The relationships between genes and DNA are best understood by mutation studies. You have studied about mutation and its effect in Chapter 5. 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.