ANS:D - DNA topoisomerase

Tightly wound DNA does not permit molecules to gain access to individual genes in order to transcribe copies of them, as required for protein synthesis. To overcome this problem, cells use specialised proteins to unwind the DNA in specific regions when it needs access to it, while keeping the rest of the DNA molecule tightly wound and out of harms way. Once the DNA is uncoiled, the DNA double helix itself needs to be unwound to separate it into two individual strands so the information it contains can be accessed. The proteins that carry out this job are collectively known as DNA topoisomerases.

DNA Topoisomerases, Unravelling DNA Strands.

DNA topoisomerases are ubiquitous enzymes found in all cell types from viruses to man. These enzymes act to regulate DNA supercoiling by catalysing the winding and unwinding of DNA strands. They do this by making an incision that breaks the DNA backbone, so they can then pass the DNA strands through one another, swivelling and relaxing/coiling the DNA before resealing the breaks. DNA topoisomerases can be divided into two groups based on the number of strands that they break.