Germ plasm
Germ plasm or polar plasm is a zone found in the cytoplasm of the egg cells of some organisms, which contains determinants that will give rise to the germ cell lineage. As the zygote undergoes mitotic divisions the germ plasm is ultimately restricted to a few cells of the embryo. These germ cells then migrate to the gonads.
Germ plasm theory
The term germ plasm was first used by the German biologist August Weismann (1834–1914). His germ plasm theory states that multicellular organisms consist of germ cells that contain and transmit heritable information, and somatic cells which carry out ordinary bodily functions.[1] In the germ plasm theory, inheritance in a multicellular organism only takes place by means of the germ cells: the gametes, such as egg cells and sperm cells. Other cells of the body do not function as agents of heredity. The effect is one-way: germ cells produce somatic cells, and more germ cells; the germ cells are not affected by anything the somatic cells learn or any ability the body acquires during its life. Genetic information cannot pass from soma to germ plasm and on to the next generation. This is referred to as the Weismann barrier.[2] This idea, if true, rules out the inheritance of acquired characteristics as proposed by Jean-Baptiste Lamarck.[3]
The part of Weismann's theory which proved most vulnerable was his notion that the germ plasm (effectively, genes) were successively reduced during division of somatic cells. As modern genetics developed, it became clear that this idea was in most cases wrong.[4] Cases such as Dolly (the famous cloned ewe) which, via somatic cell nuclear transfer, proved that adult cells retain a complete set of information – as opposed to Weismann's increasingly determined gradual loss of genetic information – putting this aspect of Weismann's theory to rest as a general rule of metazoan development. However, genetic information is really lost by somatic cells in some groups of animals. The best known example are the nematodes, in which the phenomenon of chromatin diminution was first described by Theodor Boveri.
The idea was to some extent anticipated in an 1865 article by Francis Galton, published in Macmillan's Magazine, which set out a weak version of the concept. In 1889 Weismann wrote to acknowledge that "You have exposed in your paper an idea which is in one essential point nearly allied to the main idea contained in my theory of the continuity of germ-plasm".[5]
The notion that the germ cells are unaffected by somatic cells or their environment is also proving not absolute. Chemical modification of the nucleotide bases that constitute the genetic code such as methylation of cytosines as well as modifications of the histones around which the DNA is organized into higher-order structures are influenced by the metabolic and physiologic state of the organism and in some cases can be heritable. Such changes are called epigenetic because they do not alter the nucleotide sequence.
Germplasm, other usage
- Germplasm, the collection of genetic resources
References
- ^ 1892. Das Keimplasma: eine Theorie der Vererbung. Fischer, Jena.
- ^ Germ-Plasm, a theory of heredity (1893)- Full online text
- ^ Huxley, Julian 1942. Evolution, the modern synthesis. p17
- ^ For example, by studies of polytene chromosomes in salivary glands (i.e. somatic cells) of larval Drosophila.
- ^ The Rough Guide to Evolution: Galton or Weismann first to continuity of the germ-plasm?
External links
- . New International Encyclopedia. 1906.