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CBSE · Class 12 · Biology

Molecular Basis of Inheritance

Standard difficulty · 20 questions · 40 marks

Name: Date: Score: ____ / 40

Show your working for Sections B and C. Answers are on the last page — try every question first!

Section A — Choose the correct answer8 × 1 = 8 marks
1.
The double-helix model of DNA was proposed by:
  1. Griffith
  2. Hershey and Chase
  3. Meselson and Stahl
  4. Watson and Crick (1953)
2.
The template strand of a gene reads 5′-TCATTTGCCCAT-3′. The mRNA transcribed from this gene reads:
  1. 5′-AUGGGCAAAUGA-3′
  2. 5′-UACCCGUUUACU-3′
  3. 5′-ATGGGCAAATGA-3′
  4. 5′-AGUAAACGGGUA-3′
3.
The mRNA 5′-AUGUUUAAGUAA-3′ is translated. The polypeptide formed is:
  1. Met–Phe–Lys–Stop
  2. Lys–Phe–Met
  3. Phe–Lys
  4. Met–Phe–Lys
4.
The genome of rice has 3.9 × 108 base pairs. Its length as a stretched double helix is about: (0.34 nm per base pair)
  1. 0.0663 m
  2. 132.6 m
  3. 1.326 × 10−4 m
  4. 0.1326 m
5.
E. coli grown for many generations in 15N is moved to 14N. After 3 generations, the fraction of DNA molecules that are hybrid (15N14N) is:
  1. 1/4
  2. 1/2
  3. 0
  4. 1
6.
If each nucleosome, with its linker, takes up about 200 base pairs, a stretch of 26,000 base pairs of DNA forms about how many nucleosomes?
  1. 65
  2. 260
  3. 3,250
  4. 130
7.
Which of these adds nucleotides in the 5′ → 3′ direction during replication?
  1. DNA polymerase
  2. Aminoacyl-tRNA synthetase
  3. Lactose (allolactose)
  4. RNA polymerase III
8.
Assertion (A): The codons UUA and UUC code for different amino acids.
Reason (R): Every codon that starts with the same two bases codes for the same amino acid.
  1. Both A and R are true, and R is the correct explanation of A.
  2. Both A and R are true, but R is not the correct explanation of A.
  3. A is true, but R is false.
  4. A is false, but R is true.
Section B — Short answer8 × 2 = 16 marks
9.
What is the central dogma of molecular biology?
10.
Translate the mRNA 5′-UAUGAUUCGAUCCCGUGGCUGA-3′. How many amino acids does the polypeptide have?
11.
The mRNA 5′-AUGACUGCGGCUGAGUGA-3′ changes to 5′-AUGACUGCGGCUGCGUGA-3′ by a point mutation. Give both polypeptides and say what kind of mutation this is.
12.
One strand of a DNA segment is 5′-GCCATTAACCTC-3′. Write the other strand and find the number of hydrogen bonds between them.
13.
One 15N-labelled DNA molecule replicates 3 times in a 14N medium. How many DNA molecules are formed, and how many of them are hybrid and how many light?
14.
Why must the genetic code be a triplet code? How many bases of mRNA, including the stop codon, code for a polypeptide of 340 amino acids?
15.
The mouse haploid genome has about 2.7 × 109 base pairs. Find the length of this DNA. (0.34 nm per base pair)
16.
In the lac operon, what are the roles of the promoter and the operator?
Section C — Word problems4 × 4 = 16 marks
17.
Describe the double-helix model of DNA.
18.
The coding strand of a gene reads 5′-ATGACGCGTAGACCATGA-3′. Write (a) the template strand, (b) the mRNA, (c) the polypeptide, (d) the number of amino acids.
19.
Describe the Meselson–Stahl experiment. If 50 molecules of 15N DNA replicate for 6 generations in 14N, how many molecules form, and what fraction of the DNA is hybrid and what fraction is light? What would these fractions be if replication were conservative?
20.
The mRNA 5′-AUGAUCGCGGGUCGUUAG-3′ gains an extra C after the sixth base. Give the normal and the mutant polypeptides, name the kind of mutation, and explain why it is usually more harmful than a substitution.
AmbifulWorksheets
Practice that never repeats
CBSE · Class 12 · Biology

Answer key

Molecular Basis of Inheritance · AW-12B-MBI-2026-V1 · For parents — fold back or tear off before handing the sheet to your child.

QAnswerWorking
1(d) Watson and Crick (1953)—
2(a) 5′-AUGGGCAAAUGA-3′the mRNA is complementary to the template strand and runs antiparallel to it
3(d) Met–Phe–LysAUG Met, UUU Phe, AAG Lys, UAA stop
4(d) 0.1326 mlength = number of base pairs × 0.34 × 10−9 m
5(a) 1/4semi-conservative: always 2 hybrid molecules out of 23
6(d) 130the histone octamer and linker hold about 200 bp
7(a) DNA polymerase—
8(c) A is true, but R is false.A is true: UUA = Leu, UUC = Phe. R is false.
9Genetic information flows from DNA to RNA (transcription) and from RNA to protein (translation). In some viruses information flows from RNA to DNA (reverse transcription).—
10Met–Ile–Arg–Ser–Arg–Gly; 6 amino acidsstart at AUG, read in triplets, stop at UGA
11normal: Met–Thr–Ala–Ala–Glu; mutant: Met–Thr–Ala–Ala–Ala; a base substitution that changes one amino acid (a missense mutation, like the one in sickle-cell anaemia)codon 5: GAG → GCG
123′-CGGTAATTGGAG-5′; 30A–T: 2 hydrogen bonds; G–C: 3
138 molecules: 2 hybrid (15N14N) and 6 light (14N14N)semi-conservative replication keeps each pair of old strands in two molecules
14There are 20 amino acids; a doublet code would give only 42 = 16 codons, a triplet gives 43 = 64, enough for all; (340 + 1) × 3 = 1,023 bases—
150.918 m2.7 × 109 × 0.34 × 10−9 m
16The promoter is the site where RNA polymerase binds; the operator is the site where the repressor binds—
17Two polynucleotide chains wind round each other in a right-handed helix with the sugar–phosphate backbone outside and bases inside. The chains are antiparallel (5′ → 3′ and 3′ → 5′). Bases pair by hydrogen bonds: A with T (two bonds) and G with C (three), so a purine always faces a pyrimidine and the width is uniform. The pitch is 3.4 nm with about 10 base pairs per turn, 0.34 nm apart.—
18(a) 3′-TACTGCGCATCTGGTACT-5′; (b) 5′-AUGACGCGUAGACCAUGA-3′; (c) Met–Thr–Arg–Arg–Pro; (d) 5mRNA = coding strand with U for T; translation from AUG to the stop codon
193,200 molecules. E. coli was grown in 15NH4Cl, moved to 14NH4Cl, and the DNA separated by density in CsCl. Semi-conservative: hybrid 1/32, light 31/32. Conservative: no hybrid DNA at all; heavy 1/64 and light 63/64—
20normal: Met–Ile–Ala–Gly–Arg; mutant (reading frame shifted): Met–Ile–Arg–Gly–Ser–Leu; a frameshift (insertion) mutation, which changes every codon after it and often gives a non-functional proteinread the mutant in triplets from AUG