My-oh-sis! I'm half the cell I used to be.....

Meiosis I (first cell division)

.....and then.....

Meiosis II (second cell division)

Not only does cell division replace cells that needed for growth and repair, but a special type of cell division is necessary to produce gametes, or reproductive cells. In males, we call these gametes sperm cells, while they are referred to as egg cells in females. Meiosis is the special type of cell division that only produces reproductive cells.

Why do we need a different type of cell division for reproductive cells? Well, consider this: Mitosis produces daughter cells that are identical to the original parent cell, right? If our reproductive cells were produced through mitosis, we would have the same number of chromosomes in our sperm and egg cells as we would every other cell in our body, which, for humans, is 46. If a man and woman, each containing 46 chromosomes in their sperm and egg cells, conceive a child, his sperm cell would combine with her egg cell. So, his 46 + her 46 would = 92 chromosomes in their child! Not good, since each species is supposed to maintain the same number of chromosomes (humans 46, fruit flies 8, etc). Can you imagine if the 96 chromosome child grew up and reproduced on her own, conceiving a child with a 96 chromosome man?? Their child would have 192 chromosomes! It would never end!!!!

So, how do we maintain a consistent number of chromosomes in our species? The answer is simple: meiosis produces reproductive cells that contain half the number of chromosomes as our regular body cells. This means that human sperm and egg cells each have 23 chromosomes. Let's revisit our example from the previous paragraph.....If a man and woman, each containing 23 chromosomes in their sperm and egg cells, conceive a child, his sperm cell would combine with her egg cell. So, his 23 + her 23 would = 46 chromosomes in their child. Good? You bet!

How is meiotic cell division different than mitotic cell division?

1. Meiosis only produces cells that are to become gametes. All other cells divide by mitosis.

2. In meiosis, the cells divide twice. However, the chromosomes are only copied before the first division. Mitosis includes only one cell division, which always includes chromosome copying.

3. Daughter cells produced by meiosis are haploid (n), meaning that they contain half the number of chromosomes as the parent cell. In humans, this is 23. Mitosis produces diploid (2n) daughter cells, meaning that the cells are identical to the parent. These cells have a double set of chromosomes. In humans, this is 46.

4. Meiosis produces four daughter cells at its completion. Mitosis only produces two.

During Wednesday's class, we will be visiting The Biology Project's Meiosis Tutorial and completing work to learn more about meiotic cell division. Thursday's quiz will focus on identification of the phases of mitosis, as well as the major differences between mitosis and meiosis (as listed above).

Once we finish cell division, we will get into Mendelian genetics and really begin to practice the meat and potatoes of Genetics! I'm excited for some punnett squares....are you?