Molecular Biology and Biotechnology
What you must know
DNA structure (double helix, sugar-phosphate backbone, complementary base pairing) and semi-conservative replication. The central dogma at school level: transcription in the nucleus producing mRNA, translation at the ribosome with tRNA carrying amino acids, and the properties of the genetic code (degenerate, non-overlapping, universal). Mutations: substitution, insertion and deletion, and their consequences for the protein produced.
Core techniques: PCR (denaturation near 95 degrees Celsius, primer annealing near 55, extension near 72 with Taq polymerase); gel electrophoresis (DNA is negatively charged and moves toward the positive electrode, and small fragments travel further); restriction enzymes, DNA ligase, plasmid vectors, transformation and marker genes; DNA profiling, gene therapy, stem cells, monoclonal antibodies and CRISPR. Candidates should interpret a gel or a sequencing result and discuss the ethical issues raised by biotechnology.
Traps and a worked example
Traps: confusing DNA polymerase with RNA polymerase; forgetting that PCR requires primers and a thermostable polymerase; assuming larger DNA fragments move faster through a gel; thinking transcription happens at the ribosome in eukaryotic cells.
Worked example: PCR doubles the number of target molecules each cycle, so copies = 2 to the power n. Starting from one molecule: 1, then 2, then 4, then 8 after three cycles; after 30 cycles about 1.07 x 10 to the power 9 copies. Rule of thumb: for 'how many cycles' questions, use powers of two and round up to the next whole cycle.