Like in mitosis, cells are in interphase before meiosis begins, during which DNA replication occurs. Thus meiosis begins with DNA in the form of sister chromatids (see The difference between chromatin, chromatid and chromosome). Meiosis then takes places in two major phases: meiosis I and meiosis II.

Meiosis I
At the start of meiosis I (prophase I), homologous chromosomes find each other and pair up in the cell. They then exchange bits of DNA segments, a process called crossing over. Crossing over allows for greater genetic variation, as the resulting chromosome will have some genes from the maternal chromosome and some genes from the paternal homologous chromosome. At metaphase I, the homologous chromosome pairs line up in the equator of the spindle. Then, at anaphase I, the pairs are separated and moved to opposite poles. The cell then undergoes cytokinesis to divide into two daughter cells, the chromosomes decondense into chromatin strands, and the nuclear membrane reforms around the chromatin (telophase I). This means that at the end of meiosis I, the resulting cells will already have half the amount of chromosomes. However, a second phase is needed to separate the sister chromatids that form each chromosome.

Meiosis II
Meiosis II is very similar to mitosis. The chromosomes (sister chromatids) are separated into two individual chromatid strands so that there are 23 chromatids per daughter cell.

Thus at the end of meiosis, 4 haploid cells (gametes) are produced.

This image from the Molecular Biology of the Cell (Alberts, Garland Science 2002) shows clearly the steps…

Related posts:

1. The difference between mitosis and meiosis
2. The difference between chromatin, chromatid and chromosome
3. Mitosis

Meiosis is a special form of cell division that only takes place in the sex organs (gonads) of organisms that undergo sexual reproduction, in order to produce sperm or eggs (gametes). Here, the resulting sperm/egg cells are not identical to the original dividing cell, but only have half the amount of chromosomes as our normal cells (haploid).

Human somatic cells normally have 46 chromosomes; 23 of them come from the mother, and the other 23 from the father. These somatic cells are referred to as being diploid, meaning they have 2 copies of each chromosome. Meiosis is basically a halving process so that the sperm or egg will finally end up with 23 chromosomes each (haploid), so that when they fuse during fertilization, the result is 46 chromosomes again.

Human somatic cells have 2 copies of 23 chromosomes (one copy from the mother, and one copy from the father), totalling 46 chromosomes

Read more about the process of mitosis and meiosis.

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1. Meiosis
2. Mitosis
3. The difference between chromatin, chromatid and chromosome

DNA, the blueprint of life, is organized into structures called chromosomes. In prokaryotic cells, chromosomes are circular, whereas in eukaryotic cells, they are linear strands. Different organisms have different numbers of chromosomes: human cells usually have 46 chromosomes, dogs have 78 chromosomes, while kangaroos have only 12 chromosomes!

This karyotype of a human male cell shows the 46 chromosomes.

When you add all these chromosomes up, each cell actually contains about 2m of DNA! And all this DNA has to fit into a tiny nucleus of 5-10um in diameter. This is like trying to stuff a piece of string 2km long (it will take you about 20 minutes to walk from one end to the other) into a tiny bead smaller than 1cm!!! To do this seemingly impossible feat, cells devised an ingenious packaging system: it wraps DNA around proteins called histones. The resulting DNA-protein complex is called chromatin.

At the beginning of cell division (S-phase), the DNA is replicated, producing two identical copies of DNA, which are connected to each other at the centromere. This replicated X-like structure is now called a sister chromatid pair. A chromatid is therefore just one of the strands.

During mitosis, the sister chromatid pair condenses further, giving rise to the fat X chromosomes that you can see in the karyotype above. Therefore, chromosomes can be found in 3 forms: thread-like chromatin (during interphase), thread-like sister chromatids (during S-phase) and the condensed, visible form (during mitosis).

When a cell divides, the sister chromatids separate, and each daughter cell receives one of the strands. The chromatid then decondenses back into a long single chromatin strand when the new cell goes into interphase.

For more information about DNA, check out this Scitable entry: DNA Is a Structure That Encodes Biological Information

Related posts:

1. Meiosis
2. The difference between mitosis and meiosis
3. Interphase

We can describe mitosis as consisting of 4 phases: prophase, metaphase, anaphase and telophase.

Quite a number of things happen during prophase.

1. Chromatin (thread-like DNA) is supercoiled and condensed into chromosomes.
2. The nuclear envelope breaks down, allowing the chromosomes to fill the entire cell space.
3. The two centrosomes (or microtubule organizing centers, MTOCs) move apart to opposite ends of the cell.
4. Microtubules from the centrosomes form mitotic spindles that can attach to the chromosomes to move them.

PROPHASE

The chromosomes are then moved by the mitotic spindle until they are all lined up in the middle of the cell, forming what is known as a metaphase plate. This stage of mitosis is called metaphase.

METAPHASE

Once all the chromosomes are “captured” by microtubules, the cell can proceed to anaphase. The mitotic spindles shorten, splitting the chromosome into two chromatids, and pulling each one to opposite ends of the cells.

If chromosomes are not segregated accurately, the resulting daughter cells will have errors and could become cancerous. To make sure that cells divide correctly, there are some internal checkpoints that stop the cell from continuing in the cell cycle if prior conditions are not met. For example, anaphase cannot take place until all chromosomes are attached to microtubules.

ANAPHASE

At the end of mitosis (telophase) we have two identical sets of DNA at the poles of the cell. This DNA starts to decondense back to the thread-like chromatin structure. A nuclear envelope reassembles around each set, forming 2 nuclei.

TELOPHASE

After mitosis has completed, the cell then divides into 2 daughter cells, a stage known as cytokinesis. Read more about this final stage of the cell cycle.

Animations make it really easy to understand the different phases of mitosis. Here’s a really good one!

* All images here were modified from Molecular Biology of the Cell, 4th edition, Garland Science

Related posts:

1. The difference between mitosis and meiosis
2. Meiosis
3. Interphase

In animal cells, the cell forms a contractile ring in the middle of the dividing cell. As the ring contracts, it creates a cleavage furrow, which eventually pinches the cell in two.

In plant cells, vesicles migrate to the middle of the cell and fuse to form a cell plate (which eventually becomes the plasma membrane). A cell wall then develops between the two cells.

Once cytokinesis is complete, cells once again return to interphase.

Related posts:

1. Cell Division
2. Interphase
3. Mitosis

The first part of interphase, the G1 phase (Gap 1), is the period during which cells are growing, synthesizing proteins and making more organelles. Cells need to double their organelles and size before dividing, otherwise they will get smaller after each division!

Cells then duplicate their DNA in S phase (Synthesis). This takes around 10-12 hours in human cells. Finally, it goes into a further growth phase, the G2 phase, before entering mitosis.

During interphase, the centrosome (or microtubule organizing center, MTOC) is also duplicated.

Here is an image of a cell in interphase. You can see its nucleus clearly, which contains chromatin (DNA in thread-like form, in blue).

A cell in interphase. DNA (blue) is contained within the nuclues, and microtubules (green) are not organized. From Molecular Biology of the Cell, 4th ed, by Alberts et al.

Continue reading about mitosis to see the structural changes that occur in the cell as it divides.

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1. Cell Division
2. Mitosis
3. Cytokinesis

Sometimes cells go a little hay-wire and start to divide uncontrollably. This can occur in any organ or tissue, and the result is a cancerous tumour. To learn more about cancer, click here.

To duplicate, cells go through a cell cycle which is divided into a number of stages: interphase, mitosis and cytokinesis.

For more information on each section, please click and read the individual topics

Related posts:

1. Mitosis
2. Meiosis
3. Interphase