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NEL Molecular Genetics 673

Section20.2

Met

A site

3
5

5
3
mRNA

P site

(a)
tRNA

UAC

AGG AGG AUG GCA AUA

Met

Ala

A site

ribosome shifts

P site

(b)

UAC

AGG AGG AUG GCA AUA

3 
 CGU 5 


5 
 3 
 mRNA

ribosome shifts ribosome shifts

(c)

A site

peptide

bond

P site

CGU

AGG AGG

3 
 UAU 5 

5
3
mRNA

Met Ala

Ile

AUG GCA AUA

Met

Arg

Ala Ile

P site A site

tRNA

(d)

Ile

tRNA

UAU

UAC

3 
 UAUUUG 5 


5 
 AGG AGG AUG GCAAUA AAC CUA 3 
 mRNA

polypeptide chain

A site

stop codon

P site

(e)

GCU

5 
 GUU ACUAGU CGA UAG 3 
 mRNA

Val

Ser

Thr

Asn

polypeptide

chain

large subunit

of ribosome

small subunit of ribosome

P

site

A

site

(f) tRNA

5 
 3 
 mRNA

3 
 UGU 5 

Figure 9 AGU CGA UAG

Protein synthesis

(a)The first tRNA that is brought into the P site carries methionine because the start codon is AUG.

(b)The second tRNA enters the A site.

(c)A peptide bond forms between methionine and alanine. The ribosome shifts one codon over and the next

tRNA brings in the appropriate amino acid into the A site.

(d)The ribosome moves the mRNA and another amino acid is added to the chain.

(e)The process is repeated until the ribosome reaches a stop codon for which no tRNA exists.

(f)A release-factor protein helps break apart the ribosome–mRNA complex, releasing the polypeptide chain.