THE ESSENTIALS (memorize these before NEET):
PROOF OF DNA:
- Griffith (1928) → transformation; Avery (1944) → DNase only; Hershey-Chase (1952) → ^32P pellet, ^35S supernatant
DNA STRUCTURE:
- Right-handed double helix; antiparallel; A=T (2H); G≡C (3H); B-form is physiological
CHARGAFF: A=T, G=C → if A=x%, G = (50-x)%
NUCLEOSOME: ~200 bp DNA + octamer (2×H2A, H2B, H3, H4); H1 = linker (NOT in octamer)
REPLICATION:
- Semiconservative (Meselson-Stahl, 1958; CsCl gradient; ^15N/^14N)
- Direction: always 5'→3' synthesis (reads template 3'→5')
- Leading: continuous; Lagging: Okazaki fragments (5'→3' away from fork)
- Enzymes: Helicase → SSB → Topoisomerase → Primase → Pol III → Pol I → Ligase
TRANSCRIPTION:
- RNA Pol: NO primer needed; reads template 3'→5'; makes RNA 5'→3'
- Euk processing: 5' cap + poly-A + splicing (introns out, exons in)
GENETIC CODE:
- 64 codons = 61 sense + 3 stop (UAA, UAG, UGA)
- AUG = Start = Methionine
- Degenerate, universal, non-overlapping, non-ambiguous, comma-less
TRANSLATION:
- Ribosome: A (arrival) → P (peptide) → E (exit)
- Start: AUG + Met-tRNA + small subunit + mRNA → large subunit joins
- Peptidyl transferase = 23S rRNA (ribozyme)
LAC OPERON:
- lacI → repressor (always made); repressor binds OPERATOR (not promoter)
- Lactose → allolactose (true inducer) → binds repressor → operator freed → lacZ, lacY, lacA expressed
- Order: lacI — Promoter — Operator — lacZ — lacY — lacA
HGP: 3.2 × 10^9 bp; ~20,000–25,000 genes; <2% coding
DNA FINGERPRINTING: VNTRs + restriction enzymes + gel electrophoresis + Southern blotting