Question

During cell division, DNA undergoes replication. DNA is transcribed into mRNA and the genetic code is translated into a polypeptide sequence. Out of these three processes, which is most likely to be the site a deletion, frameshift, insertion missense, nonsense, point and silent mutation or alteration occurred? Explain why you have chosen this process. 15 marks

Answer 1

The answer is DNA replication.

The DNA is replicated by the action of DNA polymerase. The DNA replication occurs in the replication fork. The DNA polymerase 1 adds the nucleotides in the newly replicated DNA. The DNA polymerase 3 helps in removal of okazaki segments. DNA polymerase 3 also helps in repair activity. If there is found wrong nucelotide in the DNA or it also fills the gap created.

If there is found mutation in the DNA polymerase 3, then there is found missense, nonsense, point and silent mutation.

* Silent mutation: When there is found the the mutation in the nucleotide in coding region occurs in such a way that same amino acid is encoded by the DNA even after replication, then the silent mutation occurs. For example: ATA is mutated to ATG. Then mRNA has changed from UAU to UAC. The tyrosine amino acid is encoded in both cases.

* Point mutation: In this case, the single nucelotide change occurs in the coding region of DNA. For example T is mutated to G.

* Missense: In this​​​​​​ case, the mutation occurs in such a way that single amino acid change is encoded by the mutation. For example: ATA is mutated to the AGA, then mRNA codon UAU is change to UCU. The amino acid is changed from tyrosine to serine amino acid.

*Non sense muttaion: The mutation in coding region occur in such a way that stop codon is encoded by this mutation. For example: ATA is mutated to ATT. The mRNA is mutater from UAU to UAA. Here UAA acts as stop codon. It does not encode any amino acid.

But when DNA polymerase 1 skips the one nucelotide addition with respect to parental strand, then there is found deletion of one nucelotide. And if it adds one extra nucleotide, then there is found one extra nucleotide and insertion occurs. The insertion and deletion leads to change the whole sequence of coding region of gene. That's why when the DNA is transcribee into mRNA, then there is found insertion or deltion of nucleotide which causes the change the codon sequence in mRNA. And the different tRNA pairs with mRNA codon. This leads to produce different polypeptide chain.