A mutation is a change in the DNA of an organism. The change may or may not result 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 RNA cause point mutation

Choose the correct answer from the options given below:

C and D

The correct answer is Option (3) → C and D

A point mutation is a change in a single nucleotide in the DNA sequence. This can occur due to:

• Substitution (one base is swapped for another)
• Insertion or deletion of a single nucleotide (if only one base is added or removed, it can still be considered a point mutation)

Analyzing Each Statement:

Statement A: "It is a change of one base to a different base in tRNA"

• Why it's incorrect:
• Point mutations happen in DNA, not specifically in tRNA.
• While point mutations in DNA can later affect RNA, this is not a mutation in tRNA itself. Thus, this statement misrepresents where point mutations occur.

Statement B: "The insertion of additional base into DNA causes point mutation"

• Why it's incorrect:
• Insertion mutations involve adding one or more bases into the DNA sequence.
• If only one base is inserted, it may be classified as a point mutation. However, in general, insertion mutations of more than one nucleotide usually lead to frameshift mutations, not just point mutations.
• Thus, this statement is misleading because it implies that any insertion, regardless of length, would be a point mutation.

Statement C: "The deletion of a base from DNA causes point mutation"

• Why it's correct:
• The deletion of a single base from DNA is a form of point mutation.
• This type of mutation also has the potential to cause a frameshift effect if it occurs within a gene coding region, as it shifts the reading frame.
• However, the initial change is a point mutation (single base change).

Statement D: "Substitution mutation by incorrect copying of RNA causes point mutation"

• Why it's correct:
• Although DNA is the original site for point mutations, incorrect substitution during replication (from incorrect copying of bases in DNA) can lead to point mutations.
• It’s not that RNA itself is mutated; rather, the mutation in DNA could later be transcribed into RNA with the wrong nucleotide.
• Therefore, a substitution mutation at the DNA level (even if indirectly due to RNA transcription error) is a classic form of point mutation.