Question

Examine the image of a set of DNA sequences.

What happens that influences viral DNA variation?

n+1 5 3' Backward slippage 5 n+1 Mutation 3' 5' 3' n+1 5 Normal t 3' 5 2012 FLVS

completion of the viral DNA

creation of an extra copy of the viral DNA

deletion of the viral DNA

point mutation within the viral DNA

Answer 1

The template strand (green) is the one, which is in 3’ to 5’ direction. The newly synthesized daughter DNA strand (Yellow) is in 5’ to 3’ direction in first round of replication. Backward slippage occurs in the 5’ to 3’ direction, where repeat nucleotide CAG slips. Hence, the yellow daughter strand has three nucleotides more than the template strand. The presence of nearby duplicate bases stabilizes the slippage. Thus, the replication proceeds. In the next round of replication, the daughter strand from first replication will replicate and form DNA that has one set of extra nucleotide (CAG), resulting in seven CAG repeats. The original parental green template strand will form a normal copy of DNA with six CAG repeats. Hence, there is a new mutant strand formed that has extra copy of CAG repeat in the viral DNA. Thus, there is creation of extra copy of viral DNA. Backward slippage requires region-containing repeats. Hence, this example is not of point mutation.

If the backward slippage occurs in template strand, deletion of viral DNA occurs. It is also not completion of viral DNA as mutated DNA and normal DNA is obtained.

Right option is creation of an extra copy of the viral DNA.