Cell Division and Mitosis

Why is cell division important?

• What do you, an octopus, and an oak tree have in common?

• You share many characteristics, but an important one is that you are made of cells– trillions of cells.

• Many organisms start out as one cell. That cell divides and becomes two, two becomes four, four becomes eight, and so on.

• Many celled organisms grow because cell division increases the total number of cells in an organism.

Even after growth stops, cell division is still important.

Everyday, billions of red blood cells in your body wear out and are replaced.

In a few seconds, your bone marrow has produced about six million red blood cells.

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The Cell Cycle

• A living organism has a life cycle.

• Cells also have a life cycle.

• The cell cycle is a series of events that takes place from one cell division to the next.

Cells in animal embryos divide quickly and can complete their cycles in less than 20 minutes.

In some human cells, the cell cycle takes about 16 hours.

Cells in humans that are needed for repair, growth, or replacement, like skin and bone cells, constantly repeat the cycle.

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Mitosis

• Most of the life of any eukaryotic cell is spent in a period of growth and development called interphase.

Cells in your body that no longer divide, such as nerve and muscle cells, are always in interphase.

Skin cells prepare for cell division during interphase.

In interphase, chromosomes are copied.This must happen so that each of the two cells will get a complete copy. Each cell needs a complete set of hereditary material to carry out life functions.

After interphase, cell division begins.

Cell division is the process in which one cell divides into two cells.

Cell division occurs in a series of stages or phases.

Mitosis is the process in which the nucleus divides to form two identical nuclei.

Each new nucleus is identical to the original nucleus.

When the nucleus divides, the chromosomes play the important part.

A chromosome is a structure in the nucleus that contains hereditary material.

When the nucleus is ready to divide, each duplicated chromosome coils tightly into two thickened, identical strands called chromatids.

The first phase of mitosis is prophase.

In prophase, the nuclear membrane disappears. The chromosomes become visible. Spindle fibers appear that stretch across the cell.

The second phase of mitosis is metaphase.

In metaphase, the chromosomes line up along the middle of the cell and attach to the spindle fibers.

The third phase of mitosis is anaphase.

In anaphase, the chromosomes split apart. Two identical sets of chromosomes are formed. Each set is pulled away from each other. The sets move to opposite sides of the cell.

The fourth phase of mitosis is telophase.

In telophase, the spindle fibers disappear and two nuclei form. Each nucleus contains one set of chromosomes. A cell membrane begins to form through the center of the cell. This divides the cytoplasm between the two nuclei. Two cells are formed.

In most cells, after the nucleus divides, the cytoplasm separates and the two cells form. This is called cytokinesis. The cell membrane pinches in the middle and the cytoplasm divides. After the cytoplasm divides, most new cells begin a growth period, or interphase.

Results of Mitosis and Cell Division

• Mitosis is the division of a nucleus.

• Mitosis produces two new nuclei that are identical to each other and the original nucleus.

• The original cell no longer exists.

Asexual Reproduction• Mitosis is a form of asexual

reproduction.• Asexual reproduction is reproduction

which involves only one parent.• The new organism will be identical to

the parent.

Examples of Asexual Reproduction

• Budding- type of asexual reproduction in which an outgrowth forms on the parent cell.

• Budding yeast cells

Regeneration- type of asexual reproduction in which organisms replace lost or damaged parts.

Fission- a type of asexual reproduction in which a one-celled organism with no nucleus copies its genetic material and then divides into two identical organisms.