Biology MUTATIONS AND tRNA

### KEY TOPICS

star Mutations
star Mutations through Insertions and Deletions
star tRNA- The Adaptor Molecule

### MUTATIONS

● The relationships between color{Violet}"genes and DNA" are best understood by color{Violet}"mutation studies"..

● Effects of color{Violet}"large deletions" and color{Violet}"rearrangements" in a segment of DNA are easy to comprehend.

● It may result in color{Violet}"loss or gain" of a gene and so a function.

● The effect of color{Violet}"point mutations" will be explained here.

● A classical example of color{Violet}"point mutation" is a change of single base pair in the gene for color{Violet}"beta globin" chain that results in the change of amino acid residue color{Violet}"glutamate to valine".

● It results into a diseased condition called as color{Violet}"sickle cell anemia".

### MUTATIONS THROUGH INSERTIONS AND DELETIONS

● Effect of color{Violet}"point mutations" that color{Violet}"inserts or deletes" a base in structural gene can be better understood by following simple example.

star color{Brown}"Insertions":

● Consider a statement that is made up of the following words each having three letters like genetic code.
color{Violet}"RAM HAS" color{Violet}"RED CAP"

● If we insert a letter B in between HAS and RED and rearrange the statement, it would read as follows:
color{Violet}"RAM HAS" color{Red}"B"color{Violet}"RE DCA P"

● Similarly, if we now insert two letters at the same place, say BI'. Now it would read,
color{Violet}"RAM HAS B"color{Red}"IR" color{Violet}"EDC AP"

● Now we insert three letters together, say BIG, the statement would read
color{Violet}"RAM HAS" color{Red}"BIG" color{Violet}"RED CAP"

star color{Brown}"Deletions":

● 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:

color{Violet}"RAM HAS" color{Violet}"RED CAP"

color{Violet}"RAM HAS" color{Red}"EDC" color{Violet}"AP"

color{Violet}"RAM HAS" color{Red}"DCA" color{Violet}"P"

color{Violet}"RAM HAS CAP"

● The conclusion from the above exercise is very color{Violet}"obvious".

● color{Violet}"Insertion or deletion" of one or two bases color{Violet}"changes the reading frame" from the point of insertion or deletion.

● Insertion or deletion of color{Violet}"three or its multiple" bases insert or delete one or multiple color{Violet}"codon" hence one or multiple amino acids, and color{Violet}"reading frame remains unaltered" from that point onwards.

● Such mutations are referred to as color{Violet}"frame-shift insertion" or color{Violet}"deletion mutations".

● This forms the genetic basis of proof that color{Violet}"codon is a triplet" and it is read in a color{Violet}"contiguous manner".

● From the very beginning of the proposition of code, it was clear to Francis Crick that there has to be a mechanism to color{Violet}"read the code" and also to color{Violet}"link" it to the color{Violet}"amino acids", because amino acids have color{Violet}"no structural specialities" to read the code uniquely.

● He postulated the presence of an color{Violet}"adapter molecule" that would on one hand read the code and on other hand would bind to specific amino acids.

● The color{Violet}"tRNA", then called color{Violet}"sRNA" (color{Violet}"soluble RNA"), was known before the genetic code was postulated.

● However, its role as an color{Violet}"adapter molecule" was assigned much later.

● tRNA has an color{Violet}"anticodon loop" that has bases complementary to the code, and it also has an color{Violet}"amino acid acceptor" end to which it binds to amino acids.

● tRNAs are color{Violet}"specific" for each amino acid.

● For color{Violet}"initiation", there is another specific tRNA that is referred to as color{Violet}"initiator tRNA".

● There are color{Violet}"no tRNAs" for color{Violet}"stop codons".

● In figure, the color{Violet}"secondary structure" of tRNA has been depicted that looks like a color{Violet}"clover-leaf".

● In color{Violet}"actual structure", the tRNA is a compact molecule which looks like color{Violet}"inverted L."