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Beyond the textbook: how a labelled-nitrogen experiment is designed and what each model of replication predicts. How to reason about every mutant of the lac operon, how a change of one base changes a codon, and how to read a gel and count copies in PCR. Twelve harder problems and a full timed paper, at the level of A-Level Biology, IB Higher Level and the Olympiad.

What's inside

1. Designing and reading the classic experiments: A good experiment is designed by first writing down what each hypothesis predicts. Each model of replication predicts different bands in the tube of Meselson and Stahl, and heating the DNA to separate the strands tests the dispersive model again. Taylor's labelled chromatids halve in number at each division. The purification of the transforming principle is followed by its activity for each milligram.
2. Operon logic and the effect of mutations: The repressor of the lac operon acts in trans, on any copy of the operator in the cell. A damaged operator or promoter acts in cis, only on the DNA that carries it. These rules give the result for every mutant, alone and in a partial diploid. A change in the third base of a codon is usually silent. A frameshift reads almost random codons until a stop codon appears.
3. Reading a gel and counting copies in PCR: DNA is negative, so in a gel it runs to the positive end, and the distance is a straight line against the logarithm of the size. A ladder of known sizes gives the size of an unknown band. Restriction enzymes cut a ring into as many pieces as there are cuts. PCR copies a target by cycles of heating, annealing of primers and extension by a heat-stable polymerase, and the copies double in each cycle.