Cell Cycle: Mitosis and Meiosis Explained

Posted on Feb 11, 2025 in Biology

The Cell Cycle

The cell cycle is the sequence of modifications that a cell undergoes from its formation until it splits to form two daughter cells. The duration depends on the cell type and environmental conditions. It consists of two main phases:

• Interphase: The initial, long-duration phase where the interphase nucleus is visible. The cell grows and performs many functions. DNA replication takes place at the end of this phase.
• Division Phase (M Phase): The final, short-duration stage where the nucleus disappears, and chromosomes become visible. It involves the division of the nucleus (mitosis) and cytoplasmic division (cytokinesis).

Cell Division

In the division phase (M phase), the mother cell gives rise to two daughter cells with the same number of chromosomes as the parent cell.

Mitosis: Division of the Nucleus

Before the nucleus splits, DNA is duplicated. The centrosome and centrioles (in animal cells) are also duplicated.

• Prophase: Chromatin condenses, forming chromosomes. The nucleolus and nuclear membrane begin to disappear, and the spindle forms.
• Metaphase: Chromosomes arrange themselves in the center of the cell, forming the equatorial plate, and attach to spindle fibers.
• Anaphase: Sister chromatids of each chromosome separate and move to opposite poles of the cell, carried by the spindle filaments.
• Telophase: The nucleolus reappears, and the nuclear membrane begins to form. The spindle disappears, and the chromosomes decondense to form chromatin again.

Cytokinesis: Cytoplasmic Division

After mitosis, the cytoplasm divides, and organelles are distributed between the two new daughter cells:

• In animal cells: Cytokinesis occurs by strangulation at the level of the cell’s equator.
• In plant cells: A partition called a phragmoplast forms between the daughter cells.

Meiosis

Meiosis is a type of reduction division. From a diploid (2n) stem cell, haploid (n) daughter cells are formed, with half the DNA content of the progenitor cell. It takes place in the gonads, and the cells formed are gametes. During meiosis, there are two successive nuclear divisions, forming four haploid daughter cells:

• Interphase: Before the first meiotic division, DNA is duplicated.
• Prophase I: Chromatin condenses, and chromosomes become visible. Homologous chromosomes pair and exchange DNA fragments (crossing over).
• Metaphase I: Homologous chromosomes are arranged in pairs at the cell’s equator.
• Anaphase I: Pulled by the spindle fibers, homologous chromosomes separate and move to opposite poles of the cell.
• Telophase I and Cytokinesis: Two haploid daughter cells form, with half the number of chromosomes as the mother cell.
• Prophase II: The spindle reforms, and the chromosomes move toward the equatorial plate.
• Metaphase II: Chromosomes are arranged at the cell’s equator.
• Anaphase II: Chromatids separate, and each moves to one end of the cell.
• Telophase II and Cytokinesis: Four different haploid cells are obtained, each with half the number of chromosomes as the parent cell.

Comparison Between Mitosis and Meiosis

Mitosis:

• Occurs in somatic cells.
• Short process.
• Occurs in diploid and haploid cells.
• The nucleus divides once.
• No crossing over.
• In anaphase, sister chromatids separate.
• Two identical daughter cells are produced, identical to each other and to the mother cell.

Meiosis:

• Occurs in the stem cells of the gametes.
• Long process.
• Occurs in diploid cells.
• The nucleus divides twice.
• Crossing over occurs in the first meiotic division.
• In the first meiotic division, homologous pairs of chromosomes separate. In the second division, chromatids separate.
• Four daughter cells are produced, different from each other and from the mother cell.

Significance of Mitosis and Meiosis

• In multicellular organisms, mitosis allows for the growth of the individual through successive divisions from a single cell, the zygote. It also allows for the renewal of lost or damaged cells. In single-celled organisms, mitosis is a mechanism of asexual reproduction, increasing the number of individuals in a population.
• Meiosis produces haploid gametes. Upon fertilization, a diploid zygote results. This ensures a constant number of chromosomes in sexually reproducing organisms and promotes genetic variability.