Molecular Biology Fundamentals and Nucleic Acid Chemistry
Core concepts
Candidates must be fluent in the structure and physical chemistry that underpins every downstream assay: Watson-Crick base pairing and hydrogen-bond counts (G≡C three, A=T two), base stacking as the dominant stabilizing force, the antiparallel double helix, and the distinction between the major and minor grooves. Know the difference between the phosphodiester backbone (covalent, nuclease-labile) and the hydrogen-bonded base pairs (noncovalent, denaturable). Understand Tm and the variables that shift it: length, GC content, monovalent salt (higher salt raises Tm), denaturants (formamide, urea lower it), and mismatches (each mismatch lowers Tm by roughly 1-1.5 °C).
Nucleic acid chemistry to know
Distinguish DNA from RNA chemistry: the 2'-OH of ribose makes RNA susceptible to alkaline hydrolysis and gives it an A-form helix; DNA adopts B-form. Know the common modified bases (5-methylcytosine, 5-hydroxymethylcytosine, bisulfite conversion chemistry) and the effect of uracil-DNA glycosylase (UNG) contamination control. Understand polymerase enzymology at a conceptual level: 5'→3' synthesis, exonuclease activities (proofreading 3'→5'; nick translation 5'→3'), and why hot-start and reverse transcriptase enzymes differ.
Traps
Questions exploit confusion between denaturation and degradation, between Tm and annealing temperature, and between Cq/Ct and absolute quantity. Watch for items where a high-GC target demands a higher denaturation temperature or where secondary structure (hairpins, G-quadruplexes) suppresses amplification.
Rule of thumb
For a quick Tm estimate of a short oligo, use the Wallace rule (2 °C per A/T, 4 °C per G/C) and remember it underestimates for longer probes; for a longer duplex, use nearest-neighbor thermodynamics. When a question asks why a GC-rich amplicon fails to melt, suspect incomplete denaturation before suspecting the primers.
Worked example
A 20-mer with 12 G/C and 8 A/T: Wallace Tm = (12×4)+(8×2)=64 °C. In 50 mM salt with a single internal mismatch, expect roughly 62-63 °C; design annealing ~5 °C below the lowest primer Tm.