Cells do not just suddenly appear, they come form a mother cells or progenitor cell. In the case of the eukaryotes, they divide (duplicate), transmit their characteristics and give was to two or more daughter cells. This process is called mitosis and is what assures growth, renovation and repairs of the cell, which are necessary for life to continue. The interval between each mitotic division is known as cell cycle.
Mitosis goes through the following phases:
Interphase: It can be referred to as the rest stage, when the cell is waiting to split. During this phase, the cell replicates its genetic material and prepares for mitosis.
Prophase: It is the first phase of mitosis, when the centriloes duplicate and migrate to the opposite poles of the cell. The chromosomes condense, form the cytoplasmic spindle and make their double structure (chromatins) visible. The cell membrane tends to disintegrate. The cellular organelles, unlike the mitochondria, seem to disappear.
Metaphase: The chromosomes line up with the spindle fibers in the center of the cell.
Anaphase: The chromatids divide and move towards the cell poles with the spindle fibers. At the end of this stage, a complete set of chromosomes group together in each cell pole.
Telophase: The sets of chromosomes are grouped in the cell poles and the nuclear envelope surrounds each set. The chromosomes unroll and two nuclei, identical to the original, appear.
There are cells that do not have a normal amount of chromosomes and they are called sex cells or male and female gametes (spermatozoon and ovum), both containing half of the chromosome dowry. Meosis consists of two successive nuclear divisions, meiosis I (which separates the chromosomes which have been paired) and meiosis II (which then separates them in half). Each phase has the following stages: prophase, anaphase and telphase. Finally, meiosis combined with fertilization is the foundation sexual reproduction.