Mitosis and Meiosis

Interphase precedes both mitosis and meiosis and is the period between cell divisions during which time the chromosomes replicate and the chromosomes are not visible (loosely packed). During interphase, two pairs of centrioles lie next to each other, just outside the nucleus.

Mitosis is a process where in, one parent cell gives rise to two identical daughter cells. Mitosis can be divided into four stages: Prophase, Metaphase, Anaphase and Telophase.

Prophase: Chromosomes (two identical copies) condense, each chromosome has two arms and each copy of chromosome is called Chromatid. Spindle fibers form at centriole and centriole begin to separate. In addition, nuclear membrane disappears.

A short period just before metaphase, called prometaphase, comprises movement of centrioles to opposite ends of the cell and attachment of spindle fibers to each of the chromatids.

Metaphase: Chromosomes line up along an imaginary line, called the metaphase plate that divides the cell into two. The spindle fibers begin to pull the chromosomes to the opposite ends of the cell.

Anaphase: Spindle fibers separate sister chromatids to opposite ends of the cell.

Telophase: Chromatids, now called chromosomes move to each pole and new nuclear membranes form.

Once mitosis is complete, the rest of the cell divides, by a process called cytokinesis (division of the cytoplasm) and cell division is complete.

Meiosis is a type of cell division that is specific to reproduction and results in 4 daughter cells that have half the number of unidentical chromosomes (genetic information is contained from both parents). Meiosis is divided into two phases: Meiosis I and Meiosis II.

Meiosis I: comprises Prophase I, Metaphase I, Anaphase I and Telophase I

Prophase I: Chromosomes attach to nuclear membrane and pair up with corresponding chromosome (to from a tetrad) from the other parent. Homologous recombination occurs between chromosome pairs and genetic material exchange takes place.

Prometaphase I: Similar to prometaphase I in mitosis except, one chromosome (instead of chromatid) from the homologous pair is attached to each centriole. Therefore, 23 chromosomes (in humans) attach to fibers from one centriole and remaining 23 attach to the fibers from the other centriole.

Metaphase I: Chromosome pairs line up along the metaphase plate on either side.

Anaphase I: Chromosome pairs separate. One half of the chromosomes goes to one pole and the other half to the other pole.

Telophase I: Chromosomes reach opposite ends of the cell and a nuclear membrane forms marking the end of Meiosis I.

There is a major distinction between sperm and egg cells at this stage. While in sperm cells the cytoplasm is equally divided between the two emerging daughter cells, in oocytes, the cytoplasm is concentrated in one of the emerging daughter cells resulting in a large and a small daughter cell called the polar body.

Telophase I is followed by cytokinesis resulting in two daughter cells in case of sperms and one large cell and one small cell (polar body) in the case of the egg (primary oocyte to be precise).

Meiosis II follows a very short Interphase II but chromosome replication does not take place unlike in Mitosis and Meiosis I.

Meiosis II can also be divided into four phases: Prophase II, Metaphase II, Anaphase II and Telophase II. Meiosis II is very similar to Mitosis

Prophase II: Chromosomes condense, spindles form centrioles begin to separate and the nuclear membrane disappears. There is no homologous recombination.

Prometaphase II: Spindle fibers attach to chromatids and centrioles move to opposite ends of cell.

Metaphase II: Chromosomes align along the metaphase plate and fibers begin to pull at the chromosomes.

Anaphase II: Sister chromatids are pulled apart toward opposite ends of the cells.

Telophase II: Chromatids arrive at opposite poles, nuclear membranes form. Again, as in Telophase I, in the female cell, the emerging daughter cells will have unequal distribution of the cytoplasm resulting in one large and another small cell. The resulting large cell becomes the egg or ovum and the smaller cell is called the polar body. The first polar body formed at the end of Meiosis I also divides to form two polar bodies. Therefore, in females, at the end of Meiosis, there is one egg cell and three polar bodies.

Cytokinesis follows Telophase II to mark the completion of cell division.

Main differences between Mitosis and Meiosis I:

Prophase

Mitosis: Chromatids of chromosome begin to separate. There is no exchange of any genetic material
Meiosis I: Pairing of homologous chromosomes, tetrad formation and homologous recombination (exchange of genetic material) take place

Metaphase

Mitosis: Chromosomes line up along metaphase plate
Meiosis I: Chromosome pairs line up along metaphase plate

Anaphase

Mitosis: Sister chromatids pulled to opposite ends of cell
Meiosis I: Separation of chromosome pairs to opposite ends of cell

Telophase and Cytokinesis

Mitosis: Two daughter cells with identical chromosomes and exact number of chromosomes as parent cells
Meiosis I: Two daughter cells with chromosomes from both parents and half the number as parent cells and this is followed by Meiosis II

PS: Opposite ends of cell and opposite poles have been used interchangeably