Sexual Reproduction

Male and female reproductive cells combine to produce a zygote:

In sexual reproduction, each of the two parents contributes characteristics to the offspring. These sex cells are called gametes and are different from the rest of the cells in our body, as they contain different numbers of chromosomes. The male parent contributes one gamete, which is the sperm cell. The female parent contributes the other gamete, called the ovum or egg cell. In humans, sperm is produced in the testes and eggs are produced in the ovaries.

Contact between the gametes allows the sex cells’ genetic material to combine to produce a cell that eventually develops into an offspring. The process which male and female gametes combine is called fertilization. The nuclei of the two gametes fuse together to produce a single cell called a zygote. The zygote is the first cell that develops into a new organism containing genetic information from both the sperm and the egg cell. Sperm reach a jelly-like coating surrounding the egg cell and release substances that digest a path through the coating. This helps sperm cells get closer to the cell membrane of the egg. The head of ONE sperm cell eventually enters the egg cell, where the sperm nucleus fuses with the egg nucleus.

Reproductive cells are formed by a cell-dividing process called meiosis.

Humans have 46 chromosomes that are organized into 23 pairs. Chromosomes that are paried are called homologous chromosomes. They have similar features and carry similar genetic information During fertilization, each parent contributes one chromosome of each pair.Since gametes combine in fertilization, each gamete must have HALF the number of chromosomes of other body cells. This ensures the correct number of chromosomes in each offspring. Cells with half the normal number of chromosomes (gametes) are called haploid cells. Cells that have the full number of chromosomes are called diploid cells. The process of producing of a diploid organism producing haploid gametes is called MEIOSIS

Meiosis produces unique gametes.Meiosis involves a diploid cell dividing twice to produce four haploid cells. Offspring are genetically different from one another because the gametes that a parent produces are not all genetically the same.

Meiosis I Meiosis II

Meiosis I:prophase I - The nuclear membrane begins to disappear.

- DNA condenses into duplicated chromosomes and homologous chromosomes are pairedmetaphase I - Spindle fibres guide chromosome movement

- Homologous chromosome pairs line up along the middle of the cellanaphase I -Homologuous chromosome pairs separate and go to each end of the celltelophase I - Two nuclei form and each nucleus contains a complete copy of the cell’s DNA

- The cell divides

Meiosis II:Prophase II - The nuclear membrane begins to disappear

- DNA exists as chromosomesmetaphase II - The chromosmes line up along the middle of the cellanaphase II - The copies of DNA are separated and go to each end of the celltelophase II - Four nuclei form

- the cells divide, forming four new cells.