Question

construct a model that would explain how transcriptionally active DNA is repaired preferentially over transcriptionally silent DNA.

Suppose you were to compare autoradiographs of two cells that had been exposed to Thymine, one that was engaged in DNA replication (S phase) and another that was not. How would you expect autoradiographs of these cells to differ? .

Answer 1

When a DNA is transcriptionally active, it produces proteins when the mRNA reaches the cytoplasm and ribosomes. Errors in the mRNA will result in errors in protein synthesised. This will cause changes in the phenotypic expression of the gene which may be harful. So to prevent this harm caused to the organism, any DNA which is transcriptionally active will get repared immediately as the replication is going on. There are different DNA polymerases which will are involved in DNA repair. Errors do happen when the replication of the DNA is occurring. These errors are corrected as and when they come up.

TRanscriptionally inactive DNA also may under go errors during replication. They are also corrected during the DNA repair work But preferrence is for transcriptionally active DNA. DNA which is transcriptionally inactive, does not produce any proteins or does not regulate any protein synthesis. Errors in these parts of the DNA does not affect the phenotype of the organism. That is why such errors remain unnoticed and unrectified.

2. When a cell is undergoing replication, Thymine will pair wth Adenine instead of the original DNA nucleotide. If it is transversion, guanine may have got replaced by thymine. In such a case, instead of Cytosine, there is adenide because of the Thymine replacement in one of the strands. So this is the change we expect when a cell is replicating.

A cell which is not replicating, will have thymine and cytosine in the opposite strand.