26 free Biology MCQs written in the Class 12 Boards exam pattern, each with a step-by-step solution. Tap an option for instant feedback — no sign-up, no ads.
Revise with smart repetition →✍️ These are original practice questions written in the Class 12 Boards pattern for the Class 12 board exams (CBSE pattern) — not questions from any official past paper.
Q1. The structural and functional unit of the kidney is the:
B. Nephron — The nephron is the functional unit of the kidney; each kidney has about a million nephrons that filter blood and form urine.
Q2. In humans, fertilisation normally takes place in the:
C. Fallopian tube (oviduct) — Fertilisation of the ovum by the sperm occurs in the ampulla of the fallopian tube (oviduct); the embryo then implants in the uterus.
Q3. Who proposed the double-helix model of DNA?
A. Watson and Crick — James Watson and Francis Crick proposed the double-helix structure of DNA in 1953, using X-ray data from Rosalind Franklin.
Q4. Mendel's Law of Segregation states that:
B. Alleles of a gene separate during gamete formation — The Law of Segregation: the two alleles of a gene separate during gamete formation, so each gamete carries only one allele.
Q5. Which type of cell division produces gametes and halves the chromosome number?
B. Meiosis — Meiosis produces four haploid gametes from one diploid cell, halving the chromosome number; mitosis keeps the number the same.
Q6. Which hormone lowers blood glucose levels?
B. Insulin — Insulin, secreted by the β-cells of the pancreas, lowers blood glucose. Glucagon does the opposite (raises it).
Q7. The theory of natural selection was proposed by:
B. Charles Darwin — Charles Darwin proposed evolution by natural selection in 'On the Origin of Species' (1859).
Q8. A test cross is a cross between an organism with a dominant phenotype and one that is:
B. Homozygous recessive — A test cross pairs the unknown genotype with a homozygous recessive individual to reveal whether it is homozygous or heterozygous.
Q9. The largest gland in the human body is the:
B. Liver — The liver is the largest gland (and largest internal organ) in the human body.
Q10. In a food chain, green plants are called:
A. Producers — Green plants make their own food by photosynthesis and form the base of the food chain, so they are producers (autotrophs).
Q11. The entry of a pollen tube into the ovule through the micropyle is called:
A. Porogamy — Porogamy is pollen-tube entry via the micropyle; chalazogamy is through the chalaza and mesogamy through the integuments.
Q12. In a typical angiosperm embryo sac, the primary endosperm nucleus (PEN) formed during double fertilisation is:
C. Triploid — One haploid male gamete fuses with the two haploid polar nuclei (triple fusion), giving a triploid (3n) primary endosperm nucleus.
Q13. Ovulation in the human menstrual cycle is triggered by a rapid mid-cycle surge of:
D. Luteinizing hormone — A sharp mid-cycle LH surge induces rupture of the Graafian follicle and release of the ovum (ovulation).
Q14. Copper-releasing intrauterine devices (such as Cu-T) prevent conception mainly by:
B. Releasing Cu ions that reduce sperm motility and fertilising capacity — Copper ions released by Cu-IUDs are spermicidal, suppressing sperm motility and their fertilising capacity; they do not primarily block ovulation or the tubes.
Q15. In snapdragon, a cross between red-flowered and white-flowered plants produces pink F1 plants. The F2 phenotypic ratio will be:
B. 1 red : 2 pink : 1 white — This is incomplete dominance; heterozygotes are pink, so the F2 genotypic and phenotypic ratios are both 1:2:1 (red : pink : white).
Q16. The AB blood group in humans, in which both A and B antigens are fully expressed on the red cells, is an example of:
D. Codominance — Alleles I^A and I^B are codominant: in genotype I^A I^B both antigens appear together rather than blending, which is codominance.
Q17. Down's syndrome in humans results from the presence of an extra copy of:
C. Chromosome 21 — Down's syndrome is trisomy 21, an additional copy of chromosome 21 usually caused by non-disjunction during gamete formation.
Q18. During DNA replication, the discontinuously synthesised Okazaki fragments are produced on the:
A. Lagging strand — Because DNA polymerase works only 5'-to-3', the lagging strand is made in short Okazaki fragments that are later joined by ligase; the leading strand is continuous.
Q19. Because most amino acids are specified by more than one codon, the genetic code is described as:
A. Degenerate — Degeneracy means several codons can code for the same amino acid; the code is unambiguous (each codon specifies only one amino acid) and largely universal.
Q20. The wings of a bird and the wings of an insect perform the same function but differ in structure and origin. They are best described as:
B. Analogous organs showing convergent evolution — Different structures adapted for a similar function are analogous and arise by convergent evolution, unlike homologous organs which share a common ancestral plan.
Q21. According to the Hardy-Weinberg principle, allele frequencies in a population stay constant across generations provided that:
A. No mutation, migration, selection or drift acts and mating is random — Genetic equilibrium requires no mutation, no gene migration, no selection, no genetic drift, and random mating; any of these forces changes allele frequencies (evolution).
Q22. The transfer of antibodies from a mother to her foetus across the placenta gives the newborn:
C. Passive natural immunity — Ready-made antibodies are received without the infant's own immune system responding, so it is passive; because it occurs naturally through the placenta, it is passive natural immunity.
Q23. The stage of Plasmodium transmitted to a human through the bite of an infected female Anopheles mosquito is the:
B. Sporozoite — Infective sporozoites present in the mosquito's saliva enter the human bloodstream during the bite and then infect liver cells.
Q24. Biogas (gobar gas) obtained from cattle dung is produced mainly by the activity of:
A. Methanogens — Methanogens (e.g., Methanobacterium) in anaerobic sludge break down organic matter and produce methane-rich biogas.
Q25. Restriction endonucleases cut DNA at palindromic recognition sites in a staggered fashion, generating:
D. Sticky (cohesive) ends — Staggered cuts leave short single-stranded overhangs (sticky ends) whose complementary sequences allow foreign DNA to be joined into a vector.
Q26. The thermostable DNA polymerase used in PCR, which withstands the high denaturation temperature, is isolated from:
C. Thermus aquaticus — Taq polymerase from the thermophilic bacterium Thermus aquaticus remains active at ~90 degrees C, so it need not be replenished after each denaturation step.
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