01 · Structure
From DNA to chromosome: four levels of packaging
Each human chromosome contains a single DNA molecule up to ~250 Mb long (~8.5 cm if stretched out). To fit into a nucleus 6 µm across, DNA is compacted ~10,000-fold through four levels of organisation:
Double helix
Raw DNA molecule, 2 nm wide. The familiar Watson-Crick double helix. See DNA glossary entry.
Nucleosome
~147 bp of DNA wrapped ~1.75 times around an octamer of 8 histone proteins. The "beads on a string" structure, 11 nm wide. Compacts ~6–7×.
30 nm chromatin fibre
Nucleosomes coil into a solenoid (6 nucleosomes/turn). Additional ~40× compaction. Linker histone H1 stabilises this level.
Metaphase chromosome
Further looping around a protein scaffold. The final ~10,000× compacted structure visible at metaphase — two sister chromatids joined at the centromere.
Chromatin is not merely structural packaging — it regulates gene expression. Histone modifications (acetylation, methylation, phosphorylation) alter how tightly DNA is packaged, determining which genes are accessible for transcription. This is the basis of epigenetics.
02 · Karyotype
Chromosome numbers across species
| Feature | Human | Fruit fly | Potato | Wheat |
|---|---|---|---|---|
| Diploid number (2n) | 46 | 8 | 48 | 42 |
| Haploid number (n) | 23 | 4 | 24 | 21 |
| Sex determination | XY / XX | XY / XX | None (asexual) | None (hermaphrodite) |
| Genome size (Mb) | 3,200 | 140 | 844 | 17,000 |
Chromosome number is not a reliable indicator of organism complexity. Adders' tongue fern has 2n = 1260; dogs have 2n = 78. Chromosome number is a species characteristic; having more chromosomes does not mean more genes or greater complexity.
03 · Cell division
Chromosomes in mitosis and meiosis
Chromosomes become visible under a light microscope only when compacted during cell division. The rest of the cell cycle they exist as diffuse chromatin — though individual chromosomes occupy distinct territories within the nucleus even then.
In mitosis, each chromosome is duplicated during S phase (forming two identical sister chromatids joined at the centromere). During M phase, the spindle pulls sister chromatids to opposite poles, producing two daughter cells with identical chromosome complements. See Mitosis and meiosis for the full phase-by-phase account.
In meiosis, homologous chromosome pairs (one from each parent) first pair up (synapsis) and exchange segments via crossing over (recombination). This creates new combinations of alleles on each chromosome — a major source of genetic variation. Two rounds of division produce four haploid gametes, each with a unique combination of chromosomes. This is the chromosomal basis of Mendel's laws.
Chromosome — common questions
- Humans have 46 chromosomes (diploid, 2n = 46): 22 autosome pairs and XX or XY. Gametes are haploid (n = 23). Fertilisation restores the diploid number. Karyotyping — see mitosis and meiosis — visualises chromosome number and structure to detect aneuploidy (e.g. trisomy 21, Down syndrome).
- A chromosome is a single long DNA molecule + histone proteins (chromatin). A gene is a region of that DNA with a specific sequence that is transcribed. Each human chromosome carries hundreds to thousands of genes. The locus of a gene is its specific chromosomal location. See DNA, genes and heredity.
- Mitosis: DNA replication → sister chromatids → aligned at metaphase plate → pulled to poles → two identical diploid daughters. Meiosis: two rounds of division — meiosis I separates homologues; meiosis II separates sister chromatids → four haploid gametes. Crossing over in prophase I recombines alleles on homologous chromosomes. See Mitosis and meiosis.
- Non-disjunction in meiosis I or II produces gametes with extra or missing chromosomes. Examples: trisomy 21 (Down syndrome), 47,XXY (Klinefelter syndrome), 45,X (Turner syndrome). Structural abnormalities — translocations, deletions — are often associated with cancer (mutations at breakpoints can activate oncogenes). See Mitosis and meiosis.