Meiosis and the Cell Cycle for the ESAT

Many eukaryotic organisms, including humans, reproduce by sexual reproduction. This process requires a specific form of cell division called meiosis, which produces specialised reproductive cells known as gametes. These cells contain half the normal complement of chromosomes found in a standard body cell.

Sexual Reproduction and the Role of Gametes

In sexual reproduction, gametes formed by the parents fuse together at fertilisation to form a new individual called a zygote. This fusion combines the genetic material of both parents, creating an offspring that is genetically unique.

Chromosome Number and Fertilisation

When a male and a female gamete fuse during fertilisation, their nuclei combine. This means the total number of chromosomes in the zygote is the sum of those found in the two gametes. For the chromosome number to remain stable across generations, the gametes must carry only half the usual number of chromosomes.

In humans, an ovum (egg cell) contains $23$ chromosomes in its nucleus, and a sperm cell also contains $23$ chromosomes. When these gametes fuse, the resulting human zygote has $46$ chromosomes in its nucleus, organised into $23$ pairs.

Plants and animals whose zygotes contain two full sets of chromosomes are described as diploid. One chromosome from each pair originates from the father (via the sperm) and the other from the mother (via the ovum). This arrangement allows offspring to display characteristics inherited from both parents. Mature gametes, which contain only one set of chromosomes, are described as haploid.

The Meiotic Cell Cycle

To produce haploid gametes, cells undergo a meiotic cell cycle. This cycle begins with interphase, a period of preparation. During interphase, the cell grows in size, respires to provide the necessary energy for division, and replicates its DNA. Replication is vital because it ensures that a full set of chromosomes is available to be distributed among the daughter cells.

Following interphase, the cell enters meiosis, which involves two distinct cell divisions:

1. The first division separates the pairs of chromosomes.
2. The second division separates the replicated copies of each chromosome.

In most diploid species, this process results in four daughter cells, each with a single copy of each chromosome.

The Importance of Meiosis

Meiosis is essential for two main reasons:

• Reductional Division: It halves the total number of chromosomes per cell. This prevents the chromosome number from doubling every time fertilisation takes place.
• Genetic Variation: The gametes produced are genetically different from each other and from the parent cell. This creates genetic variation in the offspring, which is crucial for natural selection and the adaptation of species.

Comparing Mitosis and Meiosis

It is important to distinguish between the two types of cell division found in eukaryotes:

Characteristic	Mitosis	Meiosis
Number of daughter cells produced	$2$	$4$
Daughter cells genetically identical to parent cell?	Yes	No
Daughter cells genetically identical to each other?	Yes	No
Chromosome number in daughter cells	Same as parent cell (diploid)	Half the number of parent cell (haploid)
When used?	Growth, repair, and asexual reproduction	Gamete formation for sexual reproduction

Worked Example: Understanding Meiosis Outcomes

Exercise:

a. Name the daughter cells formed by meiosis. b. State two ways in which these daughter cells are different from the parent cell.

Solution:

a. The daughter cells are gametes, also known as sex cells (sperm and ova). b. Firstly, the daughter cells are haploid (containing $23$ chromosomes) whereas the parent cell is diploid (containing $46$ chromosomes). Secondly, the daughter cells are genetically unique, while the parent cell contains its own specific combination of genetic material.