CBSE Class 12 Biology Board Paper 3 — MCQs with Answers
Original paper · CBSE patternWritten by our subject team, not a reprint of an official sample paper.
- Questions
- 20
- Time
- 30 min
- Marking
- +1, no negative marking
- Mix
- 4 Easy · 8 Medium · 8 Hard
Board Paper 3 — CBSE Class 12 Biology is a full 20-question objective paper for Class 12 Biology, written in the CBSE board pattern and marked the way the board marks: +1 for a correct answer and 0 for a wrong one — no negative marking, exactly like the real board paper. It follows Section A of a real CBSE paper: Q1–Q16 are single-correct MCQs, Q17–Q18 are case-based questions built on a short source, and Q19–Q20 are assertion–reason items using the board's own four option strings.
The difficulty sits deliberately a notch above the real board — 4 Easy · 8 Medium · 8 Hard — because a paper you can clear comfortably tells you nothing on exam day. You get 30 minutes — about 90 seconds a question, the pace the real paper demands. 8 of the questions are solved in full below, with the correct option and a worked explanation for each; the remaining 12 are timed and scored when you sign in free.
Sit it under exam conditions rather than open-book. The score you get on a timed full-length paper is the only honest signal of whether your Biology revision is holding together across chapters — which is precisely what the board tests and what chapter-wise practice cannot tell you.
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.
- Q1Easy
The innermost layer of the anther wall, whose cells nourish the developing pollen grains, is the:
A.epidermisB.endotheciumC.tapetum✓ CorrectD.middle layerAnswer: 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.
- Q2Easy
The number of individuals of a species present per unit area at a given time is called:
A.natalityB.carrying capacityC.biotic potentialD.population density✓ CorrectAnswer: 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.
- Q3Easy
The enzyme that catalyses the polymerisation of ribonucleotides during transcription in bacteria is:
A.DNA polymerase IIIB.DNA ligaseC.DNA-dependent RNA polymerase✓ CorrectD.reverse transcriptaseAnswer: 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.
- 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✓ CorrectB.the hepatitis B virusC.Plasmodium vivaxD.Salmonella typhiAnswer: 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.
- Q5Medium
Which contraceptive method works mainly by making implantation impossible rather than by preventing fertilisation?
A.condomB.copper-releasing intrauterine device✓ CorrectC.vasectomyD.diaphragmAnswer: 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.
- Q6Medium
Golden rice was engineered chiefly to combat a widespread nutritional deficiency. Its grain accumulates:
A.iron in the endospermB.lysine in the endospermC.vitamin B12 in the aleurone layerD.β-carotene, a precursor of vitamin A, in the endosperm✓ CorrectAnswer: 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.
- 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✓ CorrectB.4C.2D.12Answer: 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.
- 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.1/4B.1/2C.2/3✓ CorrectD.1Answer: 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.
Sit the full paper — free
All 20 questions on a 30-minute timer, scored automatically, with a worked solution for every one — including the 12 not shown above. Sign in with Google — no card needed.
Attempt this paper free →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
Is this an official CBSE sample paper?
No — and that is deliberate. This is an original paper written in the CBSE board pattern by our subject team, not a copy of CBSE's official Sample Question Paper. It means you get fresh questions you have not already seen on a dozen other sites, with instant scoring and worked solutions rather than a PDF.
How is this Class 12 Biology paper marked?
+1 for a correct answer and 0 for a wrong one — no negative marking, exactly like the real board paper. So attempt every question — there is no penalty for a wrong answer on a board paper, and leaving a question blank can only cost you.
How long should this paper take?
30 minutes for 20 questions, roughly 90 seconds each. The timer runs whether or not you are watching it, which is the point: board marks are lost to pacing at least as often as to gaps in knowledge.
Do I need to pay to attempt it?
No. Sign in free with Google and the full paper opens with a live timer, automatic scoring and a worked solution for every question — including the ones not shown on this page.