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Molecular Basis of Inheritance – Class 12 Biology notes, important questions and MCQs

NCERT Class 12 Biology, chapter 5. Notes, key terms, important questions with answers and MCQs.

Chapter overview

DNA is the genetic material. It replicates semi-conservatively, is transcribed into RNA and translated into proteins using the genetic code; gene expression is regulated.

DNA and replication

DNA is a double helix (Watson and Crick) with A=T (2 H-bonds) and G≡C (3 H-bonds); Chargaff: A + G = T + C.
Hershey–Chase showed DNA is the genetic material; Meselson–Stahl proved semi-conservative replication.
DNA polymerase works 5′ → 3′; Okazaki fragments on the lagging strand are joined by ligase.

Expression

Transcription: DNA → mRNA by RNA polymerase; eukaryotic mRNA undergoes splicing, capping and tailing.
Genetic code: triplet, degenerate, universal, non-overlapping; AUG start; UAA, UAG, UGA stop.
Lac operon (Jacob and Monod) is an inducible operon; Human Genome Project found about 20,000–25,000 genes.

Key terms

Codon: triplet of bases on mRNA coding for one amino acid
Operon: cluster of genes controlled by one promoter and operator
DNA fingerprinting: technique using VNTRs to identify individuals

Important questions with answers

Why is DNA a better genetic material than RNA?2 marks

DNA is more stable: it is double-stranded, lacks the reactive 2′-OH group, and has thymine instead of uracil, so it mutates less and is less easily degraded.

What are the steps of DNA fingerprinting?3 marks

Isolate DNA; digest it with restriction enzymes; separate fragments by gel electrophoresis; transfer them to a membrane (Southern blotting); hybridise with labelled VNTR probes; detect bands by autoradiography and compare patterns.

Describe the process of transcription in prokaryotes and the post-transcriptional processing of mRNA in eukaryotes.5 marks

Transcription has three steps. Initiation: RNA polymerase binds to the promoter with the help of the σ factor and opens the DNA. Elongation: it adds ribonucleotides complementary to the template strand in the 5′ → 3′ direction. Termination: at the terminator, with the help of the ρ factor, the RNA and polymerase are released. In eukaryotes the primary transcript (hnRNA) is processed: splicing removes introns and joins exons; capping adds methyl guanosine triphosphate at the 5′ end; tailing adds a poly-A tail at the 3′ end. The mature mRNA then moves to the cytoplasm for translation.

Quick revision

Semi-conservative replication.
AUG = start codon (methionine).
Lactose is the inducer of the lac operon.

Exam tips

In a DNA with 20% adenine, thymine = 20% and G = C = 30% each.
Remember the central dogma: DNA → (transcription) → RNA → (translation) → protein; reverse transcription occurs in retroviruses.

Practice MCQs

The base pairing in DNA is
1. A–T and G–C
2. A–G and C–T
3. A–C and G–T
4. A–U and G–C
Show answer
Answer: (a) A–T and G–C. Complementary pairing.
Start codon is
1. AUG
2. UAA
3. UGA
4. UAG
Show answer
Answer: (a) AUG. Codes methionine.
Semi-conservative replication was proved by
1. Hershey and Chase
2. Griffith
3. Watson and Crick
4. Meselson and Stahl
Show answer
Answer: (d) Meselson and Stahl. ¹⁵N experiment.
In the lac operon, the inducer is
1. glucose
2. repressor
3. lactose
4. galactosidase
Show answer
Answer: (c) lactose. Lactose (allolactose).
The enzyme that joins Okazaki fragments is
1. DNA ligase
2. helicase
3. primase
4. topoisomerase
Show answer
Answer: (a) DNA ligase. Seals nicks.
If DNA has 30% guanine, adenine is
1. 20%
2. 30%
3. 40%
4. 70%
Show answer
Answer: (a) 20%. G + C = 60%, so A = 20%.
Okazaki fragments are formed on the
1. leading strand
2. lagging strand
3. mRNA
4. tRNA
Show answer
Answer: (b) lagging strand. Discontinuous synthesis.
In the lac operon, when lactose is absent, the repressor
1. binds to the operator and blocks transcription
2. is inactive
3. binds to lactose
4. makes β-galactosidase
Show answer
Answer: (a) binds to the operator and blocks transcription. Negative regulation.

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Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.