Question: What is the process of cell duplication?
About the article "Biology", section "Cell theory".

Candidate passages:
DDC stands for "duplication-degeneration-complementation".
This model was first introduced by Force et al.
1999.
The first step is gene duplication.
The gene duplication in itself is neither advantageous, nor deleterious, so it will remain at low frequency within a population of individuals that do not carry a duplication.
According to DDC, this period of neutral drift may eventually lead to the complementary retention of sub-functions distributed over the two gene copies.
This comes about by activity reducing (degenerative) mutations in both duplicates, accumulating over time periods and many generations.
Taken together, the two mutated genes provide the same set of functions as the ancestral gene (before duplication).
However, if one of the genes was removed, the remaining gene would not be able to provide the full set of functions and the host cell would likely suffer some detrimental consequences.
Therefore, at this later stage of the process, there is a strong selection pressure against removing any of the two gene copies that arose by gene duplication.
The duplication becomes permanently established in the genome of the host cell or organism. 
Karyogamy is the final step in the process of fusing together two haploid eukaryotic cells, and refers specifically to the fusion of the two nuclei.
Before karyogamy, each haploid cell has one complete copy of the organism's genome.
In order for karyogamy to occur, the cell membrane and cytoplasm of each cell must fuse with the other in a process known as plasmogamy.
Once within the joined cell membrane, the nuclei are referred to as pronuclei.
Once the cell membranes, cytoplasm, and pronuclei fuse together, the resulting single cell is diploid, containing two copies of the genome.
This diploid cell, called a zygote or zygospore can then enter meiosis (a process of chromosome duplication, recombination, and division, to produce four new haploid cells), or continue to divide by mitosis.
Mammalian fertilization uses a comparable process to combine haploid sperm and egg cells (gametes) to create a diploid fertilized egg.
The term karyogamy comes from the Greek "karuo-" (from "karuon") meaning "nut" and "-gamos" meaning "marriage". 
The cell cycle, or cell-division cycle, is the series of events that take place in a cell leading to its division and duplication (replication) that produces two daughter cells.
In cells without a nucleus (prokaryotic), the cell cycle occurs via a process termed binary fission.
In cells with a nucleus (eukaryotes), the cell cycle can be divided in three periods: interphase—during which the cell grows, accumulating nutrients needed for mitosis preparing it for cell division and duplicating its DNA—and the mitotic (M) phase, during which the cell splits itself into two distinct cells, often called "daughter cells" and the final phase, cytokinesis, where the new cell is completely divided.
The cell-division cycle is a vital process by which a single-celled fertilized egg develops into a mature organism, as well as the process by which hair, skin, blood cells, and some internal organs are renewed.
After cell division, each of the daughter cells begin the interphase of a new cycle.
Although the various stages of interphase are not usually morphologically distinguishable, each phase of the cell cycle has a distinct set of specialized biochemical processes that prepare the cell for initiation of cell division. 
SAS-6 is necessary for centrosome duplication and functions during procentriole formation; SAS-6 functions to ensure that each centriole seeds the formation of a single procentriole per cell cycle. 
  Gene duplication, a process which can result in free mutation  Chromosomal duplication, which can cause Bloom and Rett syndrome  Polyploidy, a phenomenon also known as "ancient genome duplication"  Enteric duplication cysts, certain portions of the gastrointenstinal tract  Diprosopus, a form of cojoined twins also known as "craniofacial duplication"  Diphallia, a medical condition also known as "penile duplication"