Gene - length of DNA, codes for one or more polypeptides.
Sequence of nucleotide bases on a gene provides a code. It is a triplet code (3 nucleotide bases).
- First stage of protein synthesis. One strand of DNA is used as a template.
- Gene unwinds and unzips. H bonds between complimentary bases break.
- RNA nucleotides bind to exposed bases.
- mRNA is complementary to template DNA strand.
- mRNA is released from the DNA and passes out of the nucleus through a pore in the nuclear envelope.
Second stge of Protein Synthesis. Amino acids are arranged in a sequence dictated by the sequence of codons on mRNA.
Sequence of Amino acids is important because :
- Forms Primary structure of a protein.
- Primary structure determines tertiary structure.
- Tertiary structure allows protein to function, if altered the active site may have altered shape.
How the Polypeptide is assembled.
- Molecule of mRNA binds to a ribosome. 2 codons attached to small sub unit of the ribosome and exposed to large unit of ribosome.
- Second tRNA, with different amino acid binds to second exposed codon.
- Peptide bond forms between two adactent amino acids.
- Ribosome moves along the mRNA reading the next codon. Third tRNA brings another amino acid and a peptide bond forms between it and the dipeptide. First tRNA leaves and can collect and bring another amino acid.
- Polypeptide chain grows until a stop codon is reached.