Cell Division: Understanding Mitosis and Meiosis

Posted on Mar 5, 2025 in Biology

Mitosis

  1. Interphase: Visible nucleolus, chromosomes or diffuse chromatin.
  2. Prophase: Nucleolus disappears. Centrioles and asters appear and separate. The spindle forms, and each chromosome that constitutes two chromatids reappears together by the centromere.
  3. Metaphase: The nuclear membrane disappears, and the spindle extends to the poles. Chromosomes align on the spindle.
  4. Anaphase: Each centromere divides. Sister chromatids separate and go to opposite poles. The separated chromatids are now considered chromosomes.
  5. Telophase: Asters and spindle disappear. Nucleoli reappear, and the nuclear membrane forms. Cytokinesis starts.
  6. Interphase: Cytokinesis is complete. Chromosomes return to the interphase condition.

Meiosis

  1. Premeiotic Interphase: DNA duplicates.
  2. Prophase I: Synapsis of homologous chromosomes forms tetrads (crossing over occurs). The nuclear envelope degrades.
  3. Metaphase I: Tetrads line up on the equatorial plane.
  4. Anaphase I: Homologous chromosomes separate and go to opposite poles.
  5. Telophase I: The homologous chromosomes are at each pole. Cytokinesis begins.

Second Meiotic Division

  1. Prophase II: Chromosomes condense again.
  2. Metaphase II: Chromosomes align.
  3. Anaphase II: Sister chromatids separate and move to opposite poles.
  4. Telophase II: Nuclei form at the poles.
  5. Four haploid cells (gametes) are produced.

Fertilization and Meiosis

Fertilization is the means by which the genetic endowments of both parents combine, forming a new genetic identity for the progeny.

Meiosis is a special type of nuclear division in which chromosomes are redistributed, and cells are produced that have a haploid chromosome number (n). Each of the haploid cells produced by meiosis contains a unique complex of chromosomes due to crossover and random segregation of chromosomes. Thus, meiosis is a source of variability in offspring.

In general, meiosis consists of two nuclear divisions. The first division is reductional, i.e., it reduces the chromosome number by half. The second meiotic division is equational; it only separates the sister chromatids of each chromosome. The result is four cells, each of which will have a single or haploid number of chromosomes.

During meiosis, each diploid nucleus divides twice, producing a total of four cores. However, chromosomes are duplicated only once, before the first nuclear division. Therefore, each of the four nuclei produced contains half the number of chromosomes present in the original kernel.

Unlike what happens in meiosis, in mitosis, after duplication of chromosomes, each core is divided only once. Consequently, the number of chromosomes remains unchanged.

Meiosis, a special type of nuclear division, consists of two successive nuclear divisions, designated conventionally as meiosis I and meiosis II. During this process of dividing, chromosomes are redistributed, and cells with a haploid number of chromosomes (n) are produced.

During interphase preceding meiosis, the chromosomes are duplicated. While homologues are paired, crossing over occurs between them, resulting in the exchange of chromosomal material.

At the end of meiosis I, homologous chromosomes are separated. There are two cores, each with a haploid number of chromosomes. Each chromosome, in turn, is composed of two chromatids. The cores can go through a period of interphase, but the chromosomal material does not double.

In the second stage of meiosis, meiosis II, the chromatids of each chromosome separate, like in mitosis. When the two nuclei divide, four haploid cells form.