In DNA, Genes and Chromosomes you saw that a cell's DNA is packed into chromosomes. Before we watch cells divide, we need to count chromosomes carefully, because the counting is where most mistakes happen.
Key idea
A body cell of a species has a set number of chromosomes, in matching homologous pairs: one chromosome of each pair came from each parent. Humans have 46, so the diploid number is $2n = 46$ and the haploid number is $n = 23$. A copied chromosome is two sister chromatids joined at a centromere.
- Homologous chromosomes are the same size and carry the same genes in the same order, but they may carry different versions (alleles) of those genes.
- Sister chromatids are two identical copies made when the DNA is copied, still joined at the centromere.
- Diploid (2n): two full sets, as in body cells. Haploid (n): one set, as in gametes.
| Species | Diploid number (2n) | Haploid number (n) |
|---|---|---|
| Fruit fly | 8 | 4 |
| Corn | 20 | 10 |
| Cat | 38 | 19 |
| Human | 46 | 23 |
| Chimpanzee | 48 | 24 |
| Dog | 78 | 39 |
Why it works
How do you count chromosomes when each one may be doubled? Count centromeres. One centromere means one chromosome, whether it holds one chromatid (before copying) or two (after). So a human cell has 46 chromosomes both before and after its DNA is copied in S phase; what changes is the number of chromatids, from 46 to 92.
Worked example: Count it right
A fruit fly body cell ($2n = 8$) has just copied its DNA. How many chromosomes, chromatids and homologous pairs does it have?
- Copying does not change the number of centromeres: 8 chromosomes.
- Each chromosome is now two chromatids: $8 \times 2 = 16$ chromatids.
- Pairs: $8 \div 2 = 4$ homologous pairs.
Answer: 8 chromosomes, 16 chromatids, 4 pairs
Worked example: From 2n to n
A dog body cell has 78 chromosomes. How many chromosomes are in one of its gametes?
- Body cells are diploid: $2n = 78$.
- Gametes are haploid: $n = 78 \div 2 = 39$.
Answer: $39$
Common mistake
Counting each sister chromatid as its own chromosome, or thinking homologous chromosomes are identical copies. Count centromeres to count chromosomes. Sister chromatids are identical copies of one chromosome; homologs are a matched pair from two parents, with the same genes but possibly different alleles.