Question

What is Mutation ? Explain with the help of an example how does a point mutation affect the genetic code. Name another type of mutation ?


class 12th CBSE Biology sample paper

Answer 1

Mutation is defined as the change in the genetic coding of DNA

Effects of large deletions and rearrangements in a segment of DNA are easy to comprehend. 

It may result in loss or gain of a gene and so a function. 

The effect of point mutations will be explained here.  A classical example of point mutation is a change of single base pair in the gene for beta globin chain that results in the change of amino acid residue glutamate to valine. It results into a diseased condition called as sickle cell anemia.

Effect of point mutations that inserts or deletes a base in structural gene can be better understood by following simple example. Consider a statement that is made up of the following words each having three letters like genetic code.

.

RAM  HAS  RED  CAP

If we insert a letter B in between HAS and RED and rearrange the statement, it would read as follows:

RAM  HAS  BRE  DCA  P

Similarly, if we now insert two letters at the same place, say BI'. Now it would read,

RAM  HAS  BIR  EDC  AP.

Now we insert three letters together, say BIG, the statement would read

RAM  HAS  BIG  RED  CAP

The same exercise can be repeated, by deleting the letters R, E and D, one by one and rearranging the statement to make a triplet word.  

RAM  HAS  EDC  AP

RAM  HAS  DCA  P

 RAM  HAS  CAP

The conclusion from the above exercise is very obvious.  Insertion or deletion of one or two bases changes the reading frame from the point of insertion or deletion. Insertion or deletion of three or its multiple bases insert or delete one or multiple codon hence one or multiple amino acids, and reading frame remains unaltered from that point onwards. Such mutations are referred to as frame-shift insertion or deletion mutations. This forms the genetic basis of proof that codon is a triplet and it is read in a contiguous manner.