Genetics and Evolution (Class 12)

Why This Chapter Matters

Genetics and Evolution carries 12-15 marks in Class 12 Biology boards. Molecular basis of inheritance (DNA replication, transcription, translation), chromosomal basis of inheritance, and evolution theories are all tested.

Core Concepts

1. DNA Structure and Replication

DNA double helix (Watson and Crick, 1953):

Two antiparallel strands. Sugar-phosphate backbone outside, bases inside.

Complementary base pairing: A=T (2 H-bonds), G≡C (3 H-bonds).

Major groove and minor groove alternating.

Chargaff's rule: A=T, G=C (in any double-stranded DNA).

DNA replication (Semi-conservative — Meselson and Stahl experiment, 1958):

Each new DNA has one old strand and one new strand.

Enzymes involved:

•Helicase: unwinds and separates strands (breaks H-bonds).

•DNA Polymerase: synthesises new strand (5'→3' direction only).

•Primase: synthesises RNA primer.

•DNA Ligase: joins Okazaki fragments on lagging strand.

•Single-strand binding proteins: stabilise separated strands.

Leading strand: synthesised continuously (same direction as replication fork).

Lagging strand: synthesised discontinuously as Okazaki fragments.

2. Transcription (DNA → RNA)

Template strand (3'→5') → mRNA (5'→3').

RNA polymerase does not need primer.

Promoter: DNA sequence where RNA polymerase binds.

In prokaryotes: Transcription and translation simultaneous. mRNA directly used.

In eukaryotes: Pre-mRNA made → processing (splicing, capping, tailing) → mature mRNA leaves nucleus.

•Introns: non-coding sequences removed.

•Exons: coding sequences retained.

•5' capping (7-methylguanosine) + 3' poly-A tail: protect mRNA.

3. Translation (mRNA → Protein)

Ribosome reads mRNA codons (triplet) and assembles amino acids.

Genetic code:

•Triplet (3 bases = 1 codon).

•64 codons total: 61 code for amino acids, 3 are stop codons (UAA, UAG, UGA).

•AUG = start codon (also codes for methionine).

•Degenerate (multiple codons for same amino acid — except Trp and Met).

•Universal (same code in almost all organisms).

•Non-overlapping and commaless.

Steps: Initiation (ribosome assembles at AUG) → Elongation (peptide chain grows) → Termination (stop codon reached, protein released).

tRNA: adaptor molecule. Anticodon pairs with mRNA codon. Carries specific amino acid.

4. Chromosomal Basis of Inheritance

Morgan's experiments on Drosophila established chromosomal theory.

Linkage: genes on same chromosome tend to be inherited together.

Recombination: crossing over during meiosis creates new combinations.

Sex determination: XX-XY (humans), ZW-ZZ (birds).

5. Evolution

Darwin's Theory (Natural Selection):

Variation exists in populations. More individuals born than survive. Survival of the fittest (better adapted survive and reproduce). Favourable variations passed to offspring.

Modern Synthetic Theory (Neo-Darwinism): Darwin's natural selection + Mendelian genetics + mutations + genetic drift.

Sources of variation:

Mutation: sudden heritable change in DNA. Gene mutation or chromosomal mutation.

Genetic recombination: crossing over.

Migration (gene flow): movement of individuals between populations.

Genetic drift: random change in allele frequency in small populations. Bottleneck effect, founder effect.

Evidence for evolution:

Homologous organs: same anatomy, different functions (forelimbs of man, whale, bat — common ancestor).

Analogous organs: different anatomy, same function (wings of bat and insect — convergent evolution).

Vestigial organs: reduced non-functional (appendix in humans, ear muscles, coccyx).

Fossil record: history of life preserved in rocks.

Biogeography: species distribution patterns.

Hardy-Weinberg Law:

Allele frequencies remain constant in a large random mating population with no evolution pressure.

p + q = 1 (p = freq of dominant allele, q = freq of recessive allele).

p² + 2pq + q² = 1 (genotype frequencies).

PYQs (CBSE)

CBSE 2023: Explain the semi-conservative nature of DNA replication. Name the experiment that proved it.

Meselson and Stahl (1958) used ¹⁵N-labelled DNA. After one generation in ¹⁴N medium, all DNA was hybrid (¹⁵N-¹⁴N). After two generations, equal amounts of hybrid and light DNA — proving each new DNA has one old and one new strand (semi-conservative).

CBSE 2022: What are introns and exons? How are they processed?

Introns: non-coding intervening sequences in pre-mRNA that are spliced out.

Exons: coding sequences that are retained and joined together.

Processing: 5' cap + poly-A tail added. Introns removed by spliceosome. Mature mRNA enters cytoplasm for translation.

Revision Notes

DNA STRUCTURE:
A=T (2 H-bonds) | G≡C (3 H-bonds) | Antiparallel strands
Chargaff: A=T, G=C

REPLICATION (Semi-conservative):
Helicase (unwind) → Primase (primer) → DNA Pol (new strand, 5'→3')
Leading (continuous) | Lagging (Okazaki fragments, joined by Ligase)

TRANSCRIPTION: DNA template (3'→5') → mRNA (5'→3')
Eukaryotes: pre-mRNA → processing (intron removal, capping, poly-A) → mature mRNA

GENETIC CODE:
Triplet | 64 codons (61 amino acid + 3 stop: UAA,UAG,UGA)
AUG = start + methionine | Degenerate | Universal

EVOLUTION:
Darwin: natural selection (variation + survival of fittest + inheritance)
Neo-Darwinism: + mutations + genetic drift + gene flow
Homologous: same structure, different function (divergent evolution)
Analogous: different structure, same function (convergent evolution)
Hardy-Weinberg: p+q=1 | p²+2pq+q²=1