Question

Its about circular macromolecules

A protein or protein complex fully encircling DNA creates a topological challenge. How are the proteins overcoming this challenge when they become bound to DNA and how are they able to dissociate from DNA. Use one of them as an example to explain such a mechanism.

And

Another example of circular molecules is cccDNA found in many bacteria. When such closed DNAs are replicated, it may happen that the circles are trapped as rings in a chain because one cccDNA encircles the strands of the other cccDNA. Explain briefly how these “catenated” DNA molecules become dis-entangled using a specific type of enzymes and how these enzymes work?

Answer 1

Protein DNA bonds can be broken by unbound protiens floating around in rhe cell. Strands of DNA have specific sites and one type of DNA-binding proteins, called transcription factors (TF), are key players in the transcription of genetic information from DNA to messenger RNA (mRNA) to produce new proteins or other types of RNA. TF proteins control the biological processes in living cells by binding and unbinding to DNA. The examples of the same are nucleases and polymerases that cleave the DNA molecules and the histones which are involved in chromosome packaging and trancription in cell nucleus.

The cccDNA which is present in the hepatitis B virus replicates through a replication pathway that involves retrotranscription of a cccDNA transcript, termed pregenomic RNA, into progeny rcDNA genomes. The conversion of rcDNA into cccDNA requires the removal of a covalently-linked copy of the polymerase from the 5' end of one of the DNA strands, and this deproteinization step generates a DNA intermediate, the deproteinized rcDNA (DP-rcDNA), as precursor for cccDNA formation. It is also found that the rcDNA deproteinization is a trigger signal for transportation of HBV nucleocapsid containing mature viral DNA into nucleus, where the rcDNA to cccDNA conversion takes place.