Hershey-Chase (asked almost every year):

• ^32P = DNA label; found in pellet = DNA enters bacteria
• ^35S = protein label; found in supernatant = protein stays outside
• Phage T2 used; technique: blending + centrifugation

Replication directionality (asked very frequently):

• DNA Pol III: synthesizes 5'→3'; reads template 3'→5'
• Leading strand: continuous toward fork; Lagging strand: Okazaki fragments away from fork
• RNA Pol: NO primer; reads 3'→5'; synthesizes 5'→3'

Lac operon (asked frequently, with traps):

• Inducer = allolactose (NOT lactose itself)
• Repressor binds OPERATOR (not promoter)
• Structural gene order: lacZ-lacY-lacA
• lacZ = beta-galactosidase; lacY = permease; lacA = transacetylase
• Super-repressor (lacI mutation, cannot bind inducer) = genes constitutively OFF

Meselson-Stahl (calculation-type questions):

• Gen 1: ALL hybrid density (1 band)
• Gen 2: 50% hybrid + 50% light (2 bands, equal amounts)
• After n generations: 2 hybrid molecules + (2^n − 2) light molecules

Nucleosome (factual, frequently tested):

• Octamer: 2×H2A + 2×H2B + 2×H3 + 2×H4 = 8 histones
• H1 = linker histone (NOT in octamer)
• ~147 bp wrapped around core; ~200 bp total per nucleosome

Genetic code (conceptual + factual):

• 64 codons = 61 sense + 3 stop (UAA, UAG, UGA)
• AUG = start + methionine
• Non-overlapping: AUGCGA → AUG-CGA (2 codons, not 7)
• Degenerate but non-ambiguous

HGP numbers (always asked as MCQ):

• 3.2 × 10^9 base pairs; ~20,000–25,000 genes; <2% coding