Genetic code

The relationship between the sequence of nucleotides on mRNA and sequence of aminoacid in the polypeptide is called GENETIC CODE.

George Gamow suggested that the code must be of 3 bases in order to code for 20 aminoacid.
Because there are only four (4) bases which code for aminoacid So codon is triplet.

Salient features of Genetic code

➤ 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 aminoacids hence, function as stop or termination codon.
➤ One codon codes for only one particular aminoacid, hence the code is unambiguous and specific.
Some aminoacids are coded by more than one codon hence, the code is degenerate.
➤ The codon is read on mRNA in a contiguous fashion that is without punctuations and thus the code is commaless.

POINT MUTATION

It is the mutation in a single base pair, which is replaced by another base.
For example:- In sickle cell anemia point mutation in ẞ-globin chain result in change of glutamate to valine,

tRNA –The adaptor molecule

• Francis crick proposed the presence of an adaptor molecule which could read the code on one end and on the other end would bind to the specific aminoacids.
• It carries the aminoacid to the ribosomes to make protein.
• There are specific t-RNA for each codon and aminoacid.

STRUCTURE OF t-RNA

The secondary structure of t-RNA is clover leaf like but the three – dimensional tesliary structure depicts it as a compact inverted L-shaped molecule.

t-RNA has five arms or loops:-

• Anticodon loop:-which has bases complementary to the code.
• Amino acid acceptor end :– to which aminoacid binds.
• T Loop :– which help in binding to ribosome.
• D loop :– which helps in binding aminoacyl su
• Variable loop:– a specific t-RNA for initiation is called initiator t- codon. RNA. There is no t-RNA fo stop codon.