Mitosis and Meiosis

Cell division is the foundational process by which organisms grow, repair damage, and reproduce. This essential mechanism is split into two primary types: mitosis and meiosis. Furthermore, cells in multicellular organisms must connect and communicate, a function managed by specialized structures called cell junctions.

Understanding Cell Division

Cell division is the process where a parent cell reproduces itself to form two or more daughter cells.

1️⃣ Mitosis: Replication for Growth and Repair

Mitosis is a type of cell division that results in two identical daughter cells, each having the same diploid number of chromosomes as the original parent cell. This process is crucial for growth, tissue repair, and replacement of old cells.

The period between two cell divisions is known as interphase, often inaccurately called the "resting stage," as the cell is actively preparing for division, including performing DNA replication to duplicate its chromosomes.

Mitosis proceeds through four main stages:

Prophase:

Chromosomes condense and become visible as short, rod-like structures.
Each chromosome consists of two identical copies (sister chromatids) joined at the centromere.
The nuclear membrane breaks down.
Centrioles move to opposite poles of the cell and organize the spindle fibers.

Metaphase:

The sister chromatid pairs line up along the cell's equator (the metaphase plate).
Spindle fibers from opposite poles attach to the centromere of each pair.

Anaphase:

The centromeres divide, separating the sister chromatids.
Each chromatid is now considered a full chromosome.
Spindle fibers pull the complete sets of chromosomes toward the opposite poles of the cell.

Telophase:

The sets of chromosomes reach the poles and begin to uncoil (decondense) back into chromatin.
A new nuclear membrane reforms around each set of chromosomes.

2️⃣ Meiosis: Generating Reproductive Cells

Meiosis is a more complex division process that results in the formation of gametes (sex cells: eggs and sperm).

One diploid parent cell divides to produce four cells, each containing a haploid number of chromosomes (half the usual number).
In humans, the normal diploid number is 46 chromosomes; meiosis reduces this to 23 chromosomes in the egg and sperm.
This reduction is why meiosis is often called reduction division.
Oogenesis is meiosis in the ovaries (women), and spermatogenesis is meiosis in the testes (men).

When the haploid sperm (23 chromosomes) fertilizes the haploid egg (23 chromosomes), the resulting fertilized egg recovers the full diploid number of 46 chromosomes.

🔗 Cell Junctions: Connecting and Communicating

In tissues, cells must be physically connected to one another and communicate effectively. Cell junctions are multi-protein complexes that act as intercellular connections between the plasma membranes of neighboring animal cells or between cells and the extracellular matrix ( $\text{ECM}$ ).

There are three main types of cell junctions:

Tight Junctions:

Formed by the fusion of the outer layers of the plasma membranes of two adjacent epithelial cells.
They create a tight barrier, preventing fluid and molecules from passing between the cells (paracellular diffusion).
They are critical for maintaining cell polarity and forming seals in organs like the stomach and bladder.

Gap Junctions:

Connect neighboring cells via small, intercellular protein channels.
They allow for direct communication by permitting the rapid passage of ions, small molecules, and electrical impulses between cells without having to cross the external environment.

Anchoring Junctions:

Provide strong mechanical attachments, connecting the cytoskeleton of one cell to the cytoskeleton of another, or to the $\text{ECM}$ .
They are vital for maintaining the structural integrity of tissues, especially those subject to stretching or abrasion (e.g., skin and heart muscle).
Examples include Desmosomes (cell-to-cell), Hemidesmosomes (cell-to- $\text{ECM}$ ), and Adherens Junctions (cell-to-cell).

Note on Anchoring Junctions: On the cytoplasmic side of the plasma membrane, a thick fibrous layer called the plaque is found. This plaque links to the cell's cytoskeleton: actin microfilaments attach to adherens junctions, while intermediate filaments attach to desmosomes and hemidesmosomes.

🆚 Mitosis vs. Meiosis: A Comparative Table

Feature	Mitosis	Meiosis
Purpose	Growth, repair, tissue renewal, and asexual reproduction.	Sexual reproduction (producing gametes: sperm and eggs).
Location	Somatic cells (body cells, e.g., skin, liver, muscle).	Germline cells (in the ovaries and testes).
Number of Divisions	One nuclear division.	Two nuclear divisions (Meiosis I and Meiosis II).
Number of Daughter Cells	Two daughter cells.	Four daughter cells.
Chromosome Number	Diploid ( $\mathbf{2n}$ ) — maintains the original number.	Haploid ( $\mathbf{n}$ ) — half the original number (reduction division).
Genetic Identity	Identical to the parent cell and to each other.	Genetically diverse from the parent cell and from each other.
Key Events	No crossing over or synapsis of homologous chromosomes.	Crossing over occurs in Prophase I, generating genetic variation. Homologous chromosomes separate in Anaphase I.