Question

The proofreading during replication of a DNA molecule to eliminate a wrong in the elongation strand is brought about by
A. The 3'exonuclease activity of DNA polymerase I and III
B. The 3'exonuclease activity of DNA polymerase I and
C. The 5'-3' exonuclease activity of DNA polymerase II
D. RNA polymerase

Answer 1

Hint: Proofreading is mainly the term we use in genetics to refer to the correction of the error in the DNA template. It mainly occurs in mRNA translation for protein synthesis. This is brought about by the help of polymerases.

Complete answer:
We know that DNA is the genetic material. While proposing the double-helical structure for DNA, Watson and Crick had immediately proposed a scheme for replication of DNA. The scheme suggests that the two strands would get separated and act as a template for the synthesis of the new complementary strand.

There are three types of RNA that are needed for protein synthesis in a cell:
a. mRNA - messenger ribosomal nucleic acid - helps in the transcription of DNA.
b. tRNA - transfer ribosomal nucleic acid - helps in translation of protein
c. rRNA - the ribosomal nucleic acid- plays a structural and catalytic role during translation.
After the completion of replication, each DNA molecule would have one parental and one newly synthesised strand. This scheme is termed semiconservative DNA replication.

Proofreading occurs in mRNA translation for protein synthesis. Most of the mistakes made during the replication of DNA are corrected in DNA polymerase by proofreading. In this, the DNA reads the newly added base-paired correctly with the base in the template strand. This is brought about by 3' exonuclease activity (which cleaves the nucleotides one at a time of DNA strand from one end) of DNA polymerase I and II.

RNA polymerase-This enzyme is used during transcription which copies DNA sequence into RNA.
The answer is A; DNA replication is a highly acute process and during polymerisation, a wrong base might get inserted. The repair mechanism corrects these mistakes, if not it might lead to mutation.
The extent of proofreading in molecules processes depends upon the effective population size of the species, number of genes being affected.

Note: Genetic code comprises the information of the protein produced from the RNA. There are three nucleotides and four nitrogenous bases, which collectively form a triplet codon that codes for one amino acid. Therefore, there are twenty naturally existing amino acids. The genetic code degenerates. This was depicted by the features of the genetic code, on the basis of which a few amino acids are coded by more than one codon thereby causing them to degenerate. Each codon codes for only one specific amino acid and these codes are considered to be universal irrespective of the type of organism.