Biology Now, 2e

182 ■ CHAPTER 10 How Genes Work

GENETICS

First letter of codon

Third letter of codon

Second letter of codon

The genetic code

Phenylalanine

(Phe) Serine (Ser)

Tyrosine (Tyr)

Stop codon Stop codon

Tryptophan (Trp)

Arginine (Arg)

Serine (Ser)

Glycine (Gly)

Cysteine (Cys)

Arginine (Arg)

Stop codon

Histidine (His)

Asparagine (Asn)

Aspartic acid (Asp)

Glutamic acid (Glu)

Lysine (Lys)

Glutamine (Gln)

Proline (Pro)

Alanine (Ala)

Threonine (Thr)

Leucine (Leu)

Isoleucine (Ile)

Valine (Val)

Methionine:

start codon

Leucine (Leu)

U C A G

U

C

A

G

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

UUU

UUC

UUA

UUG

CUU

CUC

CUA

CUG

AUU

AUC

AUA

AUG

GUU

GUC

GUA

GUG

UCU

UCC

UCA

UCG

CCU

CCC

CCA

CCG

ACU

ACC

ACA

ACG

GCU

GCC

GCA

GCG

UAU

UAC

UAA

UAG

CAU

CAC

CAA

CAG

AAU

AAC

AAA

AAG

GAU

GAC

GAA

GAG

UGU

UGC

UGA

UGG

CGU

CGC

CGA

CGG

AGU

AGC

AGA

AGG

GGU

GGC

GGA

GGG

Gene

Codon

DNA strand

mRNA strand

Protein

Codon Codon

How cells use the genetic code

... ...

... ...

Transcription

Translation

A A G G A C

Phe Thr Gln

T G T

U U C A C U C A G

UAA, UAG, and UGA do not code for

an amino acid. Translation stops

when any of these codons is reached.

Like arginine, most

amino acids are specified

by more than one codon.

In transcription,

the DNA

sequence of a

gene is used to

produce an

mRNA molecule.

In translation, the genetic code is

used to determine the amino acid

that corresponds to each codon...

...and the amino acids are linked to one

another at the ribosome to form the

protein specified by the mRNA molecule.

Figure 10.9

The genetic code

(Top) The genetic code is composed of the 64 possible codons found in the mRNA. Each codon specifies an amino acid or is a

signal that starts or stops translation. (Bottom) The genetic code is used during the translation of mRNA to protein.

Q1: How many codons code for isoleucine? For tryptophan? For leucine?

Q2: What codons are associated with asparagine? With serine?

Q3: From the partial mRNA sequence that you specified in Figure 10.6’s question 3 as being transcribed from the DNA

template strand, remove only the first A. What amino acid sequence would be translated as a result of this change? How

does that sequence compare to the amino acid sequence you translated from the original mRNA sequence? Bonus: What

kind of mutation is this? (Hint: See Chapter 9.)