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672 Chapter 20 NEL

Elongation

The first codon that is recognized by the ribosome is the start codon AUG. The AUG

codon also codes for methionine, so every protein initially starts with the amino acid

methionine. The ribosome has two sites for tRNA to attach: the A (aminoacyl) site and

the P (peptidyl) site. The tRNA with the anticodon complementary to the start codon

enters the P site, as shown in Figure 9 (a). The next tRNA carrying the required amino

acid enters the A site, as shown in Figure 9 (b).In Figure 9 (c), a peptide bond has

formed between the methionine and the second amino acid, alanine. The ribosome has

shifted over one codon so that the second tRNA is now in the P site. This action has

released the methionine-carrying tRNA from the ribosome and allowed a third tRNA to

enter the empty A site. The process is similar to a tickertape running through a ticker-

tape machine, except that the ribosome “machine” moves along the mRNA “tickertape.”

The tRNAs that have been released are recycled in the cell cytoplasm by adding new

amino acids to them. The process continues until the entire code of the mRNA has been

translated and the ribosome reaches a stop codon, as shown in Figures 9 (d)and (e).

A C C A C G G C U U U

P G

C C G A A A A A U C U

U

U

A

G

A

G G G C C

OH

aminoacyl site for attachment

of amino acid tyrosine

anticodon arm

anticodon

C C C G G

G A G

C U C G

C U A

G

C

C G

C

G

G

G

G

G G

C

C

G

C

A

A

U A U A G A U

A

U

C

U

5 
 end

3 
 end

Figure 8

The tRNA molecule has a cloverleaf structure. The molecule folds to form this structure because

of hydrogen bonding. The anticodon is located on the anticodon arm and the amino acid is

covalently bound to the adenine nucleotide at the 3
end (aminoacyl). In this case, the amino

acid that would be added is tyrosine because the anticodon is AUA.

DID YOU KNOW??

RNA Polymerase I, II, III
There are three forms of the RNA
polymerase in eukaryotes: RNA
polymerase I transcribes ribosomal
RNA; RNA polymerase II transcribes
mRNA; and RNA polymerase III
transcribes tRNA and other short
genes that are about 100 base pairs
in length.

DID YOU KNOW??

From DNA to Protein
The discovery of the relationship
between DNA, mRNA, ribosomes,
tRNA, and protein was the result of
numerous scientists working on
separate pieces of the puzzle.
Watch an online animation of their
studies.

http://www.science.nelson.com GO