Jan 33 min read

Mitosis and Meiosis Explained

Updated: 6 days ago

What is mitosis?

Mitosis is a type of cell division that creates identical daughter cells from a parent cell. Millions of cells undergo this process every day. This is important to the human body because it creates more cells, which allow the body to grow and develop.

Understanding Interphase

For a cell to prepare for mitosis, it must go through interphase. Scientists divide interphase into three different phases.

G1 Phase: This is the phase in which the cell grows in size, creates proteins, and produces organelles. Thus, the cell readies itself for the S phase.

S Phase: In this phase, the cell duplicates its DNA, creating chromosomes with two identical chromatids joined by a centromere.

G2 Phase: In the G2 phase, the cell continues to grow even more and creates these organelles, called “centrosomes”, which will be important later in mitosis.

In between phases are checkpoints. Checkpoints verify that the cell has duplicated its DNA and has completed each phase correctly. These checkpoints are essential for preventing mutations, uncontrolled growth, and cell dysfunction, which are all harmful to the body.

Steps of Mitosis

A cell enters mitosis—a four-step process—after completing interphase.

Prophase: In this stage, the chromosomes condense, the nuclear membrane breaks down, and the centrosomes form spindle fibers which attach to the chromosomes.

Metaphase: In this stage, the chromosomes line up in the middle of the cell. This line is called the metaphase plate.

Anaphase: In this stage, the spindle fibers separate the sister chromatids by moving towards opposite sides of the cell. As a result, the cell will temporarily have 92 individual chromosomes.

Telophase: In this stage, the spindle fibers disappear and two nuclear membranes form, each having 46 individual chromosomes.

After mitosis, the cytoplasm separates into two identical daughter cells, each with the same organelles and DNA in a process called cytokinesis.

What is Meiosis?

Meiosis is a type of cell division. Scientists divide meiosis into two stages: meiosis I and meiosis II. Each stage is further divided into prophase, metaphase, anaphase, and telophase. The goal of meiosis is to create gametes. Gametes have 23 chromosomes, which means they have half the amount of chromosomes compared to somatic cells. They are important for sexual reproduction and ensuring genetic diversity in future generations.

Steps of Meiosis I (First Meiotic Division)

For a cell to prepare for meiosis, it must go through interphase just like how a cell must go through interphase before mitosis.

Prophase I: In this stage, chromosomes condense. After that, homologous chromosomes, which are chromosomes that have the same genes, pair up with each other. Homologous chromosomes that pair up with each other are called bivalents. When they pair up, they exchange DNA, thus creating a more diverse gene pool. In this stage, the cell also breaks down the nuclear membrane, and the centrosomes form spindle fibers that attach to the chromosomes.

Metaphase I: In this stage, the homologous chromosome pairs (bivalents) line up in the middle of the cell in two rows. This line in between the two rows is called the metaphase plate.

Anaphase I: In this stage, the spindle fibers move to opposite sides of the cell, thus separating the homologous chromosome pairs (bivalent). This process is called disjunction.

Telophase I: In this stage, the spindle fibers disappear and two nuclear membranes form, each having 23 chromosomes.

After Meiosis I, cytokinesis occurs, which results in two cells that have a distinct combination of genes in their chromosomes from each other.

Steps of Meiosis II (Second Meiotic Division)

Prophase II: In this stage, the chromosomes condense, the nuclear membrane breaks down, and the centrosomes form spindle fibers which attach to the chromosomes.

Metaphase II: In this stage, the chromosomes line up in the middle of the cell. This line is called the metaphase plate.

Anaphase II: In this stage, the spindle fibers separate the sister chromatids by moving towards opposite sides of the cell. As a result, both cells will temporarily have 46 individual chromosomes.

Telophase II: In this stage, the spindle fibers disappear and two nuclear membranes in each cell form.

After Meiosis II, both cells will go under cytokinesis, creating four cells, each with 23 individual chromosomes made up of different genetic materials.