Mitosis produces identical daughter cells.
Meiosis produces cells with 1/2 the number of chromosomes as the parent cell.
Chromosomes double when they split at the end of metaphase.
DNA replicates during the S period of interphase.
Summary of Mitosis
prophase - coil
metaphase - align
anaphase - separate
telophase - uncoil
Meiosis functions to reduce the number of chromosomes to one half. Each daughter cell that is produced will have one half as many chromosomes as the parent cell.
In animals, meiosis occurs only when gametes (sperm, eggs) are formed.
In plants, gametes are not produced directly. Instead meiosis produces spores and then mitosis produces gametes. Although plants have an additional step, meiosis eventually results in the production of haploid gametes.
There are two divisions in meiosis; the first division is meiosis 1 and the second is meiosis 2. The phases have the same names as those of mitosis. A number indicates the division number (1st or 2nd):
In the first meiotic division, the number of cells is doubled but the number of chromosomes is not. This results in 1/2 as many chromosomes per cell.
The second meiotic division is like mitosis; the number of chromosomes does not get reduced.
The diagram below shows that the chromosome alignment pattern during metaphase of mitosis results in the chromosomes splitting (doubling). Prophase, anaphase and telophase are not shown.
Below: The alignment pattern during metaphase I of meiosis results in pairs separating; there is no doubling.
Below: Metaphase 1 and metaphase 2 of meiosisPhases of Meiosis
The events that occur during prophase of mitosis also occur during prophase I of meiosis. The chromosomes coil up, the nuclear membrane begins to disintegrate, and the centrosomes begin moving apart.
Synapsis (joining) of homologous chromosomes produces tetrads (also called bivalents).
The two chromosomes may exchange fragments by a process called crossing over.
When the chromosomes partially separate in late prophase, the areas where crossing over occurred remain attached and are referred to as Chiasmata (sing. chiasma). They hold the chromosomes together until they separate during anaphase.
Crossing over between homologous chromosomes is likely to occur at several different points, resulting in chromosomes that are mixtures of the original two chromosomes.
One kinetochore forms on each chromosome instead of on each chromatid as in mitosis.
The spindle fibers attach to the chromosomes and begin to move them to the center of the cell as they do in mitosis.
Anaphase I begins when homologous chromosomes separate.
The nuclear envelope reforms and nucleoli reappear.
This stage is absent in some species.
Interkinesis is similar to interphase except DNA synthesis does not occur.
Oogenesis occurs in the ovary.
Each of the divisions in humans is unequal. During the first meiotic division, a large secondary oocyte and a small polar body are produced. The secondary oocyte will divide to produce an egg and a polar body. The first polar body may divide to produce two more polar bodies.
Although the polar bodies are very small and nonfunctional, they contain a full set of chromosomes.
The photograph below shows a cross section of a rabbit ovary X 40. The primary oocyte is contained within a structure called a follicle. As the follicle enlarges, it produces hormones. During ovulation, the follicle
Meiosis in human females begins before person is born but stops in prophase I and does not resume until after puberty.
Each month, approximately 1000 primary oocytes will to mature but most will die.
Ovulation occurs approximately once every 28 days. Females ovulate approximately 400 times during their lifetime.
Secondary oocytes are released at ovulation. The second meiotic division resumes after penetration by sperm.
In humans, secondary oocytes are fertilized. Eggs are produced only after fertilization of a secondary oocyte.
In the diagram above, the first polar body produced after the first meiotic division did not divide again.
Women are born with all of the primary oocytes that they will ever have (2 million). At puberty, there are approximately 400,000 left.
Some chromosomal abnormalities associated with maternal age may be due to the long time they remain paired during prophase I.