CBSE Class 12 Biology – Molecular Basis of Inheritance

NCERT-aligned chapter resource | Biology Code 044

1. DNA as Genetic Material

DNA is the hereditary material in most organisms. Experiments such as those involving transformation and bacteriophages provided evidence for the role of DNA in inheritance.

2. DNA Structure

DNA consists of nucleotides containing deoxyribose, phosphate and nitrogenous bases. Two antiparallel polynucleotide strands form a double helix through complementary base pairing: adenine pairs with thymine and guanine with cytosine.

3. Replication

Each daughter DNA molecule contains one parental strand and one newly synthesised strand.

4. Transcription

During transcription, information in DNA is copied into RNA. In eukaryotes, the primary transcript undergoes processing including capping, tailing and splicing before mature mRNA is produced.

5. Genetic Code

The genetic code is triplet, degenerate and nearly universal. AUG functions as a start codon and UAA, UAG and UGA are stop codons.

6. Translation

Ribosomes read mRNA codons while tRNA molecules bring amino acids. Translation proceeds through initiation, elongation and termination.

7. Lac Operon

The lac operon demonstrates inducible gene regulation in bacteria. In the absence of inducer, the repressor prevents transcription of structural genes; when inducer is present, repression is relieved.

8. Human Genome Project and DNA Fingerprinting

The Human Genome Project generated a reference sequence and extensive information about human genes and genomic organisation. DNA fingerprinting exploits highly variable repetitive DNA regions to distinguish individuals.

Worked Concept

If the DNA template contains a base sequence, determine the complementary RNA sequence using transcription rules, then translate codons beginning at an appropriate start codon until a stop codon is reached.

NCERT & CBSE Chapter-Specific MCQs

1. DNA replication is described as: (A) conservative (B) semiconservative (C) dispersive only (D) random. Answer: B.
2. Which is a stop codon? (A) AUG (B) UAA (C) UGG only (D) UAC. Answer: B.
3. Splicing removes: (A) exons only (B) introns from the primary transcript (C) DNA bases from chromosomes (D) ribosomes. Answer: B.

Competency Questions

1. Given a DNA template sequence, derive the RNA sequence and identify the translated peptide.
2. Explain why DNA replication is called semiconservative.
3. Predict the effect of a mutation that changes a codon but does not change the encoded amino acid.