Cell Division: Mitosis and Meiosis Processes

Posted on Nov 21, 2024 in Biology

Mitosis

Cell division involves two sequential processes: mitosis (nuclear division) followed by cytokinesis (cytoplasm division). In mitosis, genetic material is equally distributed, ensuring each daughter cell receives identical information. Chromosome division occurs during the S phase.

Mitosis phases:

  • Prophase: Nuclear envelope and nucleolus disappear. Chromatin condenses into visible chromosomes, each with two chromatids joined at the centromere. The spindle forms. In animal cells, centrioles duplicate, forming centrosomes that migrate to opposite poles.
  • Metaphase: The mitotic spindle is fully developed. Chromosomes align at the cell’s equatorial plane. Microtubules from kinetochores attach to chromosomes.
  • Anaphase: Kinetochores separate, pulling chromatids to opposite poles. Kinetochore fibers shorten.
  • Telophase: Kinetochore fibers disappear as chromatids reach the poles. The spindle elongates, nuclei reappear, and chromosomes decondense into chromatin.

Cytokinesis: Cytoplasm divides, forming two daughter cells. In animal cells, this occurs through cytoplasmic constriction. In plant cells, a fragmoplast forms due to the cell wall.

Meiosis

Meiosis produces gametes, specialized cells from two parents that unite to form a zygote. This process enhances genetic diversity. Meiosis involves two cell divisions, resulting in four haploid cells.

First Meiotic Division:

  • Prophase 1:
    • Leptotene: Chromosomes condense.
    • Zygotene: Homologous chromosomes pair via the synaptonemal complex.
    • Pachytene: Crossing-over occurs between non-sister chromatids.
    • Diplotene: Synaptonemal complex disappears; chromosomes remain connected at chiasmata.
    • Diakinesis: Chromosomes condense further; nuclear membrane breaks down.
  • Metaphase 1: Chromosomes align at the equatorial plane; kinetochore fibers attach.
  • Anaphase 1: Chiasmata break; homologous chromosomes move to opposite poles.
  • Telophase 1: Nuclear envelopes reform; chromosomes decondense.

Second Meiotic Division:

  • Prophase 2: Nuclear membranes dissolve; spindle fibers form.
  • Metaphase 2: Chromosomes align at the equatorial plane.
  • Anaphase 2: Centromeres divide; chromatids move to opposite poles.
  • Telophase 2: Nuclear membranes reform; chromosomes decondense, resulting in four haploid cells.