Sec 4 Combined Science (Biology) Molecular Genetics notes

The three terms nest inside each other. DNA (deoxyribonucleic acid) is the molecule — a very long thread of coded instructions. A gene is one section of that thread: one instruction, coding for one polypeptide. A chromosome is a whole stretch of DNA thread, tightly packaged, visible in the nucleus — a human body cell holds 46 chromosomes, in 23 pairs, one of each pair from each parent. So: DNA is the book, a gene is one recipe, and a chromosome is a bound chapter of recipes.

DNA is a double helix: two strands twisted around each other like a ladder spun into a spiral. Each strand is a chain of nucleotides, and every nucleotide has exactly three parts: a sugar, a phosphate group and one of four bases — adenine (A), thymine (T), cytosine (C) and guanine (G). In each strand the sugars and phosphates form the twisted backbone, and the bases point inwards, where the two strands meet. The base is the information: which of the four sits at each position is what spells the code.

Only four letters — but with millions of positions per chromosome, four letters are enough to write every recipe in every living thing, the way two symbols suffice for all of computing.

The two strands are held together by their bases, which pair by strict shape rules: adenine pairs only with thymine (A–T), and cytosine pairs only with guanine (C–G). Each pair is called complementary — one strand's letters fully determine the other's. Read one strand as A–T–G–C–A–A and you can write the other: T–A–C–G–T–T, no guessing required.

This rule has two consequences. First, the two strands carry the same information twice — a backup. Second, DNA can be copied faithfully: each strand can rebuild its partner, letter by letter, which is how the code passes intact to new cells and to offspring.

A gene is a sequence of nucleotides that codes for one polypeptide — one chain of amino acids, which folds into a working protein — and it is the unit of inheritance, the packet that passes from parent to offspring. DNA therefore carries the genetic code: the order of bases spells the order of amino acids in the polypeptide. A different gene carries a different code, so a different polypeptide is built — haemoglobin from one gene, a digestive enzyme from another, each protein doing its own job.

No machinery details are needed at Combined level — the syllabus stops at "the code is used to synthesise specific polypeptides". What matters is the chain of logic: base sequence → code → polypeptide → protein → the traits proteins produce, from eye colour's pigments to the enzymes that digest lunch.

Genetics questions love three moves. Move one: percentages — given any base's percentage, the pairing rule delivers all four (A = T and C = G). Move two: complementary strands — rewrite one strand using A↔T and C↔G. Move three: gene-to-protein logic — explain that a change in the gene's base sequence means a different polypeptide is coded for. All three are rule application, not memory tests — the syllabus deliberately stops before the cell's copying machinery.