Mitosis and Meiosis

What is Mitosis?

Mitosis produces two daughter cells that are identical to the parent cell.  If the parent cell is haploid (N), then the daughter cells will be haploid.   If the parent cell is diploid, the daughter cells will also be diploid.

N → N

2N → 2N

This type of cell division allows multicellular organisms to grow and repair damaged tissue.
Click here to go to the chapter on Mitosis.

Summary of the Phases of Mitosis

The drawings below show chromosome movement and alignment in a cell from a species of animal that has a diploid number of 8. As you view the drawings, keep in mind that humans have a diploid number of 46.

	Interphase (G1 and G2) Chromosomes are not easily visible because they are uncoiled.
	Prophase The chromosomes begin to coil. The spindle apparatus begins to form as centrosomes move apart.
	Prometaphase The nuclear membrane disintegrates. Kinetochores form on the chromosomes. Kinetochore microtubules attach to the chromosomes.
	Metaphase The chromosomes become aligned on a plane.
	Anaphase The chromatids separate (The number of chromosomes doubles).
	Telophase The nuclear membrane reappears. The chromosomes uncoil. The spindle apparatus breaks down. The cell divides into two.
	G1 Interphase  The chromosomes have one chromatid.
	G2 Interphase  The chromosomes are replicated. Each one has two sister chromatids.

What is Meiosis?

Meiosis produces daughter cells that have one half the number of chromosomes as the parent cell.

2N → N

Meiosis enables organisms to reproduce sexually. Gametes (sperm and eggs) are haploid.
Meiosis involves two divisions producing a total of four daughter cells.

Summary of the Phases of Meiosis

A cell undergoing meiosis will divide two times; 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):

meiosis 1: prophase 1, metaphase 1, anaphase 1, and telophase 1
meiosis 2: prophase 2, metaphase 2, anaphase 2, and telophase 2

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.

	Prophase IHomologous chromosomes become paired. Crossing-over occurs between homologous chromosomes.
	Crossing over
	Metaphase IHomologous pairs become aligned in the center of the cell.
	The random alignment pattern is called independent assortment. For example, a cell with 2N = 6 chromosomes could have any of the alignment patterns shown at the left..
	Anaphase IHomologous chromosomes separate.
	Telophase IThis stage is absent in some species
	InterkinesisInterkinesis is similar to interphase except DNA synthesis does not occur.
	Prophase II
	Metaphase II
	Anaphase II
	Telophase II
	Daughter Cells

Laboratory Exercise

Mitosis in Animals

Human Chromosomes

1. View a slide of human chromosomes and draw some of the chromosomes below.

2. Do the chromosomes have one chromatid or two?

Below: Human Chromosomes Click on the photograph to view an enlargement.

Whitefish Blastula

3. The cells of a developing embryo are dividing rapidly and can be used for viewing the different stages of mitosis. Obtain a whitefish blastula (early embryo) slide and find a cell in each of these phases: interphase, prophase, metaphase, anaphase, and telophase.

4. Draw a cell in anaphase below.

Below: Whitefish blastula mitosis X 400. Click on the images to view enlargements.

   

Additional Photographs of mitosis in animals: Ascaris Megalocephala Mitosis

Mitosis in Plants

1. Cells at the tips of plant roots and stems grow rapidly and can be used for viewing the stages of mitosis. Used a slide of onion (Allium) root tip to identify interphase, prophase, metaphase, anaphase, and telophase.

2. Draw a cell in telophase below. Draw the cell plate if it is visible.

Below: Allium root tip mitosis X 400. Click on the images to view enlargements.

   

Meiosis in Animals (Gametogenesis)

Testis

1. Obtain a slide of a cross section of rat testes and observe the seminiferous tubules. Identify spermatogonia. Identify sperm.

Below: Rat testes X 100. Click on the image to view an enlargement.

Ovary

The primary oocyte is contained within a structure called a follicle. As the follicle enlarges, it produces hormones. During ovulation, the follicle ruptures and releases the secondary oocyte. 

2. View a slide of a section of a rabbit ovary under scanning magnification and observe follicles in various stages of development. Can you see an oocyte in any of the follicles?

Below: Rabbit ovary X 40. Click on the image to view an enlargement.