Put simply, the goal of cellular reproduction is to "reproduce" a copy of a preexisting cell. Cells achieve this by first copying their contents and then dividing such that each of the resulting two cells has the same components. These processes are a part of a larger cell cycle that also includes periods of preparation for synthesizing copies of cellular components and division. This cycle occurs continuously in most organisms.
Cellular reproduction is a means of creating new life. As we mentioned, cell reproduction does not always result in the creation of an entirely distinct organism. Cell reproduction is also responsible for creating supporting cells in multi-cellular organisms. In these multi-cellular organisms, many rounds of cell reproduction are often necessary to create new individuals, whereas in single-cell organisms, each round of mitosis results in an independent organism.
Since cell reproduction involves the copying old cells into new ones, resulting cells must be faithful enough copies that they can perform the same function as the cell from which it was copied. If the copying process is not fine-tuned, mutations, or errors, can occur in the offspring cells. These mutations can vary from trivial, without physical or biological manifestation, to serious, causing severe disorders or even death.
We have discussed how there are two different types of cellular reproduction. In either of these processes, a main step in the cycle of cellular reproduction is the copying of cell contents. What exactly is copied during this process? The most important cellular components are the chromosomes, which contains all the genetic information for a cell and leads to the specific features, traits, and capabilities of a cell. DNA is packaged into chromosomes in eukaryotic cells. Not all cellular components are copied like DNA. Some structures can be synthesized in a cell from DNA after cell division. Other structures, like the endoplasmic reticulum are broken down during the cell cycle and then re-synthesized after cell division.
In higher organisms each cell usually contains two similar copies of each chromosome. One of these copies is a maternal contribution and the other is a paternal contribution. Together, these are called a homologous pair and each alone is called a homologue. The haploid number of a cell refers to the total number of homologous pairs in a cell (or the number of unique chromosomes). This number varies from species to species; in humans it is 23. The diploid number of a cell refers to the total number of chromosomes in a cell and is equal to two times the haploid number. If the haploid number is thought of as N the diploid number would be 2N. In humans the diploid number is 46.