Sex is two cells fusing together and combining genetic material. Fertilization is the beginning of a new individual. Development starts the moment gametes fuse. Reproduction is the division of one cell to form a new individual.
Sexual reproduction uses meiosis and fertilization. In the process of meiosis haploid cells are produced. These haploid cells containing a single set of chromosomes (denoted as N) are called gametes. Gametes are the sex cells (egg and sperm cells). Each human gamete has 22 autosomes and a sex chromosome (X or Y). Gametes cannot survive independently of the diploid stage. In humans, meiosis occurs in the ovaries and testes to produce egg and sperm cells with 23 chromosomes. All cells produced in meiosis in the male are sperm, but only one of the four produced in the female is an egg. This contributes to the great variety of possibilities in offspring.
Fertilization is the process where the sperm and egg unite, each containing half the number of chromosomes other body cells have (Somatic cells are cells other than egg and sperm cells). Fused together they form a zygote. A zygote is a fertilized egg with 46 chromosomes. It is a diploid cell with half the genes from the mother and half from the father. A diploid cell has two sets of chromosomes (denoted as 2N). A single human egg receives one of two possible chromosomes 23 times, same for the sperm cell. The zygote contain genetic information from both gametes. The zygote develops into a multicellular organism through mitosis.
Sexual and asexual reproduction differ in many aspects. In sexual reproduction the chances that the offspring can adapt to the changes in the environment is high because of the great diversity. Asexual reproduction produces organism with uniformity, identical to the parent, and have less success adapting to changes in the environment. The methods of reproduction are also different. Sexual reproduction involves both meiosis, fertilization, and mitosis to produce a multicellular organism. Asexual uses only mitotic division to produce another copy of itself. Sexual reproduction involves gametes or reproductive cells where as asexual involves somatic or vegetative cells. Asexual reproduction is also known as vegetative reproduction. Because sexually produced organisms are diverse they have better chances of adapting to changes in the environment.
Different organisms follow different paths of reproduction. Meiosis and fertilization is the path followed by all sexually reproducing organisms, but it is the timing and order that differs in organisms. Fungi and some protist go through mitosis to produce gametes and go through meiosis right after the zygote is formed. In plants it includes both diploid and haploid stages. The diploid stage in plants is called the sporophyte and the haploid cells produced by meiosis are called spores. Spores give rise to multicellular individuals without fusing with other cells. Plants can reproduce themselves without a gamete from a different plant to fuse with them.
Fertilization
Meiosis and Sexual Life Cycles