How Meiosis Creates Genetic Variety

Before meiosis begins, the cell copies its DNA so every chromosome consists of two identical sister chromatids. Matching chromosomes then pair up. These homologous chromosomes carry the same kinds of genes, although they may contain different versions of those genes.

The cell completes two divisions. During the first division, homologous chromosome pairs separate into different cells. During the second division, the sister chromatids separate. One starting cell usually produces four cells, each containing one chromosome from every original pair.

These final cells are haploid, meaning they contain half the chromosome number of the original diploid cell. In humans, most body cells have 46 chromosomes, while sperm and egg cells have 23. When fertilisation joins two haploid cells, the diploid number is restored.

Meiosis does more than reduce chromosome number. It also creates variation. When homologous chromosomes pair, matching sections can exchange pieces in a process called crossing over. The chromosomes that separate later may therefore contain new combinations of genetic material.

The chromosome pairs also line up independently during the first division. Each pair can face either direction, so the chromosome inherited from one parent does not always travel with the same set of other chromosomes. This independent assortment creates many possible combinations in the resulting cells.

The exact cells produced are shaped by chance. That does not make meiosis disorganised. The stages are carefully controlled, but several valid arrangements are possible. Biology can be orderly while still refusing to make siblings identical. Fertilisation adds another layer of chance because any one sperm may join any one egg.

Variation gives populations a wider range of inherited traits. If conditions change, some individuals may have combinations that help them survive and reproduce. Meiosis therefore contributes raw material for evolution, although natural selection acts on the resulting differences over generations.

Keep the main sequence clear:

• •DNA is copied before meiosis begins.
• •Homologous chromosomes pair and may exchange sections.
• •The first division separates chromosome pairs.
• •The second division separates sister chromatids.
• •Four genetically varied haploid cells are produced.

Independent assortment alone gives a human cell 2 to the power of 23 ways of distributing its chromosome pairs, about 8.4 million combinations, before crossing over multiplies that again. Combine two parents and the number of possible children is effectively unlimited, which is why full siblings can look so different while identical twins, who come from one fertilised egg, do not.

The process can also fail. If a pair of chromosomes does not separate during the first division, or sister chromatids stay together in the second, a sex cell ends up with one chromosome too many or too few. This is called nondisjunction. A fertilised egg with three copies of chromosome 21 develops with Down syndrome, and most other whole-chromosome errors are not survivable at all. The risk rises with the age of the egg, which is one reason the topic matters beyond the exam.

The textbook picture of one cell giving four equal sex cells fits sperm production well. In males, meiosis runs continuously from puberty onward, and each starting cell produces four sperm. Egg production is very different. The divisions are deliberately unequal: almost all the cytoplasm goes to one cell, while the spare sets of chromosomes are pushed into tiny cells called polar bodies, which usually break down. The result is one large egg, packed with the nutrients and cell machinery an early embryo needs, rather than four small ones.

The timing is unusual too. In a female foetus, egg cells begin meiosis before birth and then pause partway through the first division, with homologous chromosomes still paired. They can stay paused for decades. Each month, one cell resumes the first division around ovulation, and the second division is completed only if a sperm fertilises it. Because the machinery holding paired chromosomes together has to last for so long, it gradually weakens with age. This is one of the main explanations for why chromosome separation errors become more common in eggs from older women.

Meiosis was observed under the microscope before anyone understood genes. In 1883 the Belgian biologist Edouard van Beneden, studying a parasitic roundworm with conveniently few, large chromosomes, saw that the egg and sperm each contributed half of the chromosomes found in the fertilised egg. Around 1902 and 1903, Walter Sutton, working with grasshoppers, and Theodor Boveri, working with sea urchins, connected this to Mendel's newly rediscovered results. Chromosomes came in pairs, separated into different sex cells and paired again at fertilisation, exactly the way Mendel's hereditary factors seemed to behave. This idea became known as the chromosome theory of inheritance.

Crossing over then became a tool for mapping genes. In 1913 Alfred Sturtevant, an undergraduate in Thomas Hunt Morgan's fruit fly laboratory, reasoned that two genes far apart on a chromosome are more likely to be separated by a crossover than two genes close together. By measuring how often pairs of traits were inherited separately, he arranged several genes in a line along the X chromosome, producing the first genetic map. Geneticists still honour the idea in a unit of map distance, the centimorgan, which corresponds to a one percent chance of crossing over between two points in a single generation.

Crossing over is often presented only as a way of mixing genes, but in most species it has a second, mechanical job. Where two homologous chromosomes have exchanged pieces, they stay physically linked at a point called a chiasma, held by the protein rings that bind sister chromatids together along their arms. That link lets spindle fibres pull the two chromosomes of a pair towards opposite ends of the cell, creating tension that the cell can detect. A pair with no crossover has nothing holding it together and is much more likely to separate wrongly, producing sex cells with missing or extra chromosomes.

This is why cells seem to guarantee at least one crossover for nearly every chromosome pair, often called the obligate crossover. A typical human egg or sperm carries the results of a few dozen crossovers, with more on average in eggs than in sperm. Even the very different X and Y chromosomes in males pair and cross over, at small matching regions near their tips. Some species have found another way. Male fruit flies make no crossovers at all, yet still separate their chromosomes correctly using special pairing proteins, which is one reason geneticists measure crossing over in female flies.

Meiosis produces haploid sex cells through two divisions and creates variation through crossing over and independent assortment. Follow what happens to chromosome pairs first, then what happens to sister chromatids, and the process becomes much easier to picture.