133223
Description
Mind Map by harry_bygraves, updated more than 1 year ago
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Created by harry_bygraves
over 11 years ago
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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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