DNA, Genes, Chromosomes and RNA
The words, in order of size
- Genome — the entire DNA of an organism.
- Chromosome — a long molecule of DNA found in the nucleus of a cell; genes are located on chromosomes.
- Gene — a section of a molecule of DNA that codes for a specific protein.
So: nucleus → chromosomes → genes → sequence of bases.
Human body cells contain 46 chromosomes (23 pairs). Each chromosome carries hundreds of genes, always in the same position on that chromosome.
Learn the definition of a gene word-for-word. "A section of DNA" alone is not enough — you must say it codes for a specific protein. Do not write "codes for a characteristic".
DNA structure (separate award only)
A DNA molecule is two strands coiled to form a double helix. The two strands are held together by a series of paired bases:
| Base | Always pairs with |
|---|---|
| adenine (A) | thymine (T) |
| cytosine (C) | guanine (G) |
This is complementary base pairing. So if one strand reads A–C–G–T, the other strand reads T–G–C–A.
The order of the bases in a gene is the code — it determines which protein is made.
RNA (separate award only)
| DNA | RNA | |
|---|---|---|
| Strands | two strands, double helix | single stranded |
| Bases | A, T, C, G | A, U (uracil), C, G |
| Contains thymine? | Yes | No — uracil replaces it |
In RNA, uracil (U) pairs with adenine (A) instead of thymine.
Quick check
A DNA strand reads A T G C. The complementary RNA made from it reads U A C G.
Examiner tip. A small number of candidates each year state it the wrong way round — that RNA contains thymine and DNA contains uracil. Fix it with one sentence: T is for DNA, U is for RNA. Also, when asked to describe a DNA molecule, you must mention both the double helix and the specific base pairs (A–T, C–G); naming one of these only will not gain full marks.