CBSE Class 12 Biology Board Paper 3 — MCQs with Answers

8 solved questions from this paper

Answer and worked explanation shown for each. The remaining 12 are timed and scored when you sign in free.

1. Q1Easy

   The innermost layer of the anther wall, whose cells nourish the developing pollen grains, is the:

   A.epidermis

   B.endothecium

   C.tapetum✓ Correct

   D.middle layer

   Answer: C. tapetum

   Explanation: The anther wall has four layers from outside inwards, epidermis, endothecium, middle layers and tapetum, and it is the innermost tapetum whose dense cytoplasm and often multiple nuclei supply nutrition and pollen wall material to the microspores. The endothecium lies just under the epidermis and helps the anther split open at dehiscence.

2. Q2Easy

   The number of individuals of a species present per unit area at a given time is called:

   A.natality

   B.carrying capacity

   C.biotic potential

   D.population density✓ Correct

   Answer: D. population density

   Explanation: Population density is a size expressed per unit area, and where counting individuals is impractical, as with a banyan tree or with grass, it is stated as percentage cover or as biomass instead. Natality is the rate at which new individuals are added by birth, which is an attribute of change rather than of size.

3. Q3Easy

   The enzyme that catalyses the polymerisation of ribonucleotides during transcription in bacteria is:

   A.DNA polymerase III

   B.DNA ligase

   C.DNA-dependent RNA polymerase✓ Correct

   D.reverse transcriptase

   Answer: C. DNA-dependent RNA polymerase

   Explanation: A single DNA-dependent RNA polymerase catalyses initiation, elongation and termination of all bacterial RNAs, associating with the σ factor to initiate and with the ρ factor to terminate. Reverse transcriptase does the opposite job, building DNA on an RNA template inside retroviruses.

4. Q4Medium

   A patient shows a steadily falling count of helper T-lymphocytes and repeatedly develops infections that a healthy person shrugs off. The causative agent multiplies inside the patient's cells after first copying its RNA into DNA. The agent is:

   A.HIV✓ Correct

   B.the hepatitis B virus

   C.Plasmodium vivax

   D.Salmonella typhi

   Answer: A. HIV

   Explanation: HIV is a retrovirus that uses reverse transcriptase to copy its RNA genome into DNA, which then integrates into the host chromosome, and it progressively destroys CD4 helper T-cells so that opportunistic infections take hold. Plasmodium and Salmonella are not viruses at all, and hepatitis B damages the liver without depleting helper T-cells.

5. Q5Medium

   Which contraceptive method works mainly by making implantation impossible rather than by preventing fertilisation?

   A.condom

   B.copper-releasing intrauterine device✓ Correct

   C.vasectomy

   D.diaphragm

   Answer: B. copper-releasing intrauterine device

   Explanation: Copper IUDs release Cu²⁺ ions that suppress sperm motility and, just as importantly, alter the endometrium so that a blastocyst cannot implant in it. Condoms and diaphragms are barriers that stop the gametes from meeting, and vasectomy prevents sperm from entering the semen at all.

6. Q6Medium

   Golden rice was engineered chiefly to combat a widespread nutritional deficiency. Its grain accumulates:

   A.iron in the endosperm

   B.lysine in the endosperm

   C.vitamin B12 in the aleurone layer

   D.β-carotene, a precursor of vitamin A, in the endosperm✓ Correct

   Answer: D. β-carotene, a precursor of vitamin A, in the endosperm

   Explanation: Genes of the carotenoid pathway were introduced so that the endosperm accumulates β-carotene, which the human body converts into vitamin A, addressing the deficiency that causes childhood blindness. The golden yellow colour of the grain comes from that carotenoid, which is why the variety got its name.

7. Q7Hard

   A segment of mRNA reads 5′-AUG GCU UUC UAA-3′. The number of amino acids in the polypeptide translated from it is:

   A.3✓ Correct

   B.4

   C.2

   D.12

   Answer: A. 3

   Explanation: The four codons are AUG for methionine, GCU for alanine, UUC for phenylalanine and UAA, which is a stop codon read by a release factor and specifying no amino acid at all. The initiator AUG does contribute its methionine to the chain, so three residues are joined, not two, and counting bases instead of codons gives the meaningless figure of twelve.

8. Q8Hard

   In a pedigree, a phenotypically normal couple have a son affected by an autosomal recessive disorder and a daughter who is phenotypically normal. The probability that the daughter is a carrier of the disorder is:

   A.1/4

   B.1/2

   C.2/3✓ Correct

   D.1

   Answer: C. 2/3

   Explanation: The affected son proves both parents to be heterozygous, so their children are expected as 1 AA : 2 Aa : 1 aa. The daughter is known to be unaffected, which removes the aa class and leaves 1 AA : 2 Aa, so two of the three remaining possibilities are carriers and the probability is 2/3; an answer of 1/2 ignores the information that she is already known to be normal.

More Class 12 Biology board papers

• Board Paper 1 — 20 questions
• Board Paper 2 — 20 questions
• Board Paper 3 — you are here
• Board Paper 4 — 20 questions
• Board Paper 5 — 20 questions

Prefer chapter-by-chapter revision first? Class 12 Biology notes & chapter MCQs →

Questions students ask about this paper