1.3. Describe the process of meiosis

This guide will help you answer 1.3. Describe the process of meiosis.

Meiosis is a type of cell division that only occurs within the reproductive organs: the testes in males and the ovaries in females. Its purpose is to produce specialised cells called gametes, which are the sperm in males and the eggs (or ova) in females. These cells are essential for reproduction. What sets meiosis apart from regular cell division (mitosis) is its ability to reduce the number of chromosomes in these gametes by half, ensuring that when fertilisation occurs, the resulting zygote has the correct number of chromosomes.

What Are Chromosomes?

Before diving into meiosis, let’s clarify what chromosomes are. They are structures within the nucleus of a cell that contain DNA. DNA holds all of the genetic information needed for the body to function. In humans, most cells have 46 chromosomes arranged in 23 pairs. One chromosome of each pair comes from the mother, and the other comes from the father.

In contrast, gametes produced through meiosis have only 23 chromosomes—a single set. This ensures that when a sperm and egg combine, their chromosome numbers total 46.

Key Features of Meiosis

Meiosis has two distinct stages of cell division, known as Meiosis I and Meiosis II. Each of these stages has several steps. Importantly, before meiosis begins, every chromosome in the cell is duplicated during a phase called interphase.

The following points highlight the main goals of meiosis:

• Reduce the chromosome number by half.
• Create four genetically unique gametes.
• Increase genetic variation through processes like crossing over.

Meiosis I: The First Division

Meiosis I separates the homologous pairs of chromosomes (matching chromosomes from the mother and father), reducing the total number of chromosomes in the resulting cells by half.

Prophase I

This is one of the most important and longest phases in meiosis. Several things happen:

1. Chromosomes Condense: The chromosomes, which were loose strands of DNA within the nucleus, coil up tightly and become visible under a microscope.
2. Homologous Chromosomes Pair Up: Each chromosome aligns with its corresponding homologous chromosome. For example, chromosome 1 from the mother pairs with chromosome 1 from the father. These pairs are called bivalents.
3. Crossing Over Occurs: While the homologous chromosomes are paired, they exchange pieces of their DNA. This is called crossing over and results in new combinations of genetic material. It is a key source of genetic variation.
4. The Nuclear Envelope Breaks Down: The membrane surrounding the nucleus disappears, allowing the chromosomes to move freely within the cell.
5. Spindle Fibres Form: Structures called spindle fibres develop and attach to the chromosomes. These fibres will later help pull the chromosomes apart.

Metaphase I

The homologous chromosomes, still paired, align randomly at the centre of the cell. This alignment is significant because the random arrangement of maternal and paternal chromosomes leads to genetic variation. This process is called independent assortment.

Anaphase I

The spindle fibres pull the homologous pairs apart. Each chromosome of a pair moves to opposite ends (poles) of the cell. Note that the chromosomes themselves are still intact at this stage—they are not split into their individual halves yet.

Telophase I

At this stage, the chromosomes reach the opposite poles of the cell. A new nuclear envelope may start to form around these chromosomes at each end. The cell divides into two through a process called cytokinesis. Each of the two resulting cells now has half the original number of chromosomes (haploid), but each chromosome is still duplicated (consisting of two sister chromatids).

Meiosis II: The Second Division

Meiosis II is similar to mitosis, but there’s one key difference: the starting cells are haploid instead of diploid. Meiosis II separates the sister chromatids (the identical halves of a duplicated chromosome). No further duplication of chromosomes occurs before Meiosis II begins.

Prophase II

1. The nuclear envelope (if it reformed in Telophase I) breaks down again.
2. Spindle fibres reform and attach to the chromosomes.
3. The chromosomes condense, becoming tightly coiled and visible.

Metaphase II

The chromosomes line up individually at the centre of the cell, similar to what happens in mitosis. Remember, there’s only one copy of each chromosome in these cells now, not pairs like in Meiosis I.

Anaphase II

The spindle fibres pull the sister chromatids apart. Each chromatid is now considered a separate chromosome and moves to opposite poles of the cell.

Telophase II

The chromosomes arrive at opposite ends of the cell. New nuclear envelopes form around them. The cell undergoes cytokinesis, splitting into two. By the end of this phase, four haploid gametes are produced. Each gamete is genetically unique due to the processes of crossing over and independent assortment.

Significance of Meiosis

Meiosis plays a key role in sexual reproduction and species survival. Here are its main contributions:

• Reduces Chromosome Number: Without meiosis, fertilised eggs would have twice the number of chromosomes, leading to unviable offspring in most species. By reducing chromosome numbers in gametes, meiosis ensures stability.
• Creates Genetic Diversity: The processes of crossing over in Prophase I and independent assortment in Metaphase I shuffle genes between chromosomes, producing gametes with unique genetic makeups. This diversity ensures populations can adapt to changes in their environment over generations.
• Prevents Genetic Disorders: Proper meiosis reduces the likelihood of errors, such as too many or too few chromosomes in gametes. Conditions such as Down syndrome occur when meiosis does not proceed correctly, resulting in an extra chromosome.

Steps in Meiosis

To recap, meiosis involves two stages of division, and the steps follow this sequence:

Meiosis I:

• Prophase I
• Metaphase I
• Anaphase I
• Telophase I

Meiosis II:

• Prophase II
• Metaphase II
• Anaphase II
• Telophase II

The end result is four genetically diverse haploid gametes.

Final Thoughts

Meiosis is a complex but fascinating biological process that ensures genetic continuity and diversity across generations. By halving the number of chromosomes and introducing genetic variation, it lays the foundation for successful sexual reproduction. Without this process, evolution and adaptation in sexually reproducing organisms wouldn’t take place as we know them today.

Understanding meiosis will not only help you answer the assessment task but also enrich your knowledge of how life is sustained and how hereditary traits are passed on.