Editing Cell growth (section)

==Cell division==

Cell reproduction is [[asexual reproduction|asexual]]. For most of the constituents of the cell, growth is a steady, continuous process, interrupted only briefly at [[M phase]] when the nucleus and then the cell divide in two.

The process of cell division, called [[cell cycle]], has four major parts called phases. The first part, called G<sub>1</sub> phase is marked by synthesis of various [[enzyme]]s that are required for DNA replication.
The second part of the cell cycle is the S phase, where [[DNA replication]] produces two identical sets of [[chromosomes]]. The third part is the G<sub>2</sub> phase in which a significant [[protein synthesis]] occurs, mainly involving the production of [[microtubules]] that are required during the process of division, called [[mitosis]].
The fourth phase, M phase, consists of nuclear division ([[karyokinesis]]) and cytoplasmic division ([[cytokinesis]]), accompanied by the formation of a new [[cell membrane]]. This is the physical division of mother and daughter cells. The M phase has been broken down into several distinct phases, sequentially known as [[prophase]], [[prometaphase]], [[metaphase]], [[anaphase]] and [[telophase]] leading to cytokinesis.

Cell division is more complex in [[eukaryote]]s than in other organisms. [[Prokaryote|Prokaryotic]] cells such as [[bacteria]]l cells reproduce by [[binary fission]], a process that includes DNA replication, chromosome segregation, and cytokinesis. Eukaryotic cell division either involves [[mitosis]] or a more complex process called [[meiosis]]. Mitosis and meiosis are sometimes called the two [[cell nucleus|nuclear]] division processes. Binary fission is similar to eukaryote cell reproduction that involves mitosis. Both lead to the production of two daughter cells with the same number of chromosomes as the parental cell. Meiosis is used for a special cell reproduction process of [[diploid]] organisms. It produces four special daughter cells ([[gamete]]s) which have half the normal cellular amount of DNA. A [[male]] and a [[female]] gamete can then combine to produce a [[zygote]], a cell which again has the normal amount of chromosomes.

The rest of this article is a comparison of the main features of the three types of cell reproduction that either involve binary fission, mitosis, or meiosis. The diagram below depicts the similarities and differences of these three types of cell reproduction.

[[File:Three cell growth types.svg|thumb|Cell growth]]

===Comparison of the three types of cell division===
The DNA content of a cell is duplicated at the start of the cell reproduction process. Prior to [[DNA replication]], the DNA content of a cell can be represented as the amount Z (the cell has Z chromosomes). After the DNA replication process, the amount of DNA in the cell is 2Z (multiplication: 2 x Z = 2Z). During Binary fission and mitosis the duplicated DNA content of the reproducing parental cell is separated into two equal halves that are destined to end up in the two daughter cells. The final part of the cell reproduction process is [[cell division]], when daughter cells physically split apart from a parental cell. During meiosis, there are two cell division steps that together produce the four daughter cells.

After the completion of binary fission or cell reproduction involving mitosis, each daughter cell has the same amount of DNA (Z) as what the parental cell had before it replicated its DNA. These two types of cell reproduction produced two daughter cells that have the same number of chromosomes as the parental cell. Chromosomes duplicate prior to cell division when forming new skin cells for reproduction. After meiotic cell reproduction the four daughter cells have half the number of chromosomes that the parental cell originally had. This is the [[haploid]] amount of DNA, often symbolized as N. Meiosis is used by [[diploid]] organisms to produce haploid gametes. In a diploid organism such as the human organism, most cells of the body have the diploid amount of DNA, 2N. Using this notation for counting chromosomes we say that human [[Somatic (biology)|somatic]] cells have [[Karyotype|46 chromosomes]] (2N = 46) while human [[Spermatozoon|sperm]] and [[Ovum|egg]]s have 23 chromosomes (N = 23). Humans have 23 distinct types of chromosomes, the 22 [[autosome]]s and the special category of [[Sex-determination system|sex chromosomes]]. There are two distinct sex chromosomes, the X chromosome and the Y chromosome. A diploid human cell has 23 chromosomes from that person's father and 23 from the mother. That is, your body has two copies of human chromosome number 2, one from each of your parents.
[[File:Single and double chromosomes.png|thumb|right|150px|Chromosomes]]
Immediately after DNA replication a human cell will have 46 "double chromosomes". In each double chromosome there are two copies of that chromosome's DNA molecule. During mitosis the double chromosomes are split to produce 92 "single chromosomes", half of which go into each daughter cell. During meiosis, there are two chromosome separation steps which assure that each of the four daughter cells gets one copy of each of the 23 types of chromosome.

===Sexual reproduction===
{{main|Evolution of sex}}
{{further|Origin and function of meiosis|Homologous recombination}}
Though cell reproduction that uses mitosis can reproduce eukaryotic cells, eukaryotes bother with the more complicated process of meiosis because [[sexual reproduction]] such as meiosis confers a [[Natural selection|selective advantage]]. Notice that when meiosis starts, the two copies of sister chromatids number 2 are adjacent to each other. During this time, there can be [[genetic recombination]] events. Information from the chromosome 2 DNA gained from one parent (red) will transfer over to the chromosome 2 DNA molecule that was received from the other parent (green). Notice that in mitosis the two copies of chromosome number 2 do not interact. [[Homologous recombination|Recombination of genetic information between homologous chromosomes]] during [[meiosis]] is a process for [[DNA repair|repairing DNA damages]]. This process can also produce new combinations of genes, some of which may be adaptively beneficial and influence the course of evolution. However, in organisms with more than one set of chromosomes at the main life cycle stage, sex may also provide an advantage because, under random mating, it produces [[homozygote]]s and [[heterozygote]]s according to the [[Hardy–Weinberg ratio]].