Q4. Briefly describe polymorphism. (NCERT Exercise 14 b)
Polymorphism is variation at the genetic level, arising from mutations. An allelic sequence variation is called a DNA polymorphism when more than one variant at a locus occurs in the human population at a frequency greater than 0.01; it is more likely in non-coding DNA, accumulates over generations, and forms the basis of genetic mapping and DNA fingerprinting (NCERT pages 105–106).
Q5. Put these steps of the original DNA fingerprinting technique in order: hybridisation with the labelled probe; electrophoresis; isolation of DNA; autoradiography; digestion with restriction endonucleases; blotting.
Isolation of DNA, digestion with restriction endonucleases, electrophoresis, blotting, hybridisation with the labelled VNTR probe, autoradiography (NCERT page 106).
Q6. Why do VNTRs give bands of different sizes in different people?
A VNTR is a short sequence repeated tandemly, and the number of copies varies from chromosome to chromosome and from person to person, so the fragments vary in size, from 0.1 to 20 kb (NCERT page 106).
Q7. How has PCR changed DNA fingerprinting?
It has increased the sensitivity of the technique, so that DNA from a single cell is enough for the analysis (NCERT page 106). Beyond the NCERT text: with so little DNA, one of a person’s two alleles at a locus can fail to show (allelic dropout), so such results are read with care (Wikipedia, “DNA profiling”, facts in our words).
Q8. Why are polymorphisms more likely in non-coding DNA?
Mutations in non-coding sequences may have no immediate effect on an individual’s ability to reproduce, so they are passed on and keep accumulating generation after generation (NCERT page 106). Beyond the NCERT text: some non-coding DNA, such as the regulatory sequences that control gene expression, does have functions (Wikipedia, “Non-coding DNA”, facts in our words).