Genetic code and regulation of gene expression:

 

Genetic Code: The relationship between the sequence of nucleotides on mRNA and sequence of amino acids in the polypeptide is called genetic code.

George Gamow: According to him the code must be of 3 bases in order to code for 20 amino acids because there are only 4 bases (i.e., 4³ or 4 x 4 x 4 = 64) which code for 20 amino acids. So, codon is a triplet.

Salient features of genetic code:

1) The codons are triplet. Out of 64 codons, 61 code for 20 amino acids and 3 codons (UAA, UGA, UAG) do not code for any amino acid hence, function as stop or terminating codons.

2) One codon codes for only one particular amino acid, hence the code is unambiguous and specific.

 3) Some amino acids are coded by more than one codon, hence the code is degenerate.

4) The codon is read on mRNA in a contiguous fashion, i.e., without punctuations. So the code is commaless.

5) The genetic code is nearly universal, i.e, a particular codon codes for the same amino acid in all organisms except in mitochondria and few protozoa.

6) AUG is a dual function codon, it codes for methionine (met) and it also acts as initiator codon.

Regulation of gene expressions:

Here, regulation means control of gene expressions which can occur at various levels.

1) Transcriptional level (at the time of formation of primary script ---RNA primarily formed from DNA)

2) Processing level (regulation of splicing)

3) Transport of mRNA from nucleus to cytoplasm.

4) Translational level (formation of polypeptide)

*Promoter gene: it tells the template and coding strand and also marks the site where RNA polymerase binds.

*Activators: They start the process(act positively in fewer of a  specific process)

*Repressors: They stop the action (act negatively againt a specific process)

*Operators: Operator gene lie adjacent to the structural gene and control their functioning.