Molecular Basis of Inheritance

DNA (deoxyribonucleic acid) is the genetic material of most organisms. It is a long polymer of nucleotides, each made of a deoxyribose sugar, a phosphate and a nitrogenous base — the purines adenine (A) and guanine (G) and the pyrimidines cytosine (C) and thymine (T). The Watson–Crick double-helix model (1953) describes DNA as two antiparallel strands (one running 5'→3', the other 3'→5') coiled into a right-handed helix, with the sugar-phosphate backbones outside and the bases paired inside.

Base pairing is specific and complementary: A pairs with T by two hydrogen bonds and G pairs with C by three hydrogen bonds. This matches Chargaff's rule that the amount of A = T and G = C. The helix has about 10 base pairs per turn, a pitch (length of one turn) of 3.4 nm and a rise of 0.34 nm per base pair.

A human cell holds about 2 metres of DNA, so it must be tightly packaged. The negatively charged DNA wraps around a core of positively charged proteins called histones. About 200 base pairs of DNA wound around a histone octamer form a nucleosome, the basic repeating unit, giving a 'beads-on-a-string' look; nucleosomes coil further into chromatin and chromosomes. Loosely-packed, transcriptionally active chromatin is euchromatin; densely-packed, inactive chromatin is heterochromatin.

How did we learn DNA is the genetic material? Griffith (1928) found a 'transforming principle' in Streptococcus pneumoniae (his transformation experiment with R and S strains). Avery, MacLeod and McCarty identified that transforming principle as DNA. Finally, Hershey and Chase (1952) used bacteriophages labelled with radioactive ³²P (DNA) and ³⁵S (protein) and showed that only the ³²P (DNA) entered the bacteria — conclusively proving DNA is the genetic material. (RNA was likely the first genetic material in evolution, but DNA, being double-stranded and chemically more stable, took over.) For NEET, fix the helix details, base-pairing/H-bonds, Chargaff's rule, the nucleosome, and the three classic experiments.

For genetic information to pass to daughter cells, DNA must be copied (replicated) before cell division. The Watson–Crick model itself suggested how: because the two strands are complementary, each can serve as a template to make the other. This gives semiconservative replication — each new DNA molecule keeps one old (parental) strand and one newly synthesised strand.

This was proved by the classic experiment of Meselson and Stahl (1958) on Escherichia coli. They grew bacteria in medium with heavy nitrogen (¹⁵N) so the DNA was heavy, then shifted them to normal nitrogen (¹⁴N). After one generation the DNA was of intermediate (hybrid) density (one heavy + one light strand), and after two generations there were equal amounts of hybrid and light DNA — exactly the pattern predicted by semiconservative replication.

Mechanistically, replication begins at a specific site, the origin of replication (ori). The enzyme helicase unwinds the helix to form a replication fork, and the main enzyme DNA polymerase adds nucleotides to build the new strands. A crucial constraint is that DNA polymerase can only synthesise in the 5'→3' direction.

Because the two template strands are antiparallel, this directionality means one new strand — the leading strand — is made continuously toward the fork, while the other — the lagging strand — is made discontinuously in short pieces called Okazaki fragments, which are later joined by DNA ligase. Replication is highly accurate, with proofreading. For NEET, fix the meaning of semiconservative replication, the Meselson–Stahl result (hybrid after one generation), and the mechanism: origin, DNA polymerase building 5'→3', leading (continuous) vs lagging (Okazaki fragments + ligase) strands.

The flow of genetic information is summarised by the central dogma (proposed by Crick): DNA → RNA → protein, i.e. DNA is transcribed into RNA, which is translated into protein. Transcription is the copying of one strand of DNA into RNA by the enzyme RNA polymerase. Only one strand, the template strand (read 3'→5'), is copied; the other is the coding strand. A transcription unit has a promoter (where RNA polymerase binds), the structural gene and a terminator.

In eukaryotes the first transcript, hnRNA, contains coding exons and non-coding introns (split genes). It is processed: the introns are removed and exons joined by splicing, a cap is added at the 5' end and a poly-A tail at the 3' end, producing mature mRNA.

The information in mRNA is read using the genetic code, whose features are heavily tested. The code is read in triplets called codons (three bases = one amino acid), giving 64 codons. It is degenerate (most amino acids have more than one codon), unambiguous (each codon specifies only one amino acid), nearly universal (same in almost all organisms), non-overlapping and comma-less. AUG is the start (initiation) codon (and codes for methionine), while UAA, UAG and UGA are the three stop (termination) codons.

The codons are decoded during translation, which occurs on the ribosome. The key molecule is tRNA (transfer RNA), the 'adaptor molecule': it has an anticodon that base-pairs with the mRNA codon and carries the matching amino acid. Translation proceeds in three steps — initiation (ribosome assembles at AUG), elongation (amino acids are added one by one, joined by peptide bonds) and termination (at a stop codon, releasing the polypeptide). For NEET, fix the central dogma, the template strand and transcription unit, splitting/splicing in eukaryotes, the features of the genetic code (with start/stop codons) and the adaptor role of tRNA.

Cells do not make every protein all the time — gene expression is regulated so that genes are switched on only when their products are needed. The classic model is the lac operon of E. coli, described by Jacob and Monod. An operon is a unit of bacterial genes controlled together. The lac operon has a regulatory gene (i) that makes a repressor, a promoter (p), an operator (o), and three structural genes — z (β-galactosidase), y (permease) and a (transacetylase) — whose enzymes metabolise lactose.

When lactose is absent, the repressor binds the operator and blocks transcription — the genes are off. When lactose is present, it acts as an inducer: it binds the repressor, inactivating it, so the repressor leaves the operator and RNA polymerase can transcribe the genes, making the lactose-digesting enzymes. Because lactose switches the operon on by removing the repressor's block, this is an example of negative regulation by induction.

On a much larger scale, the entire genetic make-up of humans was decoded by the Human Genome Project (HGP), a global effort that ran from 1990 to 2003. It sequenced the roughly 3 billion (3 × 10⁹) base pairs of the human genome and found that humans have only about 20,000–25,000 genes — far fewer than expected. Among its findings: less than 2% of the genome codes for proteins, and there are huge stretches of repetitive sequences.

One powerful application of knowing the variable parts of the genome is DNA fingerprinting, developed by Alec Jeffreys. It compares highly variable repetitive sequences called VNTRs (variable number of tandem repeats), which differ from person to person; since these patterns are unique to each individual (except identical twins), DNA fingerprinting is used in forensic identification, paternity testing and pedigree analysis. For NEET, fix the lac operon parts and its induction by lactose (negative regulation), the HGP key facts (1990–2003, ~3 billion bp, ~20,000–25,000 genes) and DNA fingerprinting using VNTRs.