Mitosis – Cell Division

All living things start life as a single cell. As an organism grows, the cells increase in number, not size.

The cell grows and eventually splits in two. Why are there limits on cell size?

1. The larger the cell becomes, the more demands the cell places on its DNA.

2. The cell has more trouble moving enough nutrients and wastes across the cell membrane.

Reasons for Cell Division Growth Repair Reproduction Asexual (binary fission) Sexual (fusion)

Surface Area to Volume Ratio As the length of a cell increases, its volume increases faster than its surface area.

This makes it more difficult to move needed materials in and wastes out.

So, the cell must divide!

Mitosis is the process by which the nucleus of a cell divides to produce two nuclei.

Each nuclei has the same type and number of chromosomes that the original cell had.

In order to divide, the cell, must follow a cell cycle. The cell cycle is a continuous process.

Periods of the cell cycle Most cells spend about 90% of their time in interphase and about 10% of their time in mitosis and cytokinesis.

The two main parts of the cell cycle are cell growth and cell division.

Cell growth – interphase - is the longest process. The cell grows and duplicates the chromosomes.

Animal Cell Interphase

Plant Cell Interphase

The second main part of the cell cycle is cell division which is called mitosis.

The phases in mitosis are prophase, metaphase, anaphase and telophase.

PMAT – Paul made a tower. Prophase, metaphase, anaphase, telophase

PROPHASE

Animal Cell Prophase

Prophase is the first stage of mitosis in which the nuclear membrane breaks down and the chromosomes become short and thick.

Individual chromosomes are visible as two parallel threads attached at a point called the centromere.

Each thread is called a chromatid.

METAPHASE

Animal Cell Metaphase

Parallel fibers called spindle fibers appear. These attach to each chromosome on either side of its centromere.

The chromosomes line up at the center or “equator” of the cell.

ANAPHASE

Animal Cell Anaphase

Anaphase is the shortest stage of mitosis and is the separation of the chromosomes.

The chromatids that were attached to the same centromere now separate and move to opposite poles of the spindle.

Now each structure is considered a separate chromosome because it has its own centromere.

The chromosomes have been separated into two genetically identical nuclei.

TELOPHASE

Animal Cell Telophase

Telophase seems to be prophase in reverse. The spindle fibers disappear, a nuclear membrane

forms around each of the two sets of chromosomes, and a nucleoli appears in each new nucleus.

The chromosomes become longer.

CYTOKINESIS

Animal Cell Cytokinesis

The final stage of the division of cells, is cytokinesis.

Cytokinesis splits the cytoplasm of dividing cell into daughter cells.

Thanks goes to Prince Georges County Framework for Learning website for pictures and information.

Mitosis Animations Cells Alive

How do cells stop dividing? When cells come into contact with other cells, they stop growing. This is called contact inhibition.

A protein called CYCLIN regulates the timing of the cell cycle.

Rates of Cell Growth Varies by organism and within organism Bacteria vs. mammal Skin vs. liver Dependent upon environmental conditions.

What happens if a cell looses control of its growth? Cancer; Cancer cells do not respond to signals that control growth.

CANCER