A restriction endonuclease cuts double-stranded DNA at one particular site because it recognises:

a specific palindromic nucleotide sequence

any sequence that happens to be six bases long

the promoter of the nearest gene

Down's syndrome results from the presence of an additional copy of chromosome 21. This condition is described as:

trisomy, an aneuploidy caused by the failure of chromosomes to separate during meiosis

polyploidy, caused by the failure of cytokinesis

a point mutation in a single gene on chromosome 21

a deletion of part of chromosome 21

In the human male reproductive system, sperm pass along the ducts in the sequence:

rete testis → vasa efferentia → epididymis → vas deferens → urethra

rete testis → epididymis → vas deferens → vasa efferentia → urethra

vasa efferentia → vas deferens → rete testis → epididymis → urethra

epididymis → rete testis → vasa efferentia → vas deferens → urethra

A eukaryotic gene contains 3 exons and 2 introns. The mature mRNA exported from the nucleus differs from the primary transcript in that:

the introns have been spliced out and the exons joined, and the transcript has been capped at the 5′ end and tailed at the 3′ end

the exons have been removed and the introns joined together

methyl guanosine has been added at the 3′ end and the poly-A tail at the 5′ end

the transcript has been translated before leaving the nucleus

A red-eyed female Drosophila heterozygous for the X-linked white-eye allele is crossed with a white-eyed male. In the progeny:

half the females and half the males are white-eyed

all the females are red-eyed and all the males are white-eyed

all the females are white-eyed and all the males are red-eyed

🧬

CBSE Class 12 Biology Board Paper 5 — 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 5 — 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
A restriction endonuclease cuts double-stranded DNA at one particular site because it recognises:
A.any sequence that happens to be six bases long
B.a specific palindromic nucleotide sequence✓ Correct
C.the origin of replication
D.the promoter of the nearest gene
Answer: B. a specific palindromic nucleotide sequence
Explanation: Restriction endonucleases bind a specific palindromic sequence, one that reads the same on both strands when each is read in the 5′→3′ direction, and cut a little away from the centre so that single-stranded sticky ends are left. EcoRI, for example, cuts at G↓AATTC, and the identical overhangs it leaves on any DNA are what allow two different fragments to be joined.
Q2Easy
A lichen, in which a fungus and an alga live together with both partners gaining from the association, illustrates:
A.commensalism
B.predation
C.mutualism✓ Correct
D.competition
Answer: C. mutualism
Explanation: The alga photosynthesises and feeds the fungus, while the fungus provides anchorage, water and minerals, so the interaction is beneficial to both partners. In commensalism only one partner would benefit while the other remained wholly unaffected, which is clearly not the case in a lichen.
Q3Easy
The idea that life reached the Earth from elsewhere in the universe in the form of spores or seeds is known as:
A.abiogenesis
B.biogenesis
C.panspermia✓ Correct
D.spontaneous generation
Answer: C. panspermia
Explanation: Panspermia holds that units of life travelled through space and seeded the Earth, an idea some early Greek thinkers held and later writers restated as spores arriving on meteorites. Spontaneous generation, the belief that life arises from decaying matter, is the theory Louis Pasteur disproved with his swan-necked flasks.
Q4Medium
In an angiosperm whose diploid chromosome number is 24, the number of chromosomes in a cell of the endosperm is:
A.12
B.24
C.48
D.36✓ Correct
Answer: D. 36
Explanation: The primary endosperm nucleus arises by triple fusion of one male gamete, which carries n = 12, with the two polar nuclei, which contribute n + n, so the endosperm is triploid with 3 × 12 = 36 chromosomes. The zygote, formed by syngamy of two gametes only, stays diploid at 24, and the integuments and nucellus remain maternal diploid tissue.
Q5Medium
Down's syndrome results from the presence of an additional copy of chromosome 21. This condition is described as:
A.trisomy, an aneuploidy caused by the failure of chromosomes to separate during meiosis✓ Correct
B.polyploidy, caused by the failure of cytokinesis
C.a point mutation in a single gene on chromosome 21
D.a deletion of part of chromosome 21
Answer: A. trisomy, an aneuploidy caused by the failure of chromosomes to separate during meiosis
Explanation: Non-disjunction leaves one gamete carrying two copies of chromosome 21, so the zygote receives three and the total count becomes 47, a gain of one chromosome rather than of a whole set, which is aneuploidy. Polyploidy means additional complete sets of chromosomes and is common in plants, so it is not the mechanism here.
Q6Medium
In the human male reproductive system, sperm pass along the ducts in the sequence:
A.rete testis → epididymis → vas deferens → vasa efferentia → urethra
B.vasa efferentia → vas deferens → rete testis → epididymis → urethra
C.epididymis → rete testis → vasa efferentia → vas deferens → urethra
D.rete testis → vasa efferentia → epididymis → vas deferens → urethra✓ Correct
Answer: D. rete testis → vasa efferentia → epididymis → vas deferens → urethra
Explanation: Sperm leave the seminiferous tubules into the rete testis, travel through the vasa efferentia into the epididymis where they mature and are stored, and then move along the vas deferens, which joins the duct of the seminal vesicle to form the ejaculatory duct opening into the urethra. Placing the epididymis before the rete testis reverses the two ends of the pathway.
Q7Hard
A eukaryotic gene contains 3 exons and 2 introns. The mature mRNA exported from the nucleus differs from the primary transcript in that:
A.the exons have been removed and the introns joined together
B.methyl guanosine has been added at the 3′ end and the poly-A tail at the 5′ end
C.the transcript has been translated before leaving the nucleus
D.the introns have been spliced out and the exons joined, and the transcript has been capped at the 5′ end and tailed at the 3′ end✓ Correct
Answer: D. the introns have been spliced out and the exons joined, and the transcript has been capped at the 5′ end and tailed at the 3′ end
Explanation: Splicing removes the non-coding introns and joins the coding exons in their original order, after which a methyl guanosine triphosphate cap is added at the 5′ end and a poly-adenylate tail at the 3′ end. Interchanging the cap and the tail, or removing exons in place of introns, are the two commonest errors in describing hnRNA processing.
Q8Hard
A red-eyed female Drosophila heterozygous for the X-linked white-eye allele is crossed with a white-eyed male. In the progeny:
A.all the females are red-eyed and all the males are white-eyed
B.all the progeny are red-eyed
C.half the females and half the males are white-eyed✓ Correct
D.all the females are white-eyed and all the males are red-eyed
Answer: C. half the females and half the males are white-eyed
Explanation: The mother passes either her wild-type X or her white-carrying X with equal chance, while the father gives his white-bearing X to every daughter and his Y to every son. Daughters are therefore half heterozygous red and half homozygous white, and sons are half red and half white depending on which X they received from the mother, so half of each sex is white-eyed.

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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.

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