Microbiology and Immunology

Translation WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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tide bond is formed between the amino acid brought in by the

second tRNA and the amino acid carried by the first tRNA.

The first tRNA is now released and the ribosome again shifts.

The second tRNA bearing two amino acids is now in the “P”

site, and a third tRNA can now bind to the “A” site. The

process of the tRNA binding to the mRNA aligns the amino

acids in a specific order. This long chain of amino acids con-

stitutes a protein. Therefore, the sequence of nucleotides on

the mRNA molecule directs the order of the amino acids in a

given protein. The process of adding amino acids to the grow-

ing chain occurs along the length of the mRNA until the ribo-

some comes to a sequence of bases that is known as a “stop

codon.” When that happens, no tRNA binds to the empty “A”

site. This is the signal for the ribosome to release the polypep-

tide chain and the mRNA.

Bacterial ribosomes are smaller than eukaryotic ribo-

somes. In some cases, bacterial ribosomes contain less than

have the total protein found in eukaryotic ribosomes. Bacteria

also respond to fewer initiation factors than do eukaryotic cells.

After being released from the tRNA, some proteins may

undergo post-translational modifications. They may be

cleaved by a proteolytic (protein cutting) enzyme at a specific

site. Alternatively, they may have some of their amino acids

biochemically modified. After such modifications, the

polypeptide forms into its native shape and starts acting as a

functional protein in the cell.

There are four different nucleotides, A, U, G and T. If

they are taken three at a time (to specify a codon, and thus,

indirectly specify an amino acid), 64 codons could be speci-

fied. However, there are only 20 different amino acids.

Therefore, several triplets code for the same amino acid; for

example UAU and UAC both code for the amino acid tyro-

sine. In addition, some codons do not code for amino acids,

but code for polypeptide chain initiation and termination. The

Illustration depicting the transcription of DNA inside the eukaryotic nucleus and the translation of the messenger RNA to form protein that occurs

outside the nucleus.

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