Translation is the process that decodes the information of mRNA into the sequence of amino acids that eventually form a protein. Translation consists of four stages:
1 initiation 3 translocation 2 elongation 4 termination
mRNA slides through gap as the code is ‘read’ in the ribosome
line of section through ribosome to show sequence of interaction sites
ribosome moves along the mRNA in a 5’ to 3’ direction,
‘reading’ the codons
site 1 – where the peptide bond is formed, by which the new amino acid is attached to the growing polypeptide
site 2 – where the anticodon of an incoming charged tRNA base pairs with a codon on the mRNA strand
‘exit site’ – where the free tRNA leaves the ribosome
mRNA 3’
5’
small subunit
large subunit
Figure 7.9 The structure of a ribosome.
7 NUCLEIC ACIDS AND PROTEINS 179
Initiation
Translation begins at a start codon (AUG) near the 5' end of the mRNA strand. This codon codes for the amino acid methionine and is a signal to begin the process of translation (Figure 7.10). This is called initiation.
The mRNA binds to the small subunit of a ribosome. Then an activated tRNA molecule, carrying the amino acid methionine, moves into position at site 1 of the ribosome. Its anticodon binds with the AUG codon using complementary base pairing. Hydrogen bonds form between the complementary bases of the mRNA and tRNA and, once this has happened, a large ribosomal subunit moves into place and combines with the small subunit.
Elongation
Initiation is followed by elongation and the formation of peptide bonds (Figure 7.11). tRNA molecules bring amino acids to the mRNA strand in the order specifi ed by the codons. To add the second amino acid, a second
Figure 7.10 mRNA ‘start’ and ‘stop’ codons.
Figure 7.11 The formation of a peptide bond by a condensation reaction.
the genetic code in this mRNA is read in this direction
A U G G A U U C C U G C U A A
this codon represents the amino acid methionine, which signals
the ‘start’ of translation
this codon represents ‘stop’, and signals the end of translation
5' 3'
one amino acid
C H2N
H2O COOH R
H
another amino acid
C
H2N COOH R
H
C H2N CO
R
H
C
HN COOH
R
H
Two atoms of hydrogen and one of oxygen are lost – they form a molecule of water.
The two amino acids are joined at this point by a peptide bond.
peptide bond
charged tRNA with the anticodon corresponding to the next codon enters site 2 of the ribosome and binds to its codon by complementary base pairing. The ribosome catalyses the formation of a peptide bond between the two adjacent amino acids (Figure 7.12). The ribosome and
growing polypeptide
tRNA leaving
Lys
tRNA moving in
Pro exitsite site 1 site 2
peptide bond Translocation
codon
AU U A G
C A U
G AU
G AU
G
C C C 3’
5’
ribosome moves in a 5’ 3’ direction along the mRNA, to the next codon
large subunit of ribosome mRNA attached to small subunit of ribosome
exitsite site 1 site 2 Elongation
Initiation
codon
mRNA Met
AU U A G
C A U A
U A
U C C C 3’
5’
exitsite site 1 site 2
codon
mRNA
AU U A G
C A A A C C C 3’
5’
Met Lys
tRNA brings second amino acid into position
peptide bond formed
Met
Figure 7.12 The stages of translation.
Look back at Table 4.1 on page 68 to see all the mRNA codons and what they represent.
7 NUCLEIC ACIDS AND PROTEINS 181 tRNA molecules now hold two amino acids. The methionine becomes
detached from its tRNA. Now the ribosome moves along the mRNA and the fi rst tRNA is released to collect another methionine molecule.
Translocation
Translocation is the movement of the ribosome along the mRNA strand one codon at a time. As the ribosome moves, the unattached tRNA moves into the exit site and is then released into the cytoplasm, where it will pick up another amino acid molecule. The growing peptide chain is now positioned in site 1, leaving site 2 empty and ready to receive another charged tRNA molecule to enter and continue the elongation process.
Figure 7.12 shows how initiation, elongation and translocation occur as mRNA is translated.
Termination
Translocation and elongation are repeated until one of the three ‘stop’
codons aligns with site 2, which acts as a signal to end translocation.
There are no tRNA molecules with anticodons corresponding to these stop codons. The polypeptide chain and the mRNA are released from the ribosome and the ribosome separates into its two subunits. This fi nal stage of translation is called termination.
Polysomes
Translation occurs at many places along an mRNA molecule at the same time. A polysome like that in Figure 7.13 is a group of ribosomes along one mRNA strand.
Figure 7.13 A polysome.
small and large subunits of ribosome +polypeptide growing
polypeptide chains
ribosomes moving
along mRNA mRNA in
cytoplasm
5 State what is meant by an ‘exon’.
6 Distinguish between ‘transcription’ and ‘translation’.
7 Distinguish between the ‘sense’ and ‘antisense’ strands of DNA.
8 Outline the role of the promoter region in transcription.
9 Draw a peptide bond between two amino acids.
10 State where the protein that is synthesised by free ribosomes is used.