The nucleus and chromosomes

The nucleus is the cell's control centre and stores its genetic information. It is wrapped in a double membrane (the nuclear envelope). The outer membrane continues into the ER. Nuclear pore complexes, built from proteins called nucleoporins, let RNA and proteins pass in and out. A mesh of fibres inside, the nuclear lamina, gives the nucleus strength.

The fluid inside, the nucleoplasm, holds a framework called the nuclear matrix, where the enzymes that copy DNA (replication) and read it into RNA (transcription) are found. The nucleolus has no membrane; it makes rRNA and assembles ribosomes at the nucleolar organising region.

A chromosome is DNA coiled with proteins. Humans have 23 pairs: 22 pairs of autosomes and one pair of sex chromosomes, 46 in all. In 1923 Painter counted 48; in 1956 Tjio corrected it to 46.

Prokaryotes usually have one circular chromosome, lying in a region called the nucleoid (though some, like Vibrio cholerae, the cholera bacterium, have more than one). In eukaryotes, DNA is wrapped around proteins called histones, forming bead-like nucleosomes, which coil into chromatin fibres. This packing matters: unwound, the DNA of one human cell would stretch to about 6 feet (nearly 2 metres).

Before a cell divides, the chromatin condenses into thick, visible chromosomes. Errors in their number or structure can cause serious problems.