Heading Image Attribution: Doc. RNDr. Josef Reischig, CSc., CC BY-SA 3.0, via Wikimedia Commons
Introduction to Meiosis

Meiosis is a special type of cell division that occurs in sexually reproducing organisms, including plants, animals, and humans. It is a crucial process in creating gametes (sperm and eggs) with half the usual number of chromosomes, ensuring that the offspring will have the correct number of chromosomes when they are fertilized. The purpose of meiosis is to produce gametes (egg and sperm cells).

Meiosis involves two successive divisions, known as Meiosis I and Meiosis II, resulting in the formation of four genetically unique daughter cells, each with half the number of chromosomes as the parent cell. Here's a simplified explanation of meiosis:

Meiosis I:
  • Prophase I: Chromosomes condense and pair up with their homologous partner, forming a structure called a tetrad. This pairing allows for genetic recombination, where segments of DNA can be exchanged between the paired chromosomes, increasing genetic diversity.
  • Metaphase I: Tetrads align at the center of the cell.
  • Anaphase I: Homologous chromosomes separate and move to opposite poles of the cell.
  • Telophase I: Two daughter cells are formed, each with a unique combination of chromosomes.

Meiosis II:
  • Prophase II: The two daughter cells from Meiosis I prepare for the second division.
  • Metaphase II: Chromosomes align at the center of each daughter cell.
  • Anaphase II: Sister chromatids (the replicated copies of each chromosome) separate and move to opposite poles.
  • Telophase II: Four haploid daughter cells (gametes) are produced, each with a single set of chromosomes.
Meiosis
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​The final result of meiosis is the creation of four genetically distinct haploid cells (gametes), each containing half the number of chromosomes as the parent cell. These cells then differentiate into either egg cells in females or sperm cells in males. When fertilization occurs during sexual reproduction, one haploid sperm cell from the father combines with one haploid egg cell from the mother, resulting in a new diploid cell with the complete set of chromosomes (one set from each parent). This new cell then develops into a new individual with a unique combination of genetic traits inherited from both parents. Meiosis is essential for maintaining genetic diversity within a species and ensuring successful sexual reproduction.

Haploid vs. Diploid
A cell with two sets of chromosomes is diploid, referred to as 2n, where n is the number of sets of chromosomes. Most of the cells in a human body are diploid. A cell with one set of chromosomes, such as gametes, are haploid, referred to as n. Sex cells are haploid. When a haploid sperm (n) and a haploid egg (n) combine, a diploid zygote will be formed (2n). In short, when a diploid zygote is formed, half of the DNA comes from each parent.
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