Since each cell needs a complete set of genes, an ancestral cell must duplicate its genome before dividing. Each of the two new cells then receives one copy of the genome. Because the genes are made of DNA and are located on the chromosomes, this means that each chromosome must be accurately copied. When a bacterial cell, with a single chromosome divides, each daughter cell receives a copy of the parental chromosome. Division of eukaryotic cells is more complex, as each cell has multiple chromosomes. Not only must all of the chromosomes be duplicated, but a mechanism is needed to ensure that both daughter cells receive identical sets of chromosomes at cell division. This complex process, known as mitosis, is described further below.
When a single-celled organism divides, the result is two new organisms, each consisting of one cell. However, in multi-cellular organisms cell division does not automatically result in the creation of new organisms. When the cells composing a multi-cellular organism divide, they increase the size and/or complexity of the original organism. A distinct process is needed to form new organisms. The term reproduction is used to signify the production of a new individual organism. Thus, in unicellular organisms, cell division and reproduction occur simultaneously, whereas in multi-cellular organisms cell division and reproduction are two different processes.
In many plants and fungi, clumps of cells may break off or single celled spores may be released from the parental organism and give rise to new individual multi-cellular organisms. This is known as asexual or vegetative reproduction as these new individuals will be genetically identical to their parents. This contrasts with sexual reproduction, where each new individual receives roughly equal amounts of genetic information from two separate parents and is therefore a novel genetic assortment. Sexual reproduction is especially characteristic of animals and also occurs in most higher plants and many fungi. Some organisms, particularly plants and fungi possess the ability to reproduce both sexually or asexually. Although they are inextricably entwined in humans and many other animals, it is important to realize that sex and reproduction are two distinct processes from a biological viewpoint.
Strictly speaking, bacteria do not reproduce sexually since new bacteria always result from the division of a single parental cell. Nonetheless, mixing of genes from two individuals may occur in bacteria. However, this occurs in the absence of cell division and involves transfer of a relatively small segment of DNA from one cell (the donor) to another cell (the recipient) (see Ch. 18 for details). Such sideways transfer of DNA, between members of the same generation is sometimes referred to as horizontal gene transmission (see Ch. 15). In contrast, vertical gene transmission is when genes are transmitted from the previous generation to the new generation. Vertical transmission thus includes all forms of cell division and reproduction that create a new copy of the genome, whether sexual or not.
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