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NEET Practice: Molecular Basis of Inheritance

Previous-year-pattern MCQs on DNA structure, replication, transcription, the genetic code, translation and gene regulation (Lac operon) — each with a full solution.

ℹ️ Original questions modelled on NEET's testing pattern for this high-yield chapter — not a copy of any single official paper. Read, then take the timed quiz.

Solved Questions

Q1 · Chargaff's rule
According to Chargaff's rule, in a double-stranded DNA molecule:
Answer: A. Chargaff's equivalence rule states that in dsDNA the amount of adenine equals thymine, and guanine equals cytosine (A = T, G = C), because of specific base pairing (A–T with 2 H-bonds, G–C with 3 H-bonds).
Q2 · DNA double helix
In the Watson–Crick model of DNA, the two strands are:
Answer: B. The two strands run in opposite directions — one 5'→3', the other 3'→5' (antiparallel) — and their bases pair specifically (A–T, G–C), making them complementary. One helical turn is 3.4 nm long and contains about 10 base pairs.
Q3 · Semi-conservative replication
The Meselson–Stahl experiment on E. coli provided evidence that DNA replication is:
Answer: B. Meselson and Stahl (using heavy ¹⁵N and light ¹⁴N isotopes) showed each daughter DNA molecule has one parental (old) strand and one newly synthesised strand — proving semi-conservative replication.
Q4 · DNA polymerase direction
During replication, DNA polymerase synthesises the new strand in which direction?
Answer: A. DNA polymerase can add nucleotides only to the free 3'-OH end, so synthesis is always 5'→3'. This is why the lagging strand is made discontinuously as short Okazaki fragments.
Q5 · Transcription unit
In a transcription unit, the strand that has the same sequence as the mRNA (except T→U) is called the:
Answer: B. The coding (sense) strand has the same sequence as the mRNA (with U in place of T). The template (antisense) strand (3'→5') is the one actually read by RNA polymerase to build the mRNA.
Q6 · Genetic code
The genetic code is degenerate. This means that:
Answer: B. Degeneracy means one amino acid can be specified by several codons (e.g. leucine has six codons). Of the 64 codons, 61 code for amino acids and 3 (UAA, UAG, UGA) are stop codons. The code is also unambiguous, non-overlapping and nearly universal.
Q7 · Start codon
The initiation (start) codon in the genetic code is:
Answer: A. AUG is the start codon; it also codes for methionine. UAA, UAG and UGA are the three stop (termination) codons and do not code for any amino acid.
Q8 · tRNA
The adapter molecule that reads the codon on mRNA and brings the correct amino acid during translation is:
Answer: C. tRNA (transfer RNA) is the "adapter" — its anticodon base-pairs with the mRNA codon while it carries the corresponding amino acid to the ribosome. rRNA forms the ribosome and has ribozyme (peptidyl transferase) activity.
Q9 · Lac operon
In the Lac operon of E. coli, the inducer that switches the operon ON is:
Answer: B. Lactose acts as the inducer: it binds the repressor protein, inactivating it so it can no longer bind the operator. RNA polymerase can then transcribe the structural genes (z, y, a). This is an example of negative regulation.
Q10 · Human Genome Project
Approximately how many genes were estimated in the human genome by the Human Genome Project?
Answer: B. The Human Genome Project estimated roughly 20,000–25,000 genes across ~3.1 billion base pairs. Chromosome 1 has the most genes; the Y chromosome the fewest. Less than 2% of the genome actually codes for proteins.

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