Genetics and Evolution · Class 12 NEET Notes

• This unit is the highest scoring block in NEET Biology. Genetics and Evolution together carry more questions than any other unit.
• Blue marks a defining fact. Red marks a trap or a number examiners love.
• Most of the text is in points. Read the point, then study the drawing beside it.
• Sections 1 to 8 cover Molecular Basis of Inheritance. Sections 9 to 12 cover Evolution.
• Section 13 is a separate quick-review list of what NEET keeps coming back to. It is deliberately at the end, so you read the chapter properly first.

• A nucleotide has three parts: a nitrogen base, a pentose sugar and a phosphate group.
• Purines are adenine and guanine, and they have two rings. Pyrimidines are cytosine, thymine and uracil, with one ring.
• Watson and Crick built the model in 1953, using Franklin's X-ray photograph.
• The backbone is sugar and phosphate. The bases point inward and pair across.

• A pairs with T using two hydrogen bonds. G pairs with C using three.
• So DNA rich in G and C takes more heat to separate. This is used in the lab all the time.
• The two strands are antiparallel: one runs 5' to 3', the other 3' to 5'.
• Chargaff found A equals T and G equals C in any double-stranded DNA. So (A + G) divided by (T + C) is always 1.

• Human DNA is about 3.3 billion base pairs long. Laid end to end that is roughly 2 metres.
• It has to fit inside a nucleus about 10 micrometres across. So it must be packed.
• DNA wraps around a histone octamer to form a nucleosome.
• The octamer is two each of H2A, H2B, H3 and H4. About 200 base pairs wrap around it.
• Histone H1 is NOT part of the octamer. It sits outside and links one nucleosome to the next.

• Histones are positively charged because they are rich in lysine and arginine. DNA is negatively charged, so the two bind strongly.
• Euchromatin is loosely packed, stains light, and is active in transcription.
• Heterochromatin is tightly packed, stains dark, and is inactive.
• Non-histone chromosomal proteins give further levels of packing.

• Replication is semiconservative. Each new double helix keeps one old strand and one new one.
• Meselson and Stahl (1958) proved it using heavy nitrogen and density gradient centrifugation.
• Taylor showed the same thing in Vicia faba, the faba bean, using radioactive thymidine.
• Helicase unzips the two strands. DNA polymerase then builds the new ones.

• DNA polymerase can only add nucleotides in the 5 prime to 3 prime direction.
• Because the strands are antiparallel, only one of them can be built continuously.
• That one is the leading strand. The other is the lagging strand.
• The lagging strand is made in short pieces called Okazaki fragments, joined by DNA ligase.
• Replication starts at a specific sequence called the origin of replication.

• Transcription copies one strand of DNA into RNA. Only one strand is copied.
• The strand that is read is the template strand, running 3' to 5'.
• The other strand is the coding strand. It is not copied, but its sequence matches the RNA, with U in place of T.
• RNA polymerase does the work. In bacteria one enzyme makes all three RNA types.

• Eukaryotes have three RNA polymerases. I makes rRNA, II makes hnRNA (the mRNA precursor), III makes tRNA and small RNAs.
• Eukaryotic genes are split genes: coding exons interrupted by non-coding introns.
• Splicing removes the introns and joins the exons.
• The transcript also gets a cap at the 5' end and a poly-A tail at the 3' end.
• Only after all this does it become mature mRNA and leave the nucleus.

• The code is read in triplets. Four bases in groups of three give 4³ = 64 codons.
• 61 codons code for amino acids. 3 are stop codons: UAA, UAG and UGA.
• AUG is the start codon, and it also codes for methionine. So it does two jobs.
• Khorana, Nirenberg and Holley worked out the code, and shared the Nobel Prize for it.

• Degenerate: most amino acids have more than one codon. This is not the same as ambiguous.
• Unambiguous: each codon specifies one amino acid only, and never two.
• Nearly universal: the same code works from bacteria to humans, with tiny exceptions such as in mitochondria.
• Non-overlapping and comma-less: it is read straight through, three at a time.
• Degenerate and ambiguous are opposites. A codon never has two meanings, but an amino acid often has several codons.

• tRNA is the adapter. It has an anticodon at one end and carries an amino acid at the other.
• Ribosomes are the factory. The small subunit binds mRNA, and the large one joins the amino acids.
• There is no tRNA for the stop codons, which is exactly why translation stops there.
• The mRNA has untranslated regions (UTRs) at both ends that are not translated.

• A bacterium does not make an enzyme it does not need. It switches genes on only when the food arrives.
• Jacob and Monod described the lac operon in E. coli.
• An operon is a group of genes controlled together, from one promoter.
• Here lactose is the substrate, and it acts as the inducer.

• i gene makes the repressor protein. It is a regulator, not part of the operon proper.
• p is the promoter, where RNA polymerase binds.
• o is the operator, the switch that the repressor sits on.
• z gives beta-galactosidase, y gives permease, a gives transacetylase.
• One mRNA is made for all three structural genes together. That is what polycistronic means.

• The Human Genome Project ran from 1990 to 2003. The genome is about 3164.7 million base pairs.
• It found roughly 20500 genes, far fewer than the 80000 to 140000 people had expected.
• Chromosome 1 has the most genes (2968). The Y chromosome has the fewest (231).
• About 1.4 million single base differences, called SNPs, were located.
• Less than 2% of the genome codes for protein. Much of the rest is repetitive.

• DNA fingerprinting was developed by Alec Jeffreys.
• It compares VNTRs, short sequences repeated a variable number of times.
• The number of repeats differs between people, so the banding pattern is individual.
• The steps are: isolate DNA, cut with restriction enzymes, separate by electrophoresis, blot onto a membrane, hybridise with a probe, then detect by autoradiography.
• Identical twins are the exception. Their fingerprints match, because their DNA does.

• Fossils give a direct record. The deeper the rock layer, the older the fossil in it.
• Homologous organs have the same basic structure but do different jobs.
• Analogous organs have different structures but do the same job.

• Homology points to a common ancestor, so it shows divergent evolution.
• Examples: the forelimbs of humans, whales, bats and cheetahs. In plants, the thorn of Bougainvillea and the tendril of Cucurbita.
• Analogy comes from a shared way of life, so it shows convergent evolution.
• Examples: the wings of butterflies and birds, the eyes of octopus and mammals, and the flippers of penguins and dolphins.
• In plants, the sweet potato is a root and the potato is a stem, yet both store food.

• Darwin's two ideas were branching descent and natural selection.
• More young are produced than can survive. They vary. The ones that fit best leave more offspring.
• Fitness in biology means reproductive success, not strength.
• Lamarck said organs change through use and disuse, and that these changes pass on. This was rejected: changes to the body during life are not inherited.

• Stabilising selection favours the average and removes both extremes. Human birth weight.
• Directional selection favours one extreme and shifts the whole population. Industrial melanism.
• Disruptive selection favours both extremes and splits the population in two.
• Selection does not create variation. Mutation and recombination create it. Selection only decides which variants persist.

• This describes a population in which allele frequencies are not changing.
• So it is a baseline. It tells you what to expect when no evolution is happening.
• p is the frequency of the dominant allele and q the recessive one, and p + q = 1.
• The genotype frequencies then follow: p² + 2pq + q² = 1.

• Five conditions must all hold: no mutation, no gene flow, no genetic drift, no natural selection and random mating.
• In the real world at least one is always broken. So the equilibrium is a model, not a fact.
• That is exactly why it is useful. Any drift away from these values measures evolution.
• Worked example: if a recessive disease affects 1 in 10000, then q² = 0.0001 and q = 0.01. So p = 0.99, and carriers are 2pq = 0.0198, which is about 2 in 100.

Several of you asked for the repeated questions to be marked. Here they are, gathered in one place rather than scattered through the notes. Cover the gold band and test yourself.