Meiosis is a specialized type of cell division that occurs in the formation of gametes such as egg and sperm. Although meiosis appears much more complicated than mitosis, it is really just two divisions in sequence, each one of which has strong similarities to mitosis. Meiosis

Differences between Meiosis and Mitosis Mitosis produces somatic cells while meiosis produces gametes (egg and sperm) Mitosis produces diploid cells (two copies of each chromosome) and meiosis produces haploid cells (one copy of each chromosome) Mitosis produces two daughter cells while meiosis produces four daughter cells Mitosis goes through one division while meiosis goes through two divisions

MEIOSIS I Meiosis I, the first of the two divisions, is often called reduction division, since it is here that the chromosome number is reduced from 2N (diploid - double) to 1N (haploid - half). Ex: Humans: diploid number = 46 (haploid = 23) *remember this must happen for us to get half our chromosomes from our dad and half from our mom!

INTERPHASE Interphase in meiosis is identical to interphase in mitosis There is no way, by simply observing the cell, to determine what type of division the cell will undergo when it does divide. Meiotic division will only occur in cells associated with male (sperm) or female (egg) sex cells.

PROPHASE I Prophase I is virtually identical to prophase in mitosis involving the appearance of the chromosomes, the development of the spindle apparatus, and the breakdown of the nuclear membrane (envelope).

PROPHASE I Here is where the critical difference between meiosis and mitosis. Homologous chromosomes pair up and chromatid arms may overlap and temporarily fuse resulting in crossing over which is important in allowing for genetic variations

METAPHASE I In Mitosis, all the chromosomes line up along the equator in no particular order. In Metaphase I (of Meiosis), the chromosome pairs are aligned on either side of the metaphase plate.

ANAPHASE I During Anaphase I the spindle fibers contract, pulling the homologous pairs away from each other and toward each pole of the cell.

TELOPHASE I Cleavage is followed by cytokinesis, but… the nuclear membrane (envelope) usually is not reformed and the chromosomes do not disappear as in mitosis. At the end of Telophase I, each daughter cell has a single set of chromosomes, half the total number in the original cell where the chromosomes were present in pairs.

While the original cell was diploid, the daughter cells are now haploid. This is why Meiosis I is often called reduction division. Diploid = 4 Haploid = 2 It was 4, now it will be 2 in each of the new cells!!

MEIOSIS II Can You Guess? Meiosis II is quite simple in that it is a mitotic division of each of the 2 haploid cells produced in Meiosis I. There is no Interphase between Meiosis I and Meiosis II and the latter begins with: Can You Guess?

PROPHASE II A new set of spindle fibers forms and the chromosomes begin to move toward the equator of the cell.

METAPHASE II All the chromosomes in the two cells align with the equator.

ANAPHASE II The centromeres split The spindle fibers shorten drawing the chromosomes toward each pole of the cell.

TELOPHASE II Cleavage (the pinching in the cell membrane) is followed by cytokinesis The nuclear membrane/envelope reforms The chromosomes begin to unwind, replaced by the granular chromatin characteristic of interphase. Total of four daughter cells, each with half the total number of chromosomes as the original cell.

In the case of male meiosis, all four cells will eventually develop into genetically different sperm cells. In females, three of the cells will typically breakdown, leaving a single cell to develop into an egg cell which is usually much larger than a typical sperm cell.