Dna Structure

Step 1: Understanding the proteins involved in branch migration.
Rec-A is a protein involved in homologous recombination, promoting the strand invasion process. However, Ruv-A and Ruv-B proteins are involved in branch migration during recombination, enhancing the rate of migration compared to Rec-A.
Step 2: Analyzing the options.
(1) Rec-B: This is incorrect as Rec-B is part of the RecBCD complex that initiates recombination but is not as effective in branch migration as Ruv-A and Ruv-B.
(2) Rec-C: Rec-C is also involved in DNA recombination but does not enhance branch migration more than Rec-A.
(3) Rec-D: Rec-D is another subunit of RecBCD, but it does not promote branch migration more than Rec-A.
(4) Ruv-A and Ruv-B: Correct — Ruv-A and Ruv-B are directly involved in branch migration during homologous recombination and promote migration at higher rates than Rec-A.
Step 3: Conclusion.
The correct answer is (4) Ruv-A and Ruv-B, as they promote branch migration more effectively than Rec-A.

Step 1: Understanding the different forms of DNA.
DNA can exist in several forms, including A-DNA, B-DNA, and Z-DNA. Z-DNA is a left-handed helix, unlike the more common B-DNA which is right-handed.
Step 2: Analyzing the options.
(1) m-RNA: mRNA is a single-stranded RNA molecule and does not exhibit a left-handed helix structure.
(2) t-RNA: tRNA also has a distinct three-dimensional structure but does not form a left-handed helix.
(3) A-DNA: A-DNA is a right-handed helix.
(4) Z-DNA: Correct — Z-DNA is a left-handed helix, characterized by its zig-zag backbone structure.
Step 3: Conclusion.
The correct answer is (4) Z-DNA, as it is the left-handed form of DNA.

Step 1: Understanding Taq polymerase.
Taq polymerase is a thermostable DNA polymerase used in the polymerase chain reaction (PCR) to amplify DNA. It has several key activities: 5'–3' polymerase activity, which allows it to synthesize DNA, and 5'–3' exonuclease activity, which allows it to degrade DNA. However, Taq polymerase lacks 3'–5' exonuclease activity, which is present in some other polymerases for proofreading.
Step 2: Analyzing the options.
(1) 5'–3' polymerase: This is present in Taq polymerase. It is responsible for adding nucleotides during DNA synthesis.
(2) 5'–3' exonuclease: This is present in Taq polymerase. It allows for the degradation of DNA.
(3) 3'–5' exonuclease: Correct — Taq polymerase lacks 3'–5' exonuclease activity, which is responsible for proofreading during DNA synthesis.
(4) Both 5'–3' polymerase and 5'–3' exonuclease: This is incorrect. Both of these activities are present in Taq polymerase.
Step 3: Conclusion.
The correct answer is (3) 3'–5' exonuclease, as Taq polymerase does not have proofreading activity.