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Protein Synthesis: Insulin
Transcription
Initially the DNA double helix is unwound and unzipped through the breaking of
hydrogen bonds by the enzyme Helicase. This occurs in the nucleus of the cell. Another
enzyme, RNA polymerase, then binds to one unwound chain and the gene is transcribed
into RNA (transcription). During transcription the triplet-codes of DNA, consisting of
three nucleotides, are copied and correspond to a codon on the RNA. The RNA then
undergoes a Processing phase and the produced mRNA is transported out of a nuclear
pore through the nuclear membrane and into the cytoplasm.
tRNA and Ribosomes
Once out of the nucleus a ribosome will bind to the mRNA in two parts, the small sub
unit and the large sub unit. The mRNA is initially bound to the aminoacyl site, or site A.
At point A the tRNA molecule carrying the correct amino acid binds to the ribosome. The
tRNA then binds to site P or peptidyl site where the amino acid is prepared and bonded to
the previous amino acid. The tRNA is then released and binds to site E where it will Exit
the process. There are many tRNA molecules and are chosen specifically for each amino
acid as coded for by the mRNA codon. This codon corresponds to the anti-codon found
on the bottom of the tRNA. These molecules bring and release the correct amino acid and
the amino acids form peptide links at site P on the ribosome. As more tRNA molecules
are processed the peptide chain becomes longer and the primary structure of the amino
acid order causes the protein to form a tertiary structure.
Specific Triplet codes
The triplet codes are specific to an amino acid required. The code consists of three
nucleotides which allow for 64 amino acids due to there being 4 different nucleotides for
DNA: Adenine Thymine Guanine and Cytosine. These correspond to the transcribed
codons for mRNA and also tRNA anti-codons. The code is degenerate which means there
are more codes possible than required and therefore there are often three or four codes for
a single amino acid. There are also three triplet codes which signal to stop transcription
and one which signals the start of transcription:
START: methionine ATG
STOP: TAA
TAG TGA
On the mRNA the codes are reversed to the paired nucleotides: A-T and G-C, although
where thymine is required, Uracil is used instead. The anti-codon is then also reversed.
The Structure of Insulin
Insulin is one of the many proteins coded for by a gene in the DNA. It has a Specific
primary structure and although it is made in the same way as described above, the amino
acid order will be different to the order of other polypeptide chains. Also the final insulin
molecule is composed of two different polypeptide chains bonded by two disulphide
bridges in a quaternary structure. The order of amino acids can be seen on the next page.
A Diagram to show the complete process of protein synthesis:
The structure of tRNA:
The structure of insulin