Reproduction is a process by which organisms produce offspring or replicate themselves, occurring asexually (where individuals genetically identical to the parent are made) or sexually.
Sexual reproduction involves the fusion of gametes, i.e. when the sperm (male gamete) fertilizes an egg (female gamete) to produce offspring. 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. Thus if an offspring received a germ cell with a complete set of genetic material from each parent, they would have twice the amount of DNA as its parents!
Haploid (n) = 1 copy of each chromosome. In humans, that is 23 chromosomes.diploid(2n) = 2 copies of each chromosome. In humans, that is 46 chromosomes.
Thus, sexual reproduction includes two distinct processes:
The production of haploid gametes from a diploid adult, i.e., a cell containing half the number of chromosomes such as sperm and ova.This occurs through meiotic cell division.
The fusing of gametes at fertilization results in restoring the diploid number of chromosomes, i.e., the normal number of chromosomes – half from the mother and half from the father!
Meiosis can be defined as a 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 cell division unique to germ cells, while Meiosis II is similar to mitosis.
The differences between meiosis and mitosis are as follows: Source
Meiosis I begins after DNA replicates during the interphase of the cell cycle. 🧠 In diploid organisms, homologous chromosome pairs have the same type of genes but come from separate parents, i.e., one gene from each parental source.
Stage I/ Prophase I: The chromosomes condense, the nuclear envelope breaks down, and the homologous chromosomes pair up. The pairs of chromosomes may then exchange bits of DNA in a process called crossing over.
Stage II/ Metaphase I: Homologous chromosomes line up in pairs next to each other along the center of the cell.
Stage III/ Anaphase I: Homologous chromosomes in each pair separate and move towards opposite ends of the cell.
Stage IV/ Telophase I: A complete set of chromosomes at each cell pole is present, creating two new nuclei and two individual cells by cytokinesis. Each has a random assortment of chromosomes, one from each homologous pair. The DNA does not replicate between Meiosis I and Meiosis II.
🧠 Meiosis 1 is where the homologous chromosomes separate, and the resulting daughter cells each have a random combination of chromosomes – one from each homologous pair. In Meiosis I, homologous chromosomes are separated. Whereas in mitosis, sister chromatids are separated.
The daughter cells from Meiosis I go on to Meiosis II, which occurs in 4 phases – this time without DNA replication! This part of Meiosis results in the halving of the chromosomes and gives rise to 4 haploid cells.
Prophase II: The nuclear envelope breaks down, and the spindle forms.
Metaphase II: The sister chromatid of each chromosome lines up along the center of the cell.
Anaphase II: Sister chromatids separate into individual chromosomes and move towards opposite cell poles.
Telophase II: Spindles break, and the nuclear envelope forms around each group of chromosomes. Cytokinesis ensures the cytoplasm of each cell splits, and four haploid daughter cells are formed.
🧠 Each cell now has a unique combination of chromosomes genetically distinct from the parent cell that gave rise to them. Since the sister chromatids are dividing, Meiosis II is a lot like mitosis.
- In a sexually reproducing organism, the diploid organisms must create haploid cells that can fuse during fertilization to produce diploid offspring.
- Meiosis is a type of reduction division that helps in ensuring the germ cells/ gametes are haploid in number.
- Meiosis is also vital as it creates new DNA combinations in the daughter cell nuclei due to the crossover and random alignment of tetrads in Meiosis I.
- The cells created by Meiosis are genetically unique and result in some of the genetic variation found in sexual reproduction!
- Mitosis and meiosis share quite a few similarities and differences.
1. When does crossing over occur in meiosis?
Crossing over is the process during which two homologous non-sister chromatids pair up with each other and exchange different segments of genetic material to form two recombinant chromosome sister chromatids. It occurs during prophase I.
Also, crossing over can only occur between homologous chromosomes. Cells become haploid after meiosis I, and can no longer perform crossing over.
2. How is meiosis different from mitosis?
Mitosis results in two identical daughter cells, whereas meiosis results in four sex cells.
3. Meiosis occurs in what type of cells?
Meiosis occurs only in reproductive cells, resulting in the creation of haploid gametes that will be used in fertilization. It is also important to note that, meiosis occurs in a diploid organism, not haploid. The process of meiosis produces four haploid cells.
4. What advantage do organisms that reproduce sexually have over organisms that reproduce asexually?
The organisms that reproduce sexually are thought to have an advantage over asexually repoducing organisms as a wide variety of gene combinations are possbile in sexually repoducing organisms.
5. How are mitosis and meiosis similar?
Mitosis and meiosis are types of cell division that involve duplication of chromosomes, a cell’s DNA content. Meiosis II of meiotic division is similar to mitotic division except that each dividing cell has only one set of homologous chromosomes.
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- Sexual Reproduction, Meiosis, and Gametogenesis. https://humanbiology.pressbooks.tru.ca/chapter/5-11-sexual-reproduction-meiosis-and-gametogenesis/. Accessed 6 Dec, 2021.
- Meiotic Cell Division. https://www.shalom-education.com/courses/gcse-biology/lessons/cell-biology/topic/meiotic-cell-division/. Accessed 6 Dec, 2021.
- What is meiosis? https://www.yourgenome.org/facts/what-is-meiosis. Accessed Dec 6, 2021.