Molecular Basis of Inheritance Test - 43

DNA synthesis takes place by a polypeptide. In nature, such molecules are created by all living cells through the process of DNA replication, with replication initiator proteins splitting the existing DNA of the cell and making a copy of each split strand, with the copied strands then being joined together with their template strand into a new DNA molecule. 

DNA ligase is a specific type of enzyme, a ligase, that facilitates the joining of DNA strands together by catalyzing the formation of a phosphodiester bond. It plays a role in repairing single-strand breaks in duplex DNA in living organisms, but some forms may specifically repair double-strand breaks, i.e., a break in both complementary strands of DNA. Single-strand breaks are repaired by DNA ligase using the complementary strand of the double helix as a template, with DNA ligase creating the final phosphodiester bond to fully repair the DNA. DNA ligase has applications in both DNA repair and DNA replication.

Correct Option: B
Explanation:

• The process of RNA synthesis from DNA is called transcription. During transcription, the complete gene is copied into a pre-mRNA along with exons and introns.
• The exons are the coding regions and the introns are the non-coding regions. The process of RNA splicing converts the pre-mRNA to mRNA and involves the removal of introns and the joining of exons.
• This results in the formation of the mature mRNA that is translated into a protein.

Wobble hypothesis is based on the observation that, the base in first position of anti-codon on tRNA is usually an abnormal base, like inosine, pseudouridine, tyrosine etc. These abnormal bases pair with more than one type of nitrogenous base in the third position of the codon on mRNA; Inosine (I) can pair with A, C or U. This base is called Wobble base which is present at position 1 of the anti-codon and position 3 of the codon. According to Wobble hypothesis, the third position of the codon on mRNA and the first position of the anticodon on tRNA are bound less tightly than the other two pair and provide unusual base combinations. Due to genetic code degeneracy, many amino acids share the third base and differ in first two bases only thereby making the third base less important. It was given by Crick.

In genetic code, three nucleotide bases code for one amino acid. With four bases, there are 64 possible combinations of three letter genetic code. Of these 64 genetic codes 61 represents 20 amino acids and three for stop signals. Thus there are more than one codon for more than one amino acid. This is called degeneracy of genetic code. The term "degenerate" simply refers to the fact that the translation from nucleotide codons to amino acids isn't one to one. Some parts of the genetic code are redundant.

DNA synthesis always occurs in the 5'-->3' because chain growth results from the formation of a phosphodiester bond between the 3' oxygen of a growing strand and $$\alpha$$-phosphate of a dNTP.  As we know that the two strands of the parental DNA duplex are antiparallel, and DNA polymerases can add nucleotides to the growing new strands only in the  5'-->3' direction. The leading strand synthesis proceeds continuously from a single RNA primer in the  5'-->3' direction. To facilitate the growth of the lagging strand in the  5'-->3' direction, copying of its template strand must somehow occur in the opposite direction from the movement of the replication fork. This is done by synthesizing a new primer every few hundred bases on the second parental strand. Each of these primers is elongated in the  5'-->3' direction and form discontinuous segments called Okazaki fragments which are later joined together by DNA ligase. DNA replication is bidirectional mechanism because two replication forks are formed at an origin which moves in opposite directions, with both template strands being copied at each fork. Option B is the wrong statement.

DNA replication is the process of producing two identical replicas from one original DNA molecule. This biological process occurs in all living organisms and is the basis for biological inheritance. 

A. Correct option- D