Sexual reproduction involves the combining of two parent cells to create a completely new cell, which becomes the offspring. Most cells in the human body contain 23 pairs of chromosomes (46 total). These cells are called a diploid cell. Since sexual reproduction involves the combining of two cells, sex cells contain 23 unpaired chromosomes. These cells are called haploid cells. Gametes are cells that contain half the usual number of chromosomes—one chromosome from each pair. An egg is a gamete that forms in the female reproductive organs. Sperm is a gamete that forms in the male reproductive organs

Fertilization is the process that takes place when a sperm and an egg combine to form one new cell. The fertilize egg (the newly formed cell) is called a zygote. In order to produce gametes, which are haploid, a different type of cell division is necessary. Meiosis is a special type of cell division that produces haploid cells. During meiosis, a single cell goes through two cell divisions— meiosis I and meiosis II and takes place only in the reproductive organs.

Meiosis I 1.Prophase I—Chromosome condense. The threads of DNA coil up and become visible. 2.Metaphase I—Chromosome pairs line up. The pairs line up in the middle of the cell. 3.Anaphase I—Chromosome pairs separate. The pairs pull apart. This step divides the alleles of each gene. 4.Telophase I and Cytokinesis—Two cells form. The chromosome are in two groups.

Meiosis II Begins with the two cells from meiosis I. 1. Prophase II—Chromosomes are still attached. Cell prepares to divide again. Each cell has just one set of chromosome, but each chromosome is still made of two copies (chromatids) attached together. 2. Metaphase II—Chromosomes line up. The unpaired chromosomes line up along the cell’s center. 3. Anaphase II—Chromatids separate. The two attached copies of each chromosome separate and go to opposite ends. 4. Telophase II and Cytokinesis—Haploid cells form. Each cell divides into two daughter cells, producing a total of 4 cells. Each haploid (daughter cell) has one copy of one unpaired set of chromosomes.

Only cells that are to become gametes go through meiosis. All other cells divide by mitosis. Meiosis involves two cell divisions, but the chromosomes are not copies before the second division, whereas in mitosis, the chromosomes are always copied before division. Daughter cells produced by meiosis are haploid cells Daughter cells produced by mitosis contain exactly the same genetic material as the parent cell.