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


● 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"`.


● 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"`.

tRNA– the Adapter Molecule

● 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."`