Germ plasm
Encyclopedia
Germ plasm or polar plasm is a zone found in the cytoplasm of the egg cells of some model organisms (such as Caenorhabditis elegans
, Drosophila melanogaster
, Xenopus laevis), 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 gonad
s.
(1834–1914). His germ plasm theory states that multicellular organisms consist of germ cell
s that contain and transmit heritable information, and somatic cells which carry out ordinary bodily functions. 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
. This idea, if true, rules out the inheritance of acquired characteristics as proposed by Jean-Baptiste Lamarck
.
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 quite wrong. 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.
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 in idea which is in one essential point nearly allied to the main idea contained in my theory of the continuity of germ-plasm".
Caenorhabditis elegans
Caenorhabditis elegans is a free-living, transparent nematode , about 1 mm in length, which lives in temperate soil environments. Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner and it has since been used extensively as a model...
, Drosophila melanogaster
Drosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...
, Xenopus laevis), which contains determinants that will give rise to the germ cell
Germ cell
A germ cell is any biological cell that gives rise to the gametes of an organism that reproduces sexually. In many animals, the germ cells originate near the gut of an embryo and migrate to the developing gonads. There, they undergo cell division of two types, mitosis and meiosis, followed by...
lineage. As the zygote undergoes mitotic
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
divisions the germ plasm is ultimately restricted to a few cells of the embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
. These germ cells then migrate to the gonad
Gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s.
Germ plasm theory
The term germ plasm was first used by the German biologist August WeismannAugust Weismann
Friedrich Leopold August Weismann was a German evolutionary biologist. Ernst Mayr ranked him the second most notable evolutionary theorist of the 19th century, after Charles Darwin...
(1834–1914). His germ plasm theory states that multicellular organisms consist of germ cell
Germ cell
A germ cell is any biological cell that gives rise to the gametes of an organism that reproduces sexually. In many animals, the germ cells originate near the gut of an embryo and migrate to the developing gonads. There, they undergo cell division of two types, mitosis and meiosis, followed by...
s that contain and transmit heritable information, and somatic cells which carry out ordinary bodily functions. 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
Weismann barrier
The Weismann barrier is the principle that hereditary information moves only from genes to body cells, and never in reverse. In more precise terminology hereditary information moves only from germline cells to somatic cells .This does not refer to the central dogma of molecular biology which...
. This idea, if true, rules out the inheritance of acquired characteristics as proposed by Jean-Baptiste Lamarck
Jean-Baptiste Lamarck
Jean-Baptiste Pierre Antoine de Monet, Chevalier de la Marck , often known simply as Lamarck, was a French naturalist...
.
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 quite wrong. Cases such as Dolly (the famous cloned ewe) which, via somatic cell nuclear transfer
Somatic cell nuclear transfer
In genetics and developmental biology, somatic-cell nuclear transfer is a laboratory technique for creating a clonal embryo, using an ovum with a donor nucleus . It can be used in embryonic stem cell research, or, potentially, in regenerative medicine where it is sometimes referred to as...
, 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.
The idea was to some extent anticipated in an 1865 article by Francis Galton
Francis Galton
Sir Francis Galton /ˈfrɑːnsɪs ˈgɔːltn̩/ FRS , cousin of Douglas Strutt Galton, half-cousin of Charles Darwin, was an English Victorian polymath: anthropologist, eugenicist, tropical explorer, geographer, inventor, meteorologist, proto-geneticist, psychometrician, and statistician...
, published in Macmillan's Magazine
Macmillan's Magazine
Macmillan's Magazine was a monthly British magazine from 1859 to 1907 published by Alexander Macmillan.The magazine was a literary periodical that published fiction and non-fiction works from primarily British authors. Thomas Hughes had convinced Macmillan to found the magazine. The first editor...
, which set out a weak version of the concept. In 1889 Weismann wrote to acknowledge that "You have exposed in your paper an in idea which is in one essential point nearly allied to the main idea contained in my theory of the continuity of germ-plasm".