

GAA

GAG

ser

arg

AGU

AGC

AGA

AGG

A

stop stop

stop

n the sequence of nucleotides in an mRNA molecule is like a

string of three-letter protein-building “words.”

n Link to Primary structure of proteins 2.11

Codons are mrna “words”

for building proteins

Each “word” in the mRNA instruc-

tions for building a protein is a

set of three nucleotide bases that

are “read” by a cell’s protein-

making machinery. These base

triplets are called codons. There

are sixty-four kinds of codons

(Figure 21.8A). Together they are

the genetic code—a cell’s direct

instructions for making proteins.

The order of different codons in an

mRNA molecule determines the

order of amino acids that are assembled into a protein

(Figure 21.8B).

Gene by gene, mRNAs carry protein-building instruc-

tions from the DNA in a cell’s nucleus to the cytoplasm

where amino acids are. A start codon marks the first

amino acid of a new polypeptide chain. Most of the twenty

kinds of amino acids can be ordered up by more than

one start codon. (For example, glutamate corresponds to

the code words GAA or GAG.) The codon AUG sets the

“reading frame” for trans lation. That is, ribosomes start

their “three-bases-at-a-time” selections at an AUG that is

the start signal in an mRNA strand. Three different stop

codons (UAA, UAG, and UGA) can signal ribosomes to

stop adding amino acids to the growing chain.

21.4

Figure 21.8 The genetic code consists of mRNA “words”

called codons. A The codons in mRNA are nucleotide bases,

“read” in blocks of three. Sixty-one of these base triplets

correspond to specific amino acids.

The left column of the diagram shows the first of the three

nucleotides in each mRNA codon. The middle columns show

the second nucleotide. The right column shows the third.

Reading from left to right, for instance, the triplet UGG

corresponds to tryptophan. Both UUU and UUC

correspond to phenylalanine.

B How genetic information is converted into a protein. First, a

DNA strand is transcribed into mRNA. Notice how the mRNA’s

nucleotide sequence is complementary to the gene sequence

in the DNA. Each mRNA codon called for one amino acid in a

growing protein (polypeptide chain). (© Cengage Learning)

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).