Cell division, also called cell reproduction, occurs in humans and other organisms at different times in their life. The type of cell division differs depending on the organism and why the cell is dividing.
For example, bacterial cells undergoing reproduction divide by one type of cell division. Eukaryotic organisms undergoing growth, development, repair, or asexual reproduction divide by a different type of cell division. And the formation of gametes involves a third type of cell division. Gametes are an organism’s reproductive cells, such as sperm or egg cells.
Prokaryotic Cell Reproduction:
Prokayotes reproduce by a type of cell division called binary fission . Binary fission is a form of asexual reproduction that produces identical offspring. In asexual reproduction, a single parent passes exact copies of all of its DNA to its offspring.
Binary fission occurs in two stages: first, the DNA is copied (so that each new cell will have a copy of the genetic information), and then the cell divides. The prokaryote divides by adding a cell membrane to a point on the membrane between the two DNA copies.
Eukaryotic Cell Reproduction
The vast amount of information encoded in DNA is organized into units called genes . A gene is a segment of DNA that codes for a protein or RNA molecule. Genes play an important role in determining how a person’s body develops and functions.
As a eukaryotic cell prepare to divide, the DNA and the proteins associated with the DNA coil into a structure called chromosomes . Before the DNA coils up, however, the DNA is copied. The two exact copies of DNA that make up each chromosome are called chromatids . The two chromatids of a chromosome are attached at a point called a centromere .
Sets of Chromosomes
Each human somatic cell (any cell other than a sperm or egg cell) normally has two copies of 23 different chromosomes, for a total of 46 chromosomes. The 23 chromosomes differ in size, shape, and set of genes.
Each of the 23 pairs of chromosomes consists of two homologous chromosomes, or homologues.
Homologous chromosomes are chromosomes that are similar in size, shape, and genetic content.
Each homologue in a pair of homologous chromosomes comes from one of the two parents.

When a cell, such as a somatic cell, contains two sets of chromosomes, it is said to be diploid . When a cell, such as a gamete, contains one set of chromosomes, it is said to be haploid . Biologists use the symbol n to represent one set of chromosomes. The haploid number in a human gamete can be written as n = 23. The diploid number in a somatic cell can be written as n = 46. The fusion of two haploid gametes—a process called fertilization—forms a diploid zygote. A zygote is a fertilized egg cell, the first cell of a new individual.
Sex Chromosomes
Of the 23 pairs of chromosomes in human somatic cells, 22 pairs are called autosomes. Autosomes are chromosomes that are not directly involved in determining the sex (gender) of an individual. The sex chromosomes , one of the 23 pairs of chromosomes in humans, contain genes that will determine the sex of the individual.
In humans and many other organisms, the two sex chromosomes are referred to as the X and Y chromosomes. The genes that cause a fertilized egg to develop into a male are located on the Y chromosome. Thus, any individual with a Y chromosome is male, and an individual without a Y chromosome is a female. In human males, the sex chromosomes are made up of one X chromosome and one Y chromosome ( XY ). In human females consist of two X chromosomes ( XX ).
Change in Chromosome Number
The presence of all 46 chromosomes is essential for normal development and function. Humans who are missing even one of the 46 chromosomes do not survive. Humans with more then two copies of a chromosome, a condition called trisomy , will not develop properly. Abnormalities in the chromosome number can be detected by analyzing a karyotype , a photo of the chromosomes in a dividing cell that shows the chromosomes arranged by size.
A portion of a karyotype from an individual with an extra copy of chromosome 21 is called Down
Syndrome , or trisomy 21. What events can cause an individual to have an extra copy of a chromosome? When a sperm and egg cell form, each chromosome and it homologue separate, and event called disjunction . If one or more chromosomes fail to separate properly—an event calle nondisjunction— one new gamete ends up receiving both chromosomes and the other gamete receives none. Trisomy occurs when the gamete with both chromosomes fuses with a normal gamete during fertilization, resulting in offspring with 3 copies of that chromosome instead of two.
Changes in an organism’s chromosome structure are called mutations .
Cell division in eukaryotic cells is more complex than cell division in bacteria because it involves dividing both the cytoplasm and the chromosomes inside the nucleus.

The Cell Cycle
The cell cycle is a repeating sequence of cellular growth and division during the life of an organism.
A cell spends 90% of its time in the first 3 phases of the cycle, which are collectively called interphase .
1.
First growth (G
1
) phase. During the G
1
phase, a cell grows rapidly and carries out its routine functions. Cell’s that are not dividing remain in the G
1
phase. Some somatic cells, such as most muscle and nerve cells, never divide.
2.
Synthesis (S) phase.
A cell’s DNA is copied during this phase. At the end of this phase, each chromosome consists of two chromatids attached at the centromere.
3.
Second growth (G
2
) phase. In the G
2 phase, Hollow protein fibers called microtubules are assembled. The microtubules are used to move the chromosomes during mitosis.
4.
Mitosis.
The process during cell division in which the nucleus of a cell is divided into two nuclei.
Each nucleus ends up with same number and kinds of chromosomes as the original cell.
5.
Cytokinesis.
The process during cell division in which the cytoplasm divides.
Mitosis
Step 1: Prophase. Chromosomes coil up and become visible during prophase. The nuclear envelope dissolves and a spindle forms.
Step 2: Metaphase. During Metaphase the chromosomes move to the center of the cell and line up along the equator. Spindle fibers link the chromatids of each chromosome to opposite poles.
Step 3: Anaphase. Centromeres divide during anaphase. The two chromatids (now called chromosomes) move toward opposite poles as the spindle fibers attached to them shorten.
Step 4: Telophase. A nuclear envelope forms around the chromosomes at each pole. Chromosomes, now at opposite poles, uncoil and the spindle dissolves. The spindle fibers break down and disappear.
Cytokinesis
As mitosis ends, cytokinesis begins. The cytoplasm of the cell is divided in half, and the cell membrane grows to enclose each cell, forming two separate cells. The end result of mitosis and cytokinesis is two genetically identical cells where only one cell existed before. In animal cells and other cells that lack a cell wall, the cell is pinched in half by a belt of protein threads.
In plant cells, vesicles formed by the Golgi apparatus fuse at the midline of the dividing cell and form a cell plate. A cell plate is a membrane-bound cell wall that forms across the middle of the plant cell.
A new cell wall then forms on both sides of the cell plate. When complete, the cell plate separates the plant cell into two new plant cells