Pocket Notes — Molecular Basis of Inheritance

DNA numbers (must memorise)

• Length between base pairs: 0.34 nm
• Pitch (one full turn): 3.4 nm
• Base pairs per turn: 10
• Diameter: 2 nm
• Human genome size: ~3.1 × 10⁹ bp
• Total DNA length in one human cell: ~2.2 m
• Human gene count: ~30,000

Base pairs

• A = T (2 H-bonds)
• G ≡ C (3 H-bonds)

Chargaff's rules

• A ≈ T · G ≈ C · Purines = Pyrimidines · A+G ≈ T+C

Landmark experiments

Year	Scientists	Finding
1928	Griffith	Transforming principle in S. pneumoniae
1944	Avery, MacLeod, McCarty	Transforming principle is DNA
1952	Hershey & Chase	³²P/³⁵S bacteriophage → DNA is genetic material
1953	Watson & Crick	Double-helix model
1958	Meselson & Stahl	Semi-conservative DNA replication
1961	Jacob & Monod	Lac operon — gene regulation
1985	Alec Jeffreys	DNA fingerprinting (VNTRs)
1990-2003	HGP	Human Genome Project

Replication enzymes (mnemonic: HP-PL-T)

• Helicase — unwinds
• Primase — RNA primer
• Polymerase — 5'→3' synthesis
• Ligase — Okazaki joins
• Topoisomerase — supercoil relief

Leading vs Lagging strand

• Leading — 5'→3', continuous.
• Lagging — 3'→5' template; synthesised as Okazaki fragments, joined by ligase.

Central dogma

DNA → transcription → RNA → translation → protein (Reverse transcription: RNA → DNA in retroviruses.)

Transcription

• Only template strand (3'→5') is read.
• Coding strand = same sequence as RNA (except T→U).
• Prokaryotes: 1 RNA pol · Eukaryotes: 3 RNA pols (I: rRNA · II: mRNA · III: tRNA & small RNAs).

Eukaryotic mRNA processing

1. 5' Capping — 7-methyl guanosine
2. 3' Polyadenylation — poly-A tail
3. Splicing — introns removed by spliceosome

Genetic code properties

• Triplet · Degenerate · Universal · Non-overlapping · Comma-less · Unambiguous.
• Start: AUG (Met/fMet).
• Stop: UAA, UAG, UGA.

tRNA

• Cloverleaf 2D · L-shape 3D.
• Anticodon loop + 3'-CCA for aa attachment.
• Charged tRNA = tRNA + aa (aminoacyl-tRNA synthetase).

Ribosome sites

• A (aminoacyl) · P (peptidyl) · E (exit)
• 23S rRNA (prokaryotic large subunit) is the peptidyl transferase — a ribozyme.

Lac operon (Jacob & Monod)

• Structural: z (β-gal), y (permease), a (transacetylase).
• Regulator (i) gene → repressor.
• Inducer: lactose (allolactose form) removes repressor.

HGP quick facts

• Duration: 1990–2003.
• Genome size: ~3.1 × 10⁹ bp.
• Gene count: ~30,000.
• Chr with most genes: Chromosome 1.
• Chr with fewest: Y.
• Coding DNA: ~2%.
• Two humans share 99.9% DNA.

DNA fingerprinting steps

1. DNA isolation
2. Digestion with restriction enzymes
3. Gel electrophoresis by size
4. Southern blotting onto membrane
5. Hybridise with VNTR probe
6. Autoradiography — band pattern = fingerprint

Common MCQ traps

• Which base has 3 H-bonds? G ≡ C
• Which strand is used as template in transcription? 3'→5' (antisense/template)
• Which enzyme has ribozyme activity? 23S rRNA (peptidyl transferase)
• Does DNA pol work 3'→5'? — No, always 5'→3'