Chromosomes

Stretched end to end, the DNA in a single human cell would reach about 2 meters.

Two meters in a nucleus

A typical cell nucleus is only a few thousandths of a millimetre across, yet the DNA inside it is enormous. You can work it out from the last lesson. One set of human DNA has about 3 billion base pairs, and each pair adds 0.34 nm, so one set is about 3,000,000,000 × 0.34 nm, roughly 1 meter. Most body cells carry two sets, one from each parent, so the total is about 2 meters. NHGRI puts it as about 6 feet. Fitting that into a nucleus is a packing problem, and chromosomes are the answer.

A chromosome is one long DNA molecule together with proteins that hold it in shape. The DNA is wound around spool-like proteins called histones, a bit like thread on bobbins. OpenStax describes the next levels: the "beads on a string" of DNA and histones fold into a fibre about 30 nm wide, and before a cell divides the fibre is folded further until each chromosome is at its most compact, about 700 nm wide. Only then are chromosomes thick enough to see in an ordinary light microscope. The name comes from the Greek for colour and body, because chromosomes soak up certain dyes strongly.

23 pairs

Humans have 23 pairs of chromosomes, 46 in total. One chromosome of each pair comes from the mother and the other from the father, which is why children inherit some traits from each parent. Pairs 1 to 22 are called autosomes and are numbered roughly from largest to smallest. The 23rd pair is the sex chromosomes. NHGRI describes the usual pattern: females have two X chromosomes, while males have one X and one Y. The Y is much shorter than the X and carries far fewer genes.

Every chromosome has a pinched region called the centromere. Despite the name it is usually not in the exact middle. Before a cell divides, each chromosome is copied, and the two identical copies, called sister chromatids, stay joined at the centromere until they are pulled apart into the two new cells. At the ends of each chromosome are telomeres, repeated stretches of DNA that NHGRI compares to the plastic tips on shoelaces. In many cell types the telomeres get a little shorter at each division.

Halving and reshuffling

Eggs and sperm are the exception to "two of each". These reproductive cells, called gametes, carry just one copy of each chromosome, 23 in total. When an egg and a sperm join, the new cell has 46 again. Which chromosome of each pair ends up in a gamete is a matter of chance, decided pair by pair. With 23 pairs, one person can make 2 multiplied by itself 23 times, that is 2²³ = 8,388,608, different combinations of whole chromosomes, before even counting the swaps of pieces between partner chromosomes. That is a big reason brothers and sisters are not identical.

The number of chromosomes is fixed for each species. NHGRI lists a fruit fly with 4 pairs, a rice plant with 12 and a dog with 39. When a person has an extra or missing chromosome, development is often affected. Down syndrome, for example, happens when a person has three copies of chromosome 21 instead of two. Questions about one's own chromosomes belong with a doctor or genetic counsellor; this lesson is general information.