Chromosomes and the cell cycle

1One cell, then thirty million million

You began as a single cell, the zygote, made when a sperm fertilised an egg. A 2016 estimate puts the number of human cells in an adult man at about $3\times {10}^{13}3\; \backslash times\; 10^\{13\}$, thirty million million, and nearly every one of them carries a copy of the instructions that were in that first cell. Each time a cell divides, those instructions must be copied exactly and shared out fairly, or the new cells will be faulty.

Yet the sperm and the egg that made you each carried only half of a parent's set of instructions. So cells need two kinds of nuclear division: mitosis, which keeps the chromosome number the same, and meiosis, which halves it. Before you can follow either, you need the words that describe chromosomes and the cycle a cell goes through between one division and the next.

2What a chromosome looks like

For most of a cell's life its chromosomes are long, thin and uncoiled, spread through the nucleus as chromatin, too fine to see as separate threads. Before the cell divides, every chromosome is copied. The copy stays attached to the original at the centromere, so the chromosome now has two sister chromatids and looks like an X or a narrow H. It is still counted as one chromosome, because it has one centromere.

Figure 1: Chromosomes drawn as they look during division, when they are coiled up. A replicated chromosome (B) has two chromatids but one centromere. A homologous pair (C) is two separate chromosomes, one inherited from each parent.

3Every species has its own number

A human body cell has 46 chromosomes: 22 pairs of autosomes and one pair of sex chromosomes (XX in a female, XY in a male, as you saw in Senior 1). The X and Y differ in size, yet they still behave as a pair when gametes are made. Every species has a fixed diploid number, and because the chromosomes come in pairs, that number is almost always even.

Notice that a dog has more chromosomes than a person, and cattle more than a chimpanzee. The number of chromosomes tells you nothing about how complex an organism is; what matters is the genes they carry.

4The cell cycle

A cell that divides does not divide all the time. It spends most of its life growing and preparing, and only a short time dividing. The whole sequence, from the moment a cell is formed until it has divided into two, is the cell cycle. It has two main parts: a long interphase and a short division phase.

Figure 2: The cell cycle, read clockwise from the top. The slices are roughly to scale for a fast-dividing human cell grown in the laboratory, whose whole cycle takes about 24 hours; G1 alone takes about 11 of them, and the division phase only about an hour.

Three points about interphase are often tested:

• Interphase is not a resting phase. The cell is growing, making new organelles such as mitochondria, and copying its DNA.
• DNA is copied only in the S phase. After it, every chromosome has two sister chromatids and the cell holds twice as much DNA as before.
• Many specialised cells, such as nerve cells and heart muscle cells, leave the cycle after G1 and stop dividing. This non-dividing state is called G0.