A mutation is a change in the DNA of an organism. The change may or may not result in a change to the physical characteristics (or phenotype) of the organism. Mutations that occur in a gamete (sperm or eggs) can be passed to the next generation, whereas mutations that occur in body (or 'somatic') cells may be harmless or may result in a disease such as cancer. Mutations can result in DNA being copied incorrectly. Mutagens can also cause mutations.
A point mutation is a change in one base in the genetic code. The change of one base to a different base (often caused by the DNA being copied incorrectly)-this is called a substitution mutation.
Identify the statements which are true with respect to point mutation:
A. It is a change of one base to a different base in tRNA
B. The insertion of additional base into DNA cause point mutation
C. The deletion of a base from DNA cause point mutation
D. Substitution mutation by incorrect copying of DNA cause point mutation
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → C and D
A point mutation is a change in a single base of DNA. NCERT gives the change of one base to another (substitution, as in sickle-cell anaemia) as the example; the insertion or deletion of bases causes frame-shift mutations.
A. It is a change of one base to a different base in tRNA — False. The change is in DNA, not in tRNA.
B. The insertion of additional base into DNA cause point mutation — False. Insertion of a base shifts the reading frame (frame-shift mutation).
C. The deletion of a base from DNA cause point mutation — True. A deletion alters a single base position in DNA; it is counted here with point changes, although it also shifts the reading frame.
D. Substitution mutation by incorrect copying of DNA cause point mutation — True. The passage gives substitution caused by incorrect copying of DNA as a point mutation.
Note: NCERT classes insertions and deletions as frame-shift mutations; statement C is accepted here because it describes a change at a single base of DNA.
NCERT Extract:
Mutation – “Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism. In addition to recombination, mutation is another phenomenon that leads to variation in DNA. As you will learn in Chapter 5, one DNA helix runs continuously from one end to the other in each chromatid, in a highly supercoiled form. Therefore loss (deletions) or gain (insertion/duplication) of a segment of DNA, result in alteration in chromosomes. Since genes are known to be located on chromosomes, alteration in chromosomes results in abnormalities or aberrations. Chromosomal aberrations are commonly observed in cancer cells. In addition to the above, mutation also arise due to change in a single base pair of DNA. This is known as point mutation. A classical example of such a mutation is sickle cell anemia. Deletions and insertions of base pairs of DNA, causes frame-shift mutations (see Chapter 5).”