Monday, November 3, 2008

Translation

Translation is the process by which a triplet base sequence of mRNA molecules are converted into a specific sequence of amino acids in a polypeptide chain in the cytoplasm of a cell. It consists of initiation, elongation, and termination.

The mRNA, a tRNA carrying the first amino acid of the polypeptide chain and the two subunits of a ribosome are brought together. After leaving the nucleus, the mRNA forms a reversible attachment to the mRNA binding site at the surface of a small ribosomal subunit. It recognizes a specific sequence on the mRNA just upstream of the initiation codon.
The initiation codon on the mRNA attracts its complementary tRNA, which carries the specific amino acid. This aminoacyl tRNA complex binds reversibly to the mRNA at the initiation site by hydrogen bonding via specific complementary base pairing with its anticodon.
It occupies the P site of a ribosome; translation starts as a translation initiation complex is formed. The initiation codon is most often AUG, which codes for the amino acid methionine.
Several ribosomes attracted to the mRNA simultaneously form a polyribosome. This is possible as once a ribosome has moved past the start codon, a second ribosome can attach to mRNA, thereby increasing efficiency of translation. The binding of the mRNA forms a translational complex and elongation of the polypeptide chain as translation proceeds. Newly formed polypeptide chains then enter the rough endoplasmic reticulum lumen and fold into its secondary/tertiary/quaternary structure in the cisternae. It is then packaged into vesicles and buds off and fuses with the soft endoplasmic reticulum and then the Golgi apparatus.

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