Chapter 21, Problem 11P

Interpretation Introduction

Interpretation:

The following situation should be explained.

Some viruses such as SV $40$, contains closed circular DNAs carrying nucleosomes. If the SV $40$ viral chromosomes are treated with topoisomerase and histone is then removed, it still remains supercoiled. If histones are removed before the topoisomerase treatment the DNA is relaxed.

Concept introduction:

Topoisomerases enzymes participate in the over winding or under winding of the DNA. During the replication of the DNA and transcription, the DNA becomes overwound ahead of the replication fork. If left in an intense torsional strain it would stop the action of the DNA or the RNA polymerases involved in the process to continue down the strand of the DNA.

To prevent and rectify the topological problems caused due to the double helix, topoisomerase binds to the DNA and cuts the phosphate backbone one or two strands of the DNA. This break in between allows the DNA to get unwound and the DNA backbone is resealed again at the end of the process. Topoisomerases are isomerases which act on the DNA topology.

Histones belong to a family of basic proteins which associate with the DNA in the nucleus and help to condense it into a chromatin. They are basic proteins and having positive charges that allows them to get associated with DNA. DNA and histones are packed together to become nucleosomes and forms a package called chromatin. The two chromatins make a chromosome.