Read the passage given below and answer the questions that follow.
DNA fingerprinting involves identifying differences in some specific regions in DNA sequence called repetitive DNA, because in these sequences, a small stretch of DNA is repeated many times. These sequences normally do not code for any proteins, but they form a large portion of the human genome. These sequences show a high degree of polymorphism and form the basis of DNA fingerprinting. Polymorphism (variation at genetic level) arises due to mutations. New mutations may arise in an individual either in somatic cells or in the germ cells (cells that generate gametes in sexually reproducing organisms). If a germ cell mutation does not seriously impair the individual’s ability to have offspring who can transmit the mutation, it can spread to the other members of the population (through sexual reproduction). The technique of DNA fingerprinting was initially developed by Alec Jeffreys. He used a satellite DNA as probe that shows a very high degree of polymorphism. It was called Variable Number of Tandem Repeats (VNTR).
DNA polymorphism is more likely to be observed in non-coding DNA sequences. Which of the following explains this?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Mutations in these sequences may not have any immediate effect on an individual’s reproductive ability.
DNA polymorphism means variation at the genetic level: an inheritable mutation found in a population at high frequency. A mutation can build up in a population only if its carriers survive and reproduce. A change in a gene that codes for a protein often harms the individual, so it is less likely to be passed on. A change in non-coding DNA usually does no such harm.
Option (1) Mutations in these sequences may not have any immediate effect on an individual’s reproductive ability. — Correct. Because the carrier can still reproduce normally, such mutations are passed on and keep accumulating generation after generation. This is why variation is seen more often in non-coding sequences.
Option (2) Mutations do not arise in these sequences in somatic cells. — Incorrect. Mutations can arise in any DNA, in somatic cells or in germ cells. Also, somatic mutations are not inherited at all, so they could not explain polymorphism in a population.
Option (3) These sequences make up less than 2 per cent of the human genome. — Incorrect. It is the protein-coding part that makes up less than 2 per cent of the genome. Repeated, non-coding sequences make up a very large portion of it.
Option (4) These sequences are removed from the DNA by splicing before they are passed on to offspring. — Incorrect. Splicing removes introns from the primary RNA transcript in eukaryotes. It acts on RNA, not on DNA, and it does not change the DNA passed to the next generation.
NCERT Extract:
DNA Fingerprinting – “… In simple terms, if an inheritable mutation is observed in a population at high frequency, it is referred to as DNA polymorphism. The probability of such variation to be observed in non-coding DNA sequence would be higher as mutations in these sequences may not have any immediate effect/impact in an individual’s reproductive ability. These mutations keep on accumulating generation after generation, and form one of the basis of variability/polymorphism.”