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Somatic cells

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Somatic cells

A somatic cell (Greek: σὠμα/soma = body) is any biological cell forming the body of an organism; that is, in a multicellular organism, any cell other than a gamete, germ cell, gametocyte or undifferentiated stem cell.[1] By contrast, gametes are cells that fuse during sexual reproduction, for organisms that reproduce sexually; Germ cells are cells that give rise to gametes; Stem cells are cells that can divide through mitosis and differentiate into diverse specialized cell types. For example, in mammals, somatic cells make up all the internal organs, skin, bones, blood and connective tissue. By contrast, mammalian germ cells give rise to spermatozoa and ova which fuse during fertilization to produce a cell called a zygote, which develops into an embryo. Somatic cells are diploid.[1]

The word "somatic" is derived from the Greek word sōma, meaning "body".

Genetics and chromosome content

Like all cells, somatic cells contain DNA arranged in chromosomes. If a somatic cell contains chromosomes arranged in pairs, it is called diploid and the organism is called a diploid organism. (The gametes of diploid organisms contain only single unpaired chromosomes and are called haploid.) Each pair of chromosomes comprises one chromosome inherited from the father and one inherited from the mother. For example, in humans, somatic cells contain 46 chromosomes organized into 23 pairs. By contrast, gametes of diploid organisms contain only half as many chromosomes. In humans, this is 23 unpaired chromosomes. When two gametes (i.e. a spermatozoon and an ovum) meet during conception, they fuse together, creating a zygote. Due to the fusion of the two gametes, a human zygote contains 46 chromosomes (i.e. 23 pairs).

However, a large number of species have the chromosomes in their somatic cells arranged in fours ("tetraploid") or even sixes ("hexaploid"). Thus, they can have diploid or even triploid germline cells. An example of this is the modern cultivated species of wheat, Triticum aestivum L., a hexaploid species whose somatic cells contain six copies of every chromatid.

Cloning

In recent years, the technique of cloning whole organisms has been developed in mammals, allowing almost identical genetic clones of an animal to be produced. One method of doing this is called "somatic cell nuclear transfer" and involves removing the nucleus from a somatic cell, usually a skin cell. This nucleus contains all of the genetic information needed to produce the organism it was removed from. This nucleus is then injected into an ovum of the same species which has had its own genetic material removed. The ovum now no longer needs to be fertilized, because it contains the correct amount of genetic material (a diploid number of chromosomes). In theory, the ovum can be implanted into the uterus of a same-species animal and allowed to develop. The resulting animal will be a nearly genetically identical clone to the animal from which the nucleus was taken. The only difference is caused by any mitochondrial DNA that is retained in the ovum, which is different than the cell that donated the nucleus. In practice, this technique has so far been problematic, although there have been a few high profile successes, such as Dolly the Sheep and, more recently, Snuppy, the first cloned dog.

References

See also

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