Question

The smallest part of DNA molecule that can be changed by point mutation is :

• A. Oligonucleotide
• B. Codon
• C. Gene
• D. Nucleotide

Hint:

The definition of a point mutation contains the answer itself: "a change in a single base pair." Since a base pair is an integral part of a single nucleotide unit, the nucleotide is the smallest unit of mutation.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks for the absolute smallest structural unit within a double-stranded DNA molecule that can undergo a modification via a point mutation.

Step 2: Key Formula or Approach:
A point mutation is defined as a genetic mutation where a single base pair in the DNA sequence is altered, substituted, inserted, or deleted.

Step 3: Detailed Explanation:
Let us look at the structural hierarchy of DNA from largest to smallest among the choices:
* Gene (Option C): A large segment of DNA containing hundreds or thousands of base pairs that encodes a functional polypeptide or RNA molecule.
* Oligonucleotide (Option A): A short polymer chain composed of several linked nucleotide units (typically 10 to 50 base pairs).
* Codon (Option B): A specific sequence of three consecutive nucleotides in mRNA (or corresponding triplets in DNA) that specifies a single amino acid during translation.
* Nucleotide (Option D): The fundamental chemical monomer building block of nucleic acids, consisting of a nitrogenous base, a pentose sugar, and a phosphate group.
Since a point mutation involves a chemical alteration to a single nitrogenous base pair (for example, the transition of an Adenine-Thymine pair to a Guanine-Cytosine pair, as seen in Sickle Cell Anemia where a single base substitution occurs in the $\beta$-globin gene), the absolute limit of resolution for such a mutation is a single nucleotide site. This site is also referred to in molecular genetics as a muton—the smallest unit of genetic material mutable by a point mutation.

Step 4: Final Answer:
The smallest unit of DNA altered by a point mutation is a single nucleotide, which matches option (D).