- Understand what haploid cells are, and the role that meiosis plays in giving rise to haploid cells for sexual reproduction.
- Understand the different phases of meiosis and the events that occur at each stage. and their importance.
- Understand the importance of meiosis especially in creating new DNA combinations.
Reproduction is a process by which organisms produce offspring or replicate themselves, and occurs in the following ways:
- Asexual reproduction produces individuals that are genetically identical to the parent
- Sexual reproduction occurs through the fusion of gametes, i.e., when the sperm (male gamete) fertilises an egg (female gamete) to produce offspring
Sexual reproduction includes two distinct processes:
- The production of haploid gametes ie. a cell containing half the number of chromosomes such as sperm and ova, occurs through meiotic cell division
- The fusion of gametes at fertilisation results in restoring the diploid number of chromosomes, i.e., the normal number of chromosomes – half from the mother, and half from the father!
Each human cell has 46 chromosomes in 23 pairs. These pairs are made up of half a set of chromosomes from each parent, i.e., 23 chromosomes from each parent
- These 23 pairs of chromosomes are said to be homologous.
Haploid cells/ gametes that are part of the sexual reproductive cycle are produced by a cell division called meiosis.
- Meiosis can be defined as a type of cell division that results in four daughter cells, each with half the number of parent cell chromosomes.
- Meiosis can be divided into meiosis I and meiosis II. Meiosis I is a type of cell division unique to germ cells, while meiosis II is similar to mitosis.
Meiosis I begins after DNA replicates during the interphase of the cell cycle. The resulting daughter cells each have a random combination of chromosomes – one from each homologous pair. Let’s look at what happens in the each stage of Meiosis I.
- The chromosomes condense, and the nuclear envelope breaks down.
- The centrioles move apart, and the spindle begins to form.
- Homologous chromosomes pair up.
- The pairs of chromosomes may then exchange bits of DNA in a process called crossing over.
- Homologous chromosomes line up in pairs next to each other along the center of the cell.
- The spindle fibers attach to one chromosome of each pair.
- Homologous chromosomes in each pair separate and move towards opposite ends of the cell as they are pulled apart by the meiotic spindles.
- The spindles break, and a complete set of chromosomes at each cell pole is present.
- A nuclear envelope forms around each group of chromosomes to create two new nuclei.
- Each has a random assortment of chromosomes, with one from each homologous pair.
- The DNA does not replicate between meiosis I and meiosis II.
- The single-cell pinches in the middle, and the cytoplasm splits, forming two haploid daughter cells.
The daughter cells from Meiosis I go on to Meiosis II, which occurs in 4 phases – this time without the replication of DNA! This part of meiosis results in the halving of the chromosomes and gives rise to 4 haploid cells.
- The nuclear envelope breaks down, and the spindle forms. The centrioles also start to separate.
- Sister chromatid of each chromosome lines up along the center of the cell.
- Sister chromatids separate and move towards opposite poles of the cell.
- Spindles break, and the nuclear envelope forms around each group of chromosomes.
- The cytoplasm of each cell splits, and four haploid daughter cells are formed. Each cell now has a unique combination of chromosomes.
- In a sexually reproducing organism, it is essential that the diploid organisms create haploid cells that can fuse during fertilisation to produce diploid offspring.
- Meiosis being a type of reduction division helps in ensuring the germ cells/ gametes are haploid in number.
- Meiosis is also important as it creates new DNA combinations in the daughter cell nuclei due to the crossover in prophase I and random alignment of tetrads at metaphase I.
- The cells that are created by meiosis are genetically unique and as a whole result in some of the genetic variation found in sexual reproduction!
1. What is meiosis?
Meiosis is a process where a single parent cell is divided into four daughter cells containing half of the original genetic material, i.e., the DNA.
2. What is the simple definition of meiosis?
Meiosis can be defined as cell division from a parent cell to form four cells in two stages. These cells contain half of the genetic materials.
3. What is the difference between meiosis and mitosis?
Meiosis consists of forming four daughter cells from one parent cell through two stages, whereas, in mitosis, cell division occurs, forming two identical daughter cells.
4. What are the 9 stages of meiosis?
The nine stages of meiosis are:
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I
- Prophase II
- Metaphase II
- Anaphase II
- Telophase II
5. What happens during pachytene?
In pachytene, each quadruple thickens, shortens, and is separated into four individual chromatids joined at the centromere. It is also the stage where homologous recombination occurs — for example, the chromosomal crossover between two non-sister chromatids.
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- Sexual Reproduction, Meiosis, and Gametogenesis. https://humanbiology.pressbooks.tru.ca/chapter/5-11-sexual-reproduction-meiosis-and-gametogenesis/. Accessed Nov 3, 2021.
- Meiotic Cell Division. https://www.shalom-education.com/courses/gcse-biology/lessons/cell-biology/topic/meiotic-cell-division/. Accessed Nov 3, 2021.
- What is meiosis? https://www.yourgenome.org/facts/what-is-meiosis. Accessed Nov 3, 2021.