Translation

Translation

Is the process by which information encoded within mRNA is used to make a specific polpeptide chain. It is a complex process which takes place in ribosomes in the cytoplasm
Ribosomes are small intracellualr organelles. Each consist of two subunits made up of proteins and ribosomal RNA (rRNA); the samll subunit has a binding site for mRNA, the large sub unit has a binding site for tRNA
mRNA carries information in the form of codons which dictate which amino acids are to be used to make a polypeptide. A ribosome moves along a strand of mRNA in the 5'-3' direction and the codons are 'read' sequently. tRNA molecules bring amino acids to the ribosomes. These are added one by one to the growing polypeptide chain. The ribosome allows two molecules of tRNA to combine with the mRNA at any time. One tRNA molecule holds the growing polypeptide chain; the other carries the next amino acid to be added to the chain.
Before a tRNA molecule moves to a ribosome, a specific enzyme makes sure it is carrying the correct amino acid specified by its anticodon. The attachment of amio acids to tRNA requires energy from ATP
The following sequences of events takes place during translation. 1. A new amino acid is added to a growing polypeptide chain when the ribosome has reached a part of the mRNA strand containign its specific codon. 2. The tRNA molecule with the complememtary anticodon and carrying the amino acid attaches onto the mRNA. 3. An enzyme catalyses the formation of a peptide bond between the amino acid on the tRNA and the amino acid at the end of the growing polypeptide chain. 4. The tRNA molecule that was holding the amino acid is released from the ribosome, and is free to carry another amino acid molecule 5. The ribosome moves one codon further along the mRNA strand, exposing the next codon so that another amino acid can be added to the chain.
Starting and stopping; polypeptide synthesis is usually intiated by the codon AUG
On leaving the ribosome, the polypeptide is processed according to the final destination of the protein. This may include folding to form the secondary and tertiary structure and, in some proteins, adding chains to form a quarternary structure. Proteins used inside the cell, such as heamoglobin are usually made on free rbosomes and released into the cytoplasm. Polypeptides of proteins that are to be exported from the cell, such as digestive enzymes, are made on the RER. As the polypeptide is mafe, it is threaded through pores in the ER and it builds up inthe cisternae. It is transported in vesicles to the golgi apparatus, where it is modified and packaged. golgi vesicles then transport the protein to the cell surface membrane from which it is secreted by exocytosis. Polypetides that form membrane proteins follow the same route, but they remain on the cell surface membrane rather than being transported

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	Created by harry_bygraves almost 11 years ago