Molecular Basis of Inheritance

DNA (Deoxyribonucleic acid) is the molecular basis of inheritance in most organisms (RNA in some viruses). Understanding DNA structure, replication, transcription, and translation is fundamental to modern biology.

DNA Structure

Watson and Crick (1953) proposed the double helix model of DNA. Key features:
Two antiparallel polynucleotide strands coiled around a common axis
Nucleotides consist of: deoxyribose sugar + phosphate group + nitrogenous base
Purines: Adenine (A), Guanine (G); Pyrimidines: Cytosine (C), Thymine (T)
Chargaff's rules: A = T, G = C (complementary base pairing — A with T by 2 hydrogen bonds; G with C by 3 hydrogen bonds)
The backbone is sugar-phosphate; bases face inward
Pitch of helix = 3.4 nm; 10 bp per turn; distance between bp = 0.34 nm

Example 1

If a DNA molecule has 30% Adenine, find % of each base: A = T = 30%; G + C = 40%, so G = C = 20%. This uses Chargaff's rule: A% = T%; G% = C%; (A+T) + (G+C) = 100%.

DNA Packaging

Human DNA (approximately 6.6 x 10⁹ bp in diploid cells) must be packaged into a nucleus of about 10 micrometres. DNA wraps around histone octamers (H2A, H2B, H3, H4 — two each) to form nucleosomes. The nucleosome is the basic unit of chromatin. H1 histone links nucleosomes. Further coiling forms solenoid structures and higher-order chromatin.

DNA Replication

Proposed by Watson and Crick — semiconservative replication confirmed by Meselson and Stahl (1958) using N¹⁵/N¹⁴ density gradient centrifugation.

1.Steps:
2.Initiation: Helicase unwinds the double helix at origin of replication; single-strand binding proteins (SSBPs) stabilise strands; RNA primase lays short RNA primer
3.Elongation: DNA polymerase III adds dNTPs in 5' to 3' direction on each template strand
4.Leading strand: synthesised continuously (same direction as helicase movement)
5.Lagging strand: synthesised discontinuously as Okazaki fragments (short stretches, 5' to 3')
6.Termination: RNA primers removed by DNA polymerase I; gaps filled; Okazaki fragments joined by DNA ligase

Example 2

If a bacterial DNA has 1000 bp and undergoes 3 rounds of replication, the total number of DNA molecules = 2³ = 8. All will have one original strand and one new strand (semiconservative).

Example 3

Meselson-Stahl experiment: bacteria grown in N¹⁵ medium (heavy) then transferred to N¹⁴ medium. After one round of replication, all DNA was hybrid (intermediate density). After two rounds, two bands: hybrid and light. This proved semiconservative replication.

Transcription

DNA is transcribed into RNA by RNA polymerase. In prokaryotes, one RNA polymerase transcribes all RNA types. In eukaryotes:
RNA polymerase I: rRNA
RNA polymerase II: mRNA (hnRNA)
RNA polymerase III: tRNA, small RNAs

Steps: Initiation (RNA pol binds promoter), Elongation (reads template 3' to 5', synthesises RNA 5' to 3'), Termination.

In eukaryotes, the primary transcript (hnRNA) undergoes RNA processing:
5' capping: addition of 7-methyl guanosine cap
3' polyadenylation: addition of poly-A tail
Splicing: removal of introns (non-coding) and joining of exons (coding)

Example 4

A DNA template strand reads 3'-ATCGTA-5'. The mRNA transcribed will be 5'-UAGCAU-3' (U replaces T in RNA; complementary and antiparallel).

Translation

mRNA is translated into protein at ribosomes. The genetic code:
Triplet codons (64 total): 61 coding + 3 stop codons (UAA, UAG, UGA)
Code is degenerate (multiple codons for same amino acid), universal (same in all organisms), non-overlapping, and non-ambiguous
AUG: start codon (codes for Methionine)

Example 5

tRNA has anticodon 3'-UAC-5'. The codon on mRNA will be 5'-AUG-3' (start codon) coding for Methionine. Anticodon is complementary and antiparallel to the codon.

Example 6

If mRNA has 300 nucleotides (excluding start and stop codons), the protein has 300/3 = 100 amino acids.

Regulation of Gene Expression — lac Operon

Jacob and Monod proposed the operon model (Lac operon in E. coli):
Structural genes (lacZ, lacY, lacA): encode enzymes for lactose metabolism
Operator: binding site for repressor
Promoter: binding site for RNA polymerase
Regulator gene: codes for repressor protein
In absence of lactose (inducer): repressor binds operator → genes OFF
In presence of lactose: lactose (inducer) binds repressor, inactivating it → RNA polymerase transcribes structural genes → genes ON

Example 7

When glucose is absent but lactose is present, lac operon is maximally induced (CAP-cAMP also activates transcription). When both are present, glucose is preferred, lac operon is partially repressed.

Human Genome Project (HGP)

Completed in 2003; sequenced all ~3 billion bp of human genome. Found approximately 20,000-25,000 genes. Key tools: EST (Expressed Sequence Tags), DNA chips, bioinformatics. Revealed that only ~2% of genome codes for proteins; large portion is repetitive DNA.

Common mistakes

In transcription, RNA polymerase reads the template strand 3' to 5', producing RNA in the 5' to 3' direction.
Introns are removed (non-coding); exons are expressed (coding).
The genetic code is degenerate (multiple codons per amino acid) but non-ambiguous (one codon codes for only one amino acid).
DNA replication is semiconservative, not conservative or dispersive.

Summary

DNA is a double helix with complementary base pairing. Replication is semiconservative. Transcription produces RNA from DNA template; in eukaryotes, RNA is processed before translation. Translation at ribosomes converts mRNA codons into amino acids. Gene expression is regulated at the level of transcription (lac operon model).