Question

2. How can simple transposition lead to copy number increase?

Briefly explain the role of reverse transcriptase and integrase in retrotransposition.

What damage does UV do to DNA?

UV damage to DNA is repaired by the Nucleotide Excision Repair Pathway. Briefly explain how UvrB and UvrC repair UV damage in E.coli.

Answer 1

1.In replicative transposition (copy and paste), transposons create a second copy of itself. In this process, the original transposon remains in place, while its copy is inserted at the new position. So, this leads to an increase in copy number of transposons in the genome.

2.Transposable elements that transpose through an RNA intermediate, by utilising a reverse transcriptase and integrase are called Retrotransposons.

Reverse transcriptase is used for making cDNA from RNA intermediate formed.

Integrase is used for inserting this transposon to other DNA sequence.

3. UV is a physical mutagen and damages DNA by causing mutations, i.e. dimerization of adjacent pyrimidine bases., especially if both are thymine.

UV radiation can also convert cytosine to cytosine hydrate , in presence of water.

4. UV induced damage is repaired by nucleotide excision repair system, in which damaged areas of DNA(oligonucleotides) are removed by breaking phosphodiester bonds on either side of the lesions.

5. UVR B and C are part of UVR system of excision repair in E. coli, which works as follows:-

Uvr A and UVR B attach to the DNA at the damaged site. Uvr A recognises the damage.

Uvr A departs , then Uvr C binds, thus forming Uvr BC dimer.

Uvr BC dimer cleaves 8th phosphodiester bond on 5' side and 4th bond on 3' side.

Uvr D is a helicase that unwinds the DNA in between the cuts.

The resulting gap is filled by DNA polymerase 1 and sealed by ligase.