THE CELL CYCLE

The Cell Cycle The life cycle of the cell is called the cell cycle. The life cycle begins when the cell is formed. It ends when the cell divides

This is a form of asexual reproduction. Mitosis is the process in which the nucleus divides to form two new nuclei. This is a form of asexual reproduction.

Three reasons why cells reproduce by asexual reproduction: 1. Growth 2 Three reasons why cells reproduce by asexual reproduction: 1. Growth 2. Repair 3. Replacement

Phases of Cell Division Interphase Prophase Metaphase Anaphase Telophase Cytokinesis mitosis http://www.cellsalive.com/mitosis.htm

Before a cell divides: The cell must make a copy of its DNA. DNA – deoxyribonucleic acid DNA is the hereditary material that directs all cell activities, including the making of a new cell.

Interphase This phase of the cell cycle is called interphase. During this phase, the cell grows and Copies its organelles and chromosomes.

The DNA of a cell is organized into structures called chromosomes. Chromosomes appear as threadlike coils (chromatin) at the start. After each chromosome is duplicated, the two copies are called chromatids

Chromotids are Held together in the centromere. The joined chromotids condense into an “x” shape.

Copying chromosomes ensures that each new cell will be an exact copy of its parents

Ninety percent or more of the cell cycle is spent in interphase. During interphase, cellular organelles double in number, the DNA replicates, and protein synthesis occurs. (These are cells from a whitefish.)

prophase – chromosomes condense from long strands into rod like structures. (this is the longest phase)

metaphase: paired chromatids join at cell equator. The nuclear membrane has been dissolved.

a nuclear membrane forms around each set of chromosomes.

In cells that do not have cell walls, division of the cytoplasm begins at the cell membrane. The cell membrane pinches all the way through the cell. Two daughters cell form.

The division of cytoplasm is called cytokinesis.

Average Cell Division Times The amount of time it takes for one cell to completely divide in most mammals, including the human body, is about 24 hours. Some cells take more or less time, depending on their purpose; a fly embryo takes only eight minutes to divide. a human liver cell could take more than a year. Gametes, cells used in sexual reproduction, can take decades to divide. Abnormal cell division can also occur, resulting in cancerous cells that multiply much more rapidly.

In the case of bacteria and rapidly growing organisms like a fetus or a young animal, cell division takes place very rapidly so the organism can grow and thrive. Most of the cells in fully-grown organisms divide on a much slower scale to slowly renew the body. The exception to this is in areas like the skin and bone marrow, which are constantly generating new cells so that, if an injury is sustained, the cells can divide quickly to heal the area.