Question

16. Which of the following polymerases do not have proof reading ability?

a. DNA polymerase I

b. DNA polymerase II

c. DNA polymerase III

d. DNA polymerase a (alpha)

e. DNA polymerase d (delta)

17. Which of teh following may detect G:T mismatch in bacterial genome?

a. exonuclease I

b. MutS

c. DNA polymerase III

d. Uracil DNA glycosidase

e. uvrABC excinuclease

18. Which of the following is the primary target of ciprofloxacin?

a. DNA topoisomerase II (DNA gyrase)

b. DNA topoisomerase I

c. Helicase

d. DnaA

e. DnaB

19. Which of the following is not a function of reverse transcriptase?

a. Synthesis of DNA complementary to RNA

b. Digestion of RNA

c. Synthesis of second strand of DNA

d. Incorporation of DNA fragment into host genome.

Answer 1

Answer 16-DNA polymerase α (alpha)

DNA polymerases are a group of enzymes that performs the most fundamental functions in DNA replication. It catalyze the synthesis of polydeoxyribonucleotides from four mono-deoxyribonucleoside triphosphates (dATP, dGTP, dTTP, dCTP).

Various types of DNA polymerase  have been found i.e. three types of Bacterial DNA polymerase (DNA Pol I, II, and III) and Five Eukaryotic DNA polymerase (DNA Pol α, β, γ, δ, and ɛ). Besides polymerization, these polymerases also carry an important function of proof reading.

DNA polymerase can only add bases in one direction i.e. from 5'→3' direction. DNA polymerases have the ability to proofread, using 3’ → 5’ exonuclease activity. When an incorrect base pair is incorporated in DNA strand, DNA polymerase reverses its direction by one base pair of DNA and excises the incorrect base.

Among the eight polymerase mentioned above, DNA polymerase α (alpha) lacks proofreading activity. Researchers found the evidence that errors incorporated by DNA Polymerase α are corrected by DNA Polymerase δ.

Answer 17- MutS

DNA mismatch repair (MMR) is a highly conserved DNA repair system that corrects incorrect base pairs incorporated by DNA replication errors. Three proteins i.e. MutS, MutH and MutL are essential in detecting the mismatch and directing repair machinery to it. Recognition of a G:T mismatch by a dimeric MutS protein initiates a cascade of reactions and results in repair of the newly synthesized strand. When a mismatched base is recognized by a MutS homodimer, it form a MutS-DNA complex. A MutL homodimer interacts with this complex alongwith activation of MutH restriction endonuclease. MutH nicks the unmethylated strand to introduce an entry point for the excision reaction. The mismatch containing strand is removed by helicases and exonucleases, and a new strand is synthesized by DNA polymerase III and ligase.

Answer 18- DNA topoisomerase II (DNA gyrase)

Topoisomerases are enzymes that helps in the overwinding or underwinding of DNA. DNA Gyrase belongs to a class of enzymes known as topoisomerases that relaxes tension by reducing supercoiling which builds up during DNA unwinding, preventing DNA breakage. Ciprofloxacin also known as topoisomerase poisons is second-generation bactericidal fluoroquinolone that interrupts DNA replication by inhibiting topoisomerase. Binding of Ciprofloxacin to the enzyme, traps the enzyme in transient step of a catalytic cycle thereby preventing the reunion of a G-segment. Inhibition of DNA gyrase blocks relaxation of supercoiled DNA and thus DNA replication gets inhibited.

Answer 19- Incorporation of DNA fragment into host genome.

Reverse transcriptase is an RNA-dependent DNA polymerase that is found in many retroviruses such as human immunodeficiency virus (HIV). The enzyme catalyzes the conversion of RNA template molecules into a DNA double helix.

Reverse transcriptase contains three enzymatic activities: (a) RNA-dependent DNA polymerase, (b) RNase H, and (c) DNA-dependent DNA polymerase.

RNA-dependent DNA polymerase synthesizes a complementary DNA strand to the RNA template. After that RNase H digest the RNA strand from the RNA–DNA hybrid double helix. Then the DNA-dependent DNA polymerase synthesize another strand of DNA using single strand DNA creating double-stranded DNA.