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Molecular basis of inheritance: NEET MCQs with answers

169 questions on this topic of Genetics and Evolution (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. If Adenine constitutes 30% of the nitrogenous bases in a double-stranded DNA molecule, find the expected percentage of Cytosine:

  1. 30 %
  2. 60 %
  3. 70 %
  4. 20 %
Show answer and explanation

Answer: (D) 20 %

By Chargaff's Rule: Adenine (A) = Thymine (T) = 30%. Thus, A + T = 60%. The remaining bases (G + C) = 100% - 60% = 40%. Since Guanine (G) = Cytosine (C), Cytosine = (100 - 60) / 2 = 50 - 30 = 20%.

Quick tip: In any double-stranded DNA, the sum of one purine and one pyrimidine always equals 50% (A + C = 50%).

medium

2. The coding region of an mRNA, from the start codon AUG up to and including the stop codon, is 555 nucleotides long. The polypeptide translated from it has how many amino acids?

  1. 185
  2. 186
  3. 555
  4. 184
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Answer: (D) 184

555 nucleotides = 185 codons. The stop codon codes for no amino acid, so 185 − 1 = 184 amino acids.

hard

3. If the coding strand of a transcription unit is 5'-ATGCATGCAT-3', the RNA transcribed will be:

  1. 5'-TACGTACGTA-3'
  2. 5'-UACGUACGUA-3'
  3. 3'-AUGCAUGCAU-5'
  4. 5'-AUGCAUGCAU-3'
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Answer: (D) 5'-AUGCAUGCAU-3'

The RNA has the same sequence as the coding strand, with uracil in place of thymine, and the same 5' → 3' polarity.

medium

4. The fact that some amino acids are coded by more than one codon means the genetic code is:

  1. Overlapping
  2. Ambiguous
  3. Degenerate
  4. Non-universal
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Answer: (C) Degenerate

Degeneracy means an amino acid may be coded by more than one codon; the code is otherwise unambiguous and specific.

easy

5. A double-stranded DNA sample contains 34% adenine. According to Chargaff's rules, the percentage of cytosine is:

  1. 34 %
  2. 16 %
  3. 32 %
  4. 66 %
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Answer: (B) 16 %

A = T and G = C, so A + T = 68% and G + C = 32%. Hence cytosine = 16%.

hard

6. In the Meselson–Stahl experiment, E. coli grown in ^15N medium was transferred to ^14N medium. After two generations, the DNA extracted would show:

  1. Only heavy DNA
  2. Only hybrid DNA
  3. Equal amounts of hybrid (intermediate) and light DNA
  4. Only light DNA
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Answer: (C) Equal amounts of hybrid (intermediate) and light DNA

After one generation all DNA is hybrid (^15N/^14N). After the second generation, half is hybrid and half is light (^14N/^14N), consistent with semi-conservative replication.

medium

7. Bacteriophage φX174 has how many nucleotides in its genome?

  1. 3.3 × 10^9
  2. 48,502
  3. 4.6 × 10^6
  4. 5,386
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Answer: (D) 5,386

φX174, a bacteriophage, has 5,386 nucleotides; lambda phage has 48,502 base pairs.

medium

8. During processing of eukaryotic hnRNA, "tailing" refers to:

  1. Joining of exons
  2. Addition of 200-300 adenylate residues at the 3' end
  3. Addition of methyl guanosine triphosphate at the 5' end
  4. Removal of introns
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Answer: (B) Addition of 200-300 adenylate residues at the 3' end

hnRNA undergoes splicing (introns removed, exons joined), capping (methyl guanosine triphosphate added at the 5' end) and tailing (200-300 adenylate residues added at the 3' end).

NCERT Class 12

medium

9. Regions of chromatin that are loosely packed, stain lightly and are transcriptionally active are called:

  1. Nucleosomes
  2. Heterochromatin
  3. Euchromatin
  4. Satellite DNA
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Answer: (C) Euchromatin

Euchromatin is loosely packed and transcriptionally active; heterochromatin is densely packed, stains dark and is inactive.

medium

10. The codon AUG has a dual function: it codes for methionine and also acts as the:

  1. Initiator codon
  2. Stop codon
  3. Anticodon
  4. Promoter
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Answer: (A) Initiator codon

AUG codes for methionine (met) and also acts as the initiator codon. The code is also degenerate, unambiguous, read without punctuation and nearly universal.

NCERT Class 12

medium

11. The permease coded by the y gene of the lac operon:

  1. Hydrolyses lactose into glucose and galactose
  2. Increases the permeability of the cell to β-galactosides
  3. Binds to the operator
  4. Transfers an acetyl group
Show answer and explanation

Answer: (B) Increases the permeability of the cell to β-galactosides

Permease (y gene) increases the permeability of the cell to β-galactosides; β-galactosidase (z gene) hydrolyses lactose into galactose and glucose.

medium

12. Commonly used vectors for cloning DNA fragments in the Human Genome Project were:

  1. Retroviruses and adenoviruses
  2. Ti plasmid and Ri plasmid
  3. pBR322 and lambda phage only
  4. BAC (bacterial artificial chromosome) and YAC (yeast artificial chromosome)
Show answer and explanation

Answer: (D) BAC (bacterial artificial chromosome) and YAC (yeast artificial chromosome)

The genome was broken into fragments and cloned in hosts using BAC and YAC vectors before sequencing.

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