Class 12 Biology – Molecular Basis of Inheritance Notes

Class 12 Biology – Molecular Basis of Inheritance Notes

1. DNA – The Genetic Material

  • DNA (Deoxyribonucleic acid) carries genetic information.
  • Structure (Watson & Crick, 1953):
    • Double helix, antiparallel strands
    • Sugar-phosphate backbone
    • Complementary base pairing: A–T, G–C
  • Function: Storage and transmission of genetic information.

2. RNA – Types and Functions

  • RNA (Ribonucleic acid): Single-stranded nucleic acid.
  • Types:
    1. mRNA (messenger RNA): Carries genetic code from DNA to ribosome.
    2. tRNA (transfer RNA): Brings amino acids to ribosome.
    3. rRNA (ribosomal RNA): Forms ribosome structure.
  • Function: Protein synthesis.

3. Replication of DNA

  • Semi-conservative replication: Each new DNA has one old and one new strand.
  • Enzymes involved:
    • Helicase: Unwinds DNA
    • DNA polymerase: Adds nucleotides
    • Ligase: Joins Okazaki fragments (lagging strand)
  • Direction: 5’ → 3’ synthesis

4. Central Dogma of Molecular Biology

  • Flow of genetic information:
    DNA → RNA → Protein
  • Process:
    1. Transcription: DNA → mRNA
    2. Translation: mRNA → Polypeptide (protein)
  • Genetic code: Triplet codon, universal, degenerate.

5. Gene Expression & Regulation

  • Gene: Sequence of DNA coding for a functional product.
  • Regulation in prokaryotes: Operon model (e.g., lac operon).
  • Regulation in eukaryotes: Transcription factors, enhancers, silencers.

6. DNA Packaging & Chromosomes

  • DNA wraps around histone proteins → nucleosomes → chromatin → chromosomes.
  • Ensures condensation, protection, and regulation of DNA.

7. Mutations and DNA Repair

  • Mutation: Permanent change in DNA sequence.
  • Types: Point mutation, insertion, deletion, frame-shift.
  • DNA repair mechanisms: Proofreading by DNA polymerase, excision repair, recombination repair.

Molecular Basis of Inheritance | MCQs

1. DNA – Structure and Function

  1. Who discovered the double helix structure of DNA?
    a) Mendel
    b) Watson & Crick
    c) Franklin & Wilkins
    d) Chargaff
    Answer: b) Watson & Crick
  2. Complementary base pairing in DNA involves:
    a) A–G, T–C
    b) A–T, G–C
    c) A–C, G–T
    d) A–U, G–C
    Answer: b) A–T, G–C
  3. DNA is composed of:
    a) Ribose, phosphate, nitrogen bases
    b) Deoxyribose, phosphate, nitrogen bases
    c) Glucose, phosphate, nitrogen bases
    d) Deoxyribose, sugar, phosphate only
    Answer: b) Deoxyribose, phosphate, nitrogen bases
  4. The strands of DNA are:
    a) Parallel
    b) Antiparallel
    c) Perpendicular
    d) Randomly oriented
    Answer: b) Antiparallel

2. RNA – Types and Functions

  1. Which RNA carries amino acids to the ribosome?
    a) mRNA
    b) tRNA
    c) rRNA
    d) snRNA
    Answer: b) tRNA
  2. rRNA is important because it:
    a) Synthesizes DNA
    b) Forms ribosomal structure
    c) Carries genetic code
    d) Modifies proteins
    Answer: b) Forms ribosomal structure
  3. mRNA is synthesized during:
    a) Translation
    b) Replication
    c) Transcription
    d) Splicing
    Answer: c) Transcription

3. DNA Replication

  1. DNA replication is described as:
    a) Conservative
    b) Semi-conservative
    c) Dispersive
    d) Random
    Answer: b) Semi-conservative
  2. The enzyme that unwinds the DNA helix is:
    a) DNA polymerase
    b) Helicase
    c) Ligase
    d) Primase
    Answer: b) Helicase
  3. Okazaki fragments are formed on the:
    a) Leading strand
    b) Lagging strand
    c) mRNA strand
    d) rRNA strand
    Answer: b) Lagging strand
  4. DNA polymerase synthesizes DNA in which direction?
    a) 3’ → 5’
    b) 5’ → 3’
    c) Both directions
    d) Random direction
    Answer: b) 5’ → 3’

4. Central Dogma and Protein Synthesis

  1. Central dogma of molecular biology is:
    a) RNA → DNA → Protein
    b) DNA → RNA → Protein
    c) Protein → RNA → DNA
    d) DNA → Protein → RNA
    Answer: b) DNA → RNA → Protein
  2. A triplet of nucleotides in mRNA that codes for an amino acid is called:
    a) Anticodon
    b) Codon
    c) Exon
    d) Intron
    Answer: b) Codon
  3. Translation occurs in:
    a) Nucleus
    b) Ribosome
    c) Mitochondria
    d) Cytoplasm only
    Answer: b) Ribosome
  4. Which RNA has an anticodon sequence?
    a) mRNA
    b) tRNA
    c) rRNA
    d) snRNA
    Answer: b) tRNA

5. Gene Regulation

  1. Lac operon is a model for:
    a) Eukaryotic gene regulation
    b) Prokaryotic gene regulation
    c) DNA replication
    d) Mutation repair
    Answer: b) Prokaryotic gene regulation
  2. In eukaryotes, enhancers are:
    a) Coding sequences
    b) DNA sequences increasing transcription
    c) Proteins that inhibit translation
    d) tRNA molecules
    Answer: b) DNA sequences increasing transcription

6. Mutations and DNA Repair

  1. A permanent change in DNA sequence is called:
    a) Recombination
    b) Mutation
    c) Replication
    d) Transcription
    Answer: b) Mutation
  2. Sickle cell anemia is caused by:
    a) Chromosomal mutation
    b) Gene mutation
    c) Frame-shift mutation only
    d) Silent mutation
    Answer: b) Gene mutation
  3. DNA repair mechanisms include:
    a) Proofreading by DNA polymerase
    b) Excision repair
    c) Recombination repair
    d) All of the above
    Answer: d) All of the above