Mitosis

Mitosis is Cell Division As cells divide, growth occurs.

Growth is Caused by Mitosis Growing bodies, fingernails, hair and new tissue to repair wounds is caused by mitosis.

Humans Continually Shed Dead Cells From Their Skin House dust is more than 70% dead skin cells. These cells are continually replaced by dividing cells beneath the outer skin layer.

The Cell Cycle A cell spends most of its time (90%) growing and functioning without dividing. Mitosis or cell division is a short phase (10%) in a cell’s life.

Cells Divide At Different Rates in Different Places Cells in the body of a person divide at different rates depending on where there cells are.

The Power of Cell Division If a cell divides once per day, after 5 days there would be 25 cells or 32 cells. After 10 days there would be 210 = 1024 cells. After 18 days, there would be 218 = 262,144 cells and they would be visible to the eye (about 1 mm wide). After 30 days they would be 230 = 1,073, 741,824 cells (4 295 m wide).

Mitosis: Keeps the Chromosomes Constant Before a cell divides, it copies its chromosomes (chromatin-DNA) so that the new cells (called daughter cells) have the same number and kinds of chromosomes as the original cell (called the mother cell).

Cell Division Without Chromosome Duplication If cells were to divide without duplicating their chromosomes, the cells from these divisions get fewer and fewer chromosomes - would soon lose most of the information they need to grow and develop.

What are Chromosomes? Chromosomes are coiled up DNA and protein.

Copying Chromatin (DNA) is Called Replication Each chromatin strand (DNA) splits apart and makes a new strand on the pattern of the original strand. Thus, two identical strands are made from the original strand.

The Cell Cycle After growing and being given a message to divide from some other cell, a cell replicates its DNA and prepares for mitosis.

The Stages of Mitosis in Brief The chromosomes become visible as the chromatin coils up. The nuclear membrane disappears. A fiber structure called the spindle appears. The duplicated chromosomes attach to the spindle and line up in the middle of the cell. Each identical chromosome pair splits apart with copies going to opposite sides. Nuclear membranes reappear and chromosomes disappear, uncoiling into chromatin.

Parts of a Chromosome Each part duplicate chromosome is called a sister chromatid. They are held together at a spot called the centromere which is attached to the spindle fibers.

The Names of Mitosis Stages Prophase is the name of the stage where the nuclear membrane disappears, chromosomes become visible and the spindle forms. In animal cells structures called centrioles (centrosomes) form the spindle and rays called asters. Chromosomes attach to the spindle fibers.

The Names of Mitosis Stages Metaphase is the stage where chromosomes line up along the middle of the cell (along the cell’s equator).

The Names of Mitosis Stages Anaphase is the stage where the centromere splits apart and the sister chromatids separate to opposite sides of the cell. They are pulled apart by the spindle fibers which shorten up.

The Names of Mitosis Stages Telophase is the stage where chromosomes disappear and nuclear membranes reform around the uncoiling chromatin. Nucleoli reappear, the spindle dissolves and the division of the cytoplasm (cytokinesis) starts.

Cytokinesis: Division of the Cytoplasm After Mitosis Animal cells pinch apart to separate their cytoplasm. Plant cells make a cell plate to separate the cytoplasm. The cell plate becomes a cell wall.

Special Proteins : Checkpoints in the Cell Cycle All along the stages of the cell cycle, different proteins get made. The nucleus checks for these and will stop the cell cycle if a necessary protein has not been made. These different proteins along the cell cycle act as checkpoints on a race to ensure that mitosis is proceeding as it should.

Cancer: Uncontrolled Cell Division Cells normally only divide if they get a chemical signal from somewhere else in the body. Normal cells usually only divide up to about 35 cell divisions and then stop dividing. Telomeres (end of chromosomes) lose bits each time they divide. When enough telomere is gone, cells stop dividing.

Cancer: Uncontrolled Cell Division Cancer cells are cells that have been damaged by radiation or mutagens (chemicals). Cancer cells have their DNA genes changed (damaged).

Cancer: Uncontrolled Cell Division Cancer cells give themselves continuous signals to divide, they no longer rely on getting a message to divide from some other cell in another part of the body.

Cancer: Uncontrolled Cell Division Normal cells lose bits of telomeres and eventually can no longer divide. Cancer cells replace lost telomeres with each division.

Cancer: Uncontrolled Cell Division Normal damaged cells commit suicide (apoptosis). Many cancer cells have lost the ability to kill themselves (apoptosis)

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