Some viruses, such as SV40, contain closed circular DNAs carrying nucleosomes. I
ID: 3166889 • Letter: S
Question
Some viruses, such as SV40, contain closed circular DNAs carrying nucleosomes. If the SV40 viral chromosome is treated with topoisomerase, and the histone is then removed, it is still supercoiled. If histones are removed before topoisomerase treatment, however, the DNA is relaxed.
Part A
Explain.
Explain.
1.
Recognition and activity sites of topoisomerases are hidden by the binding of DNA nucleosomes. With the DNA release of the histones this sequences open and relax occurs.
2.
Topoisomerase turns whole sections of DNA-linkers and nucleosomes. With the removal of histones windings that was hidden under compactization are revealed and mimic the original twists.
3.
The DNA makes superhelical turns about each nucleosome. These turns are constrained by binding to histone octamers and cannot be unwound by topoisomerase action.
4
Recognition and activity sites of topoisomerases are hidden by the binding of DNA nucleosomes. With the DNA release of the histones this sequences open and relax occurs.
2.
Topoisomerase turns whole sections of DNA-linkers and nucleosomes. With the removal of histones windings that was hidden under compactization are revealed and mimic the original twists.
3.
The DNA makes superhelical turns about each nucleosome. These turns are constrained by binding to histone octamers and cannot be unwound by topoisomerase action.
4
Gaps produced by topoisomerase cannot move on DNA molecule, stopping in places of binding with nucleosomes, and are ligated at that position thereby not remove the supercoiling.Explanation / Answer
Answer: Option 3 is correct.
Explanation:
Topoisomerases relax tension generated in the DNA duplex replication or transcription machinery by cleaving one or two DNA strands and religating them.
In nucleosomes, DNA (146 bp) is wrapped around histone octamers. These turns are constrained by binding to histone octamers and cannot be unwound by topoisomerase action.
During replication and transcription, nucleosomes are displaced by DNA/RNA polymerases. Hence topoisomerase can release tension by cleaving the upstream DNA.
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