Cell Division Processes: Mitosis and Meiosis Explained

Mitosis: The Process of Cell Duplication

All cells reproduce in sets of two from a single parent cell. These new cells, called daughter cells, must be identical to the parent cell. As genetic information is stored in the nucleus, the most important step in reproduction is its division through mitosis. Cells obtained from mitosis contain the same number of chromosomes as their parent cells.

Phases of Mitosis

Prophase

• Chromatin fibers thicken and shorten to form chromosomes.
• The nucleolus disappears, and protein fibers appear between both poles to form the mitotic spindle.
• The nuclear membrane disappears, allowing chromosomes to move freely.

Metaphase

• Chromosomes bind to the mitotic spindle fibers using their centromeres.
• This union takes place at the equator of the cell.
• Sister chromatids belonging to each chromosome point to the opposite poles of the cell.

Anaphase

• The mitotic spindle fibers break into two equal halves. This causes the chromosome's centromere to break.
• As a result, the two chromatids that made up each chromosome separate into two unconnected halves.
• The mitotic spindle fibers contract, pulling the chromatids towards the opposite poles. From this point onwards, the chromatids are considered independent chromosomes.

Telophase

• Once the chromatids move to the ends of the poles, the remains of the mitotic spindle fibers disappear.
• A new nuclear membrane surrounds each group of chromatids, forming two new nuclei.
• Chromatids expand and turn into chromatin.
• The nucleolus reappears.

Cytokinesis: Cytoplasm Division

Once mitosis ends, the division of the cytoplasm takes place, and cell organelles are shared. This process is known as cytokinesis, and it occurs in both animal and plant cells. In plant cells, a wall forms which divides the cytoplasm in half. In animal cells, the cytoplasm stretches, thins out, and eventually separates.

Meiosis: Generating Genetic Diversity

Cells divide to create gametes, but these are not identical. Gametes join during fertilization to form a zygote, which will then grow into an organism after completing cell division.

Meiosis I: The First Division

• Prophase I

  • Chromosomes appear, but they are homologous.
  • A genetic exchange between these pairs occurs, called genetic recombination or crossing over.

• Metaphase I

  • Pairs of homologous chromosomes, not individual chromosomes, bind to the mitotic spindle fibers.

• Anaphase I

  • A complete chromosome moves to each cell pole, instead of half a chromosome.

• Telophase I

  • There are no significant differences compared to mitotic telophase, but the cell divides into two haploid cells.

Meiosis II: The Second Division

Chromosomes divide into chromatids, and each daughter cell receives one set. This way, meiosis produces four haploid daughter cells, which have half the chromosomes of the original parent diploid cell. This is important because any changes in the genetic information would lead to several consequences.

Mitosis vs. Meiosis: Key Differences

Mitosis creates cells for growth and asexual reproduction, ensuring the genetic information remains the same. Meiosis is used for sexual reproduction; it ensures that daughter cells are modified by the genetic recombination process, leading to genetic diversity.