Class 12 Biology — CBSE Board Sample Paper

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Class 12 Biology — CBSE Board Sample Paper — solved practice questions

8 Class 12 Board Sample Papers (CBSE) questions with step-by-step solutions. Attempt each, then reveal the answer.

1. Q1medium

   Signals for parturition in human female originate from

   • AFully developed foetus only
   • BBoth placenta as well as fully developed foetus
   • CPlacenta only
   • DOxytocin released from maternal pituitary
   Show answer & solution

   Correct answer: (B) Both placenta as well as fully developed foetus

   The signals for parturition originate from the fully developed foetus and the placenta, which induce mild uterine contractions (the foetal ejection reflex).

2. Q2hard

   To produce 1600 seeds, the number of meiotic divisions required will be

   • A2400
   • B2000
   • C1600
   • D1800
   Show answer & solution

   Correct answer: (B) 2000

   One microspore mother cell gives 4 pollen grains per meiosis, so 400 meiotic divisions yield 1600 pollen grains; each megaspore mother cell gives 1 egg per meiosis, needing 1600 divisions. Total = 400 + 1600 = 2000.

3. Q3medium

   A sample of normal double-stranded DNA was found to have thymine content of 27%. What will be the expected proportion of guanine in this strand?

   • A23%
   • B32%
   • C36%
   • D73%
   Show answer & solution

   Correct answer: (A) 23%

   By Chargaff's rule A=T=27%, so A+T=54% and G+C=46%. Since G=C, guanine = 46/2 = 23%.

4. Q4medium

   E. coli has 4.6 x 10^6 base pairs and completes the process of replication in 18 minutes, then the average rate of polymerization is approximately

   • A2000 bp/s
   • B4000 bp/s
   • C3000 bp/s
   • D1000 bp/s
   Show answer & solution

   Correct answer: (B) 4000 bp/s

   Rate = 4.6 x 10^6 bp / (18 x 60 s) = 4.6 x 10^6 / 1080 = approximately 4259 bp/s, i.e. about 4000 bp/s.

5. Q5medium

   Suresh and Rajesh have defective haemoglobin due to genetic disorders. In Suresh, the problem is qualitative as he is having incorrectly functioning globin molecules while in Rajesh the problem is quantitative as he is having very few globin molecules. Identify the disorder they are suffering from.

   • ASuresh: Thalassemia - Autosomal Dominant blood disorder; Rajesh: Sickle Cell Anaemia - Autosomal linked Recessive trait
   • BSuresh: Sickle Cell Anaemia - Autosomal linked Dominant trait; Rajesh: Thalassemia - Autosomal Recessive blood disorder
   • CSuresh: Sickle Cell Anaemia - Autosomal linked Recessive trait; Rajesh: Thalassemia - Autosomal Recessive blood disorder
   • DSuresh: Thalassemia - Autosomal Dominant blood disorder; Rajesh: Sickle Cell Anaemia - Autosomal linked Dominant trait
   Show answer & solution

   Correct answer: (C) Suresh: Sickle Cell Anaemia - Autosomal linked Recessive trait; Rajesh: Thalassemia - Autosomal Recessive blood disorder

   A qualitative defect (incorrectly functioning globin) is Sickle Cell Anaemia (autosomal linked recessive), while a quantitative defect (too few globin molecules) is Thalassemia (autosomal recessive blood disorder).

6. Q6medium

   In E. coli, the lac operon gets switched on when lactose is

   • Apresent in the medium and it binds to the repressor.
   • Bnot present in the medium and the repressor binds to the operator.
   • Cnot present in the medium and RNA polymerase binds to the operator.
   • DActive lactose present in the medium binds to RNA polymerase.
   Show answer & solution

   Correct answer: (A) present in the medium and it binds to the repressor.

   Lactose acts as the inducer; when present it binds the repressor, inactivating it, so the repressor cannot bind the operator and the lac operon is switched on.

7. Q7medium

   Which of the following features shows the mechanism of sex determination in honey-bee? (i) An offspring formed from the union of a sperm and egg develops as a female. (ii) Males have half the number of chromosomes than that of female. (iii) The males are haploid having 32 chromosomes. (iv) All workers and males are diploid having 16 chromosomes

   • A(i) and (ii)
   • B(ii) and (iii)
   • C(i) and (iv)
   • D(ii) and (iv)
   Show answer & solution

   Correct answer: (A) (i) and (ii)

   In honey-bees (haplodiploidy), fertilised eggs become diploid females while males are haploid, developing from unfertilised eggs, so males have half the chromosome number of females. Statements (i) and (ii) are correct.

8. Q8medium

   The following fragment of DNA is going to be transcribed; the upper strand with polarity 3' to 5' is the template strand: 3' ATTGCC 5' / 5' TAACGG 3'. After transcription the mRNA can be represented by:

   • A5' AUUGCC 3'
   • B5' AUUGCC 3'
   • C5' UAACGG 3'
   • D5' GGCAAU 3'
   Show answer & solution

   Correct answer: (C) 5' UAACGG 3'

   The mRNA is complementary and antiparallel to the 3'-ATTGCC-5' template, giving 5'-UAACGG-3' (with uracil replacing thymine).

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