MBI- Genetic code

Question 1

what are the features of genetic code

Answer 1

(i) The codon is triplet. 61 codons code for amino acids and 3 codons do
not code for any amino acids, hence they function as stop codons.
(ii) Some amino acids are coded by more than one codon, hence
the code is degenerate.
(iii) The codon is read in mRNA in a contiguous fashion. There are
no punctuations.
(iv) The code is nearly universal: for example, from bacteria to human
UUU would code for Phenylalanine (phe). Some exceptions to this
rule have been found in mitochondrial codons, and in some
protozoans.
(v) AUG has dual functions. It codes for Methionine (met) , and it
also act as initiator codon.
(vi) UAA, UAG, UGA are stop terminator codons

vii)Specific, unambiguous- one codon codes for the same amino acid

Question 1

explain the work of hargobind khorana and marshall nirenberg’s contribution to the discovery of codons

Answer 1

The chemical method developed by Har Gobind Khorana was instrumental in synthesising RNA molecules with defined combinations
of bases (homopolymers and copolymers). Marshall Nirenberg’s cell-free
system for protein synthesis finally helped the code to be deciphered.
Severo Ochoa enzyme (polynucleotide phosphorylase) was also helpful
in polymerising RNA with defined sequences in a template independent
manner (enzymatic synthesis of RNA).

Question 2

what is mutation

Answer 2

Mutation is a phenomenon which results in alteration of DNA sequences
and consequently results in changes in the genotype and the phenotype
of an organism. In addition to recombination, mutation is another
phenomenon that leads to variation in DNA.

Question 3

what are chromosomal aberrations

Answer 3

one DNA helix runs continuously from
one end to the other in each chromatid, in a highly supercoiled form.
Therefore loss (deletions) or gain (insertion/duplication) of a segment of
DNA, result in alteration in chromosomes. Since genes are known to be
located on chromosomes, alteration in chromosomes results in
abnormalities or aberrations. Chromosomal
aberrations are commonly observed in cancer cells.

Question 4

what are pointmutation

Answer 4

In addition to the above, mutation also arise due
to change in a single base pair of DNA. This is known
as point mutation. It may be insertion, deletion or substitution. A classical example of such a
mutation is sickle cell anemia. Deletions and insertions
of base pairs of DNA, causes frame-shift mutations

Question 5

base substitution mutation

Answer 5

replacement of one base by the other, changes only one
codon, the polypeptide only changes by a single amino acid. Example- Sickle cell anaemia.

Question 6

explain the cause of sickle cell anemia

Answer 6

The defect is caused by the substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the beta globin chain of the
haemoglobin molecule. The substitution of amino acid in the globin
protein results due to the single base substitution at the sixth codon of
the beta globin gene from GAG to GUG. The mutant haemoglobin molecule
undergoes polymerisation under low oxygen tension causing the change
in the shape of the RBC from biconcave disc to elongated sickle like
structure

Question 7

what are frameshift insertion or deletion mutations

Answer 7

The insertion or deletion of one or bases results in a change in the reading frame of the codons from the point of insertion/deletion. This results in massive changes in the polypeptide length and chemical composition, resulting in a non functional protein that disrupts the functioning of the cell.
eg: cystic fibrosis

Question 8

non frameshift mutation

Answer 8

Insertion or deletion of
three or its multiple bases insert or delete in one or multiple codon hence
one or multiple amino acids, and reading frame remains unaltered from
that point onwards.