Two haploid gametes combine into one diploid cell known as a zygote in a process called fertilisation. Two rounds of cell division then produce four haploid gametes, each with half the number of chromosomes from each parent cell, but with the genetic information in the parental chromosomes recombined. The homologous chromosomes pair up so that their DNA sequences are aligned with each other, and this is followed by exchange of genetic information between them. In the production of sex cells in eukaryotes, diploid mother cells divide to produce haploid cells known as gametes in a process called meiosis that involves genetic recombination. Sexual reproduction does not occur in prokaryotes (organisms without cell nuclei), but they have processes with similar effects such as bacterial conjugation, transformation and transduction, which may have been precursors to sexual reproduction in early eukaryotes. Sexual reproduction is the most common life cycle in multicellular eukaryotes, such as animals, fungi and plants. Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete (such as a sperm or egg cell) with a single set of chromosomes ( haploid) combines with another to produce a zygote that develops into an organism composed of cells with two sets of chromosomes ( diploid). During " fertilisation", haploid gametes come together to form a diploid zygote, and the original number of chromosomes is restored.
In the first stage of sexual reproduction, " meiosis", the number of chromosomes is reduced from a diploid number (2n) to a haploid number (n).