Cellular Control

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).

Transcription

• 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.