What are chromosomes and genes? Give three features of chromosomes and the chromosome make-up of a human body cell.

1. 1 · Define chromosome. A thread-like body in the nucleus made of one DNA molecule and proteins.
2. 2 · Define gene. A segment of the DNA of a chromosome; the unit of heredity.
3. 3 · Three features.
   • Found in pairs, one from each parent.
   • Seen only during cell division; otherwise a network called chromatin.
   • Fixed in number for each species.
4. 4 · Human make-up. 46 = 22 pairs of autosomes + one pair of sex chromosomes (XX or XY).
5. 5 · Close in one line. So chromosomes carry genes from parents to children.

Scoring tip: Learn 2n = 46 and n = 23; examiners often ask the number in a sperm or an egg.