Question

Match column I with column II and select the correct option from the codes given below.

	Column I		Column II
A.	Exonucleases	(i)	Stable above ${90}^{o}C$
B.	Polynucleotide kinases	(ii)	Cleave the end of linear DNA
C.	Taq DNA polyrnerase	(iii)	Add phosphate to 5' OH end
D.	Terminal transferases	(iv)	add a number of nucleotides to 3' end of DNA or RNA
		(v)	Regulate the level of supercoiling of DNA molecule

• A. A - (ii), B - (iii), C - (i), D - (iv)
• B. A - (ii), B - (v), C - (i), D - (iii)
• C. A - (ii), B - (iv), C - (i), D - (v)
• D. A - (iii), B - (iv), C - (i), D - (ii)

Answer 1

The correct option is A A - (ii), B - (iii), C - (i), D - (iv)

Column I	Column II
A. Exonucleases	Exonucleases are enzymes that work by cleaving nucleotides one at a time from the end (exo) of a polynucleotide chain. A hydrolyzing reaction that breaks phosphodiester bonds at either the 3' or the 5' end occurs. Its close relative is the endonuclease, which cleaves phosphodiester bonds in the middle (endo) of a polynucleotide chain. Eukaryotes and prokaryotes have three types of exonucleases involved in the normal turnover of mRNA: 5' to 3' exonuclease (Xrn1), which is a dependent decapping protein; 3' to 5' exonuclease, an independent protein; and poly(A)-specific 3' to 5' exonuclease. So the correct option is 'cleave the end of linear DNA'.
B. Polynucleotide kinases	T4 polynucleotide kinase (Pnk), in addition to being an invaluable research tool, exemplifies a family of bifunctional enzymes with 5′-kinase and 3′-phosphatase activities that play key roles in RNA and DNA repair. T4 Pnk is a homotetramer composed of a C-terminal phosphatase domain and an N-terminal kinase domain. The 2.0 Å crystal structure of the isolated kinase domain highlights a tunnel-like active site through the heart of the enzyme, with an entrance on the 5′ OH acceptor side that can accommodate a single-stranded polynucleotide. The active site is composed of essential side chains that coordinate the β phosphate of the NTP donor and the 3′ phosphate of the 5′ OH acceptor, plus a putative general acid that activates the 5′ OH. So the correct option is 'polynucleotide kinases'.
C. Taq DNA polymerase	Taq polymerase is a thermostable DNA polymerase named after the thermophilic bacterium Thermus aquaticus from which it was originally isolated. Its name is often abbreviated to Taq Pol or simply Taq. It is frequently used in the polymerase chain reaction (PCR), a method for greatly amplifying the number of short segments of DNA. T. aquaticus is a bacterium that lives in hot springs and hydrothermal vents, and Taq polymerase was identified as an enzyme able to withstand the protein-denaturing conditions (high temperature) required during PCR. So the correct option is 'stable above 90${}^o$ C'.
D. Terminal transferases	Terminal transferase (terminal nucleotidyl transferase) a DNA polymerase that is capable of catalyzing the addition of nucleotides to the 3′-END of a single-stranded segment of a DNA molecule. The enzyme does not require a template strand, unlike normal polymerases. For example, the enzyme can be used to add a tail of adenine residues (poly(A) tail) to the 3′ends of a cloning vector and a tail of thymine residues (poly(T) tail) to the 3′-ends of insert DNA When mixed, the adenines will anneal to the thymines by complementary base pairing to generate a recombinant vector for use in gene cloning. So the correct option is 'add a number of nucleotides to 3' end of DNA or RNA'.