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Cell Division

Your body is composed of more than a billion cells. Cells are continually dying, and new cells are continually being formed. An identical copy of your hereditary material is found in the nucleus of each and every somatic cell. A somatic cell is any cell in the body except for the reproductive cells in the reproductive system.
This genetic blueprint is organized into 46 chapters or parts known as chromosomes. It is estimated that, on average, each chromosome contains between one and two thousand genes. A gene contains the information for making a single protein or RNA product.
Every time a cell divides, each chromosome must be carefully replicated (copied) and then distributed to assure that each daughter cell gets a complete and accurate set of information. Thus, nuclear division includes successive processes of chromosome replication, separation, and distribution.
Cell division is essential to growth, repair and reproduction. The process of dividing the genetic material among the daughter cells is imperitive to both types of cell division (mitosis & meiosis).
In mitosis, the daughter cells receive the same number of chromosomes as the parent cell. They are exact copies!
In meiosis, a reductive cell division occurs such that the daughter cells are haploid (or half) with respect to the parent cell.

Stages of: Mitosis

Interphase: The time before mitosis. The cells may appear inactive during this stage, but they are quite the opposite:
• This is the longest period of the complete cell cycle.
• The cells enlarge, preparing for mitosis.
• The DNA replicates, or copies itself.
• The cell grows & makes structures to use during the rest of the cell cycle
Early Prophase: During this first mitotic stage:
• The chromatin in the nucleus condenses and becomes visible chromosomes. Each replicated (copied) chromosome is made of two chromatids, both with the same genetic information.
• Spindle fibers begin to form around the centrioles.
Mitosis begins. The nuclear membrane breaks apart. Chromosomes condense into
rodlike structures. The two pairs of centrioles move to opposite sides of the cell. Fibers form between the two pairs of centrioles and attach to the centromeres.
Middle Prophase: • The nuclear membrane breaks down.
• The centrioles are moving to opposite ends of the cell.
Late Prophase: • The nuclear membrane is completely gone.
• The chromosomes have doubled, and are moving toward the middle.
• The centrioles are a little further apart.
Metaphase: The ‘middle’ phase:
• The centromere attaches the chromatids to the spindle fibers.
•Tension applied by the spindle fibers aligns all chromosomes at the center of the
cell.
Anaphase:
• The chromatids (daughter chromosomes) separate, the spindle fibers shorten &
the chromatids are pulled apart & begin moving to the cell poles.
Late Anaphase: • The spindle fibers are getting shorter.
•The daughter chromosomes arrive at the poles (opposite ends of the cell).
The chromatids separate and are pulled to opposite sides of the cell by the fibers attached to the centrioles. Before mitosis begins, the chromosomes and other cell materials are copied. The pair of centrioles, which are two cylindrical structures, are also copied. Each chromosome now consists of two chromatids.
Telophase: • The nuclear membrane forms around the chromosomes.
• The spindle fibers that have pulled them apart disappear.
• The cell membrance is beginning to pinch the cytoplasm (pinocytosis).
Late Telophase/Cytokinesis: :The middle of the ‘cell’ cleaves the cell into two cells. • The chromosomes thicken and become longer.
• The result is two identical daughter cells that are also identical to the original parent cell. The nuclear membrane forms around the two sets of chromosomes, and they unwind. The fibers disappear. Mitosis is completed. Once mitosis is completed, the cytoplasm splits in two. This process is called cytokinesis.
The result is two identical cells that are also identical to the original cell from which they were formed. After cytokinesis, the cell cycle is complete, and the new cells are at the beginning of their next cell cycle.

Mitosis: Terms to Know

anaphase - phase of mitosis where sister chromatids separate and daughter chromosomes migrate to opposite poles of the cell.
centrioles - structures in the cytoplasm from which the spindle apparatus forms.
centromere - constriction in an unreplicated chromosome which results in two regions or arms ; serves as an attachment site for sister chromatids and spindle fibers .
chromatid - one half of a replicated chromosome which is joined to the other half at the centromere.
chromosome - discrete structures which occur in eukaryotic cell nuclei, contain one or two DNA double helixes
cytokinesis - division of the cytoplasm, usually at the end of nuclear division.
daughter cell - one of two cells resulting from the division of a single cell..
gametes - reproductive cells; sperm and egg cells in animals.
gene - section of a chromosome which codes for a protein or RNA product.
interphase - the portion of the cell cycle where the cell is not dividing; includes G1, S and G2 stages.
metaphase - stage of mitosis in which replicated chromosomes align at the center of the cell.
mitosis - process of cellular division in which the daughter cells are genetically and morphologically identical to themselves and to the mother cell.
nucleus - organelle in the center of the cell which contains the chromosomes.
prophase - stage of mitosis in which replicated chromosomes condense; nuclear membrane dissociates; centrioles migrate to the poles of the cell.
spindle fibers - protein structures which move the chromosomes during cell division.
telophase - mitotic stage where nuclear membrane reforms and the spindle fibers disappear.