Meiosis

The Process of Meiosis

How exactly does one single parent cell produce four entirely unique sex cells?

• Meiosis occurs specifically in reproductive organs to produce gametes (sperm and eggs in animals; pollen and ovules in plants).
• Before the division begins, the cell must prepare by replicating its DNA. Every single chromosome duplicates to form two identical arms, called chromatids, which are joined together in an X-shape.
• After DNA replication, the cell undergoes two consecutive rounds of division.

In the first division, homologous pairs of chromosomes (one maternal and one paternal) line up in the centre and are pulled to opposite poles of the cell. During the second division, the double-armed chromatids are pulled apart.

• The final outcome is four genetically non-identical daughter cells.
• Each of these new cells contains exactly half the original set of chromosomes.

Restoring the Diploid Number

Millions of unique sperm can be produced in a single day, but only one will successfully reach the egg.

• Once a gamete is fully formed, its ultimate purpose is to undergo fertilisation, which is strictly defined as the fusion of the nuclei from a male and female gamete.
• Because meiosis has halved the genetic material, these gametes only carry one set of unpaired chromosomes ($n$).
• When these nuclei fuse, they combine their genetic material to form a single diploid cell called a zygote ($2n$).

This fusion successfully restores the full set of chromosomes, providing the zygote with two complete sets (one from each parent). After fertilisation is complete, the zygote will divide by normal mitosis to grow into an embryo.

Worked Example

How many chromosomes are present at each stage of reproduction in a mouse? (Diploid number of a mouse body cell = 40 chromosomes)

Step 1: Calculate the number of chromatids immediately after DNA replication.

• $40 \times 2 = 80$ chromatids

Step 2: Calculate the number of chromosomes in the haploid gametes (sperm/egg).

• $n = 40 \div 2$
• $n = 20$ chromosomes per gamete

Step 3: Calculate the chromosome number in the zygote after fertilisation.

• $n \text{ (sperm)} + n \text{ (egg)} = 2n \text{ (zygote)}$

• $20 + 20 = 40$ chromosomes in the zygote

Generating Genetic Variation

You share the same parents as your siblings, but unless you are identical twins, you all look completely different.

• This variation happens because meiosis guarantees every gamete is genetically unique through a shuffling of alleles.
• Alleles are different versions of the same gene, and they are mixed up in two main ways during the cell divisions.

First, homologous chromosomes can swap sections of DNA with each other, a process known as crossing over. This physically creates new combinations of alleles on the chromatids.

• Second, maternal and paternal chromosomes are distributed randomly into the new cells, which is called independent assortment.
• Finally, because any unique sperm can fuse with any unique egg, random fertilisation adds a massive third layer of genetic variation to the resulting offspring.