Chromosomes: How Two Metres of DNA Fits Inside a Cell — and Why the Packaging Matters
Chromosomes are not merely storage — the way DNA is packaged controls which genes are active. Five levels of compaction from double helix to metaphase chromosome, the human karyotype of 23 pairs, and why telomere length is a biological clock.
TL;DR
Nucleosomes, 30nm chromatin fibre, loop domains, metaphase chromosome. 46 chromosomes, 23 pairs, autosomes 1-22 plus sex pair. Centromere, telomere, origins of replication. Euchromatin vs heterochromatin, histone modifications, telomerase in cancer.
If DNA is the text, chromosomes are the books. Each chromosome is a single, enormously long DNA molecule wrapped and organised into a compact structure that can be replicated, sorted, and delivered to daughter cells with extraordinary precision. The way DNA is packaged is not incidental to gene function — it is itself a layer of the genetic code.
From naked DNA to chromosome
The two metres of DNA in a human cell must be compacted into a nucleus about six micrometres across — a compression ratio of roughly 50,000:1. This is achieved through a hierarchy of packaging. DNA first wraps twice around protein spools called histones, forming structures called nucleosomes — the basic unit of chromatin, resembling beads on a string. The nucleosome chain is then folded and coiled through successive levels of higher-order organisation until the full chromosome is formed.
Human chromosomes: 23 pairs
Humans carry 46 chromosomes in 23 pairs. For each pair, one chromosome came from the mother's egg and one from the father's sperm. Pairs 1 through 22 are called autosomes — they are present in both males and females in matching pairs. The 23rd pair is the sex chromosomes: females carry two X chromosomes (XX), males carry one X and one Y (XY). The chromosome number, shape, and banding pattern together form the karyotype — a cellular signature of the genome's structural organisation.
The centromere, telomeres, and chromosome architecture
Every chromosome has three functionally essential elements. The centromere is a constricted region where the two sister chromatids are joined after DNA replication and where the kinetochore — the attachment point for the spindle fibres that pull chromosomes apart during cell division — assembles. The telomeres are repetitive DNA sequences (TTAGGG repeated thousands of times) that cap each chromosome end, protecting it from degradation and preventing chromosome ends from being recognised as DNA breaks. Origins of replication are the sites where DNA replication initiates along each chromosome.
Telomere length is now understood as a biological clock of cellular ageing. Each round of DNA replication shortens the telomere slightly — because the replication machinery cannot copy the very end of a linear chromosome. When telomeres shorten to a critical length, cells cease dividing. Telomerase, an enzyme that extends telomeres, is active in stem cells and germline cells but largely absent from differentiated somatic cells — which is why most adult cells have a finite replicative lifespan. Cancer cells, notably, reactivate telomerase, achieving the immortality of uncontrolled division.