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In biology and genetics, the germline is the population of a multicellular organism's cells that develop into . In other words, they are the cells that form (Egg cell and sperm), which can come together to form a zygote. They differentiate in the from Germ cell into Gametogonium, which develop into , which develop into the final gametes. This process is known as gametogenesis.
Germ cells pass on genetic material through the process of sexual reproduction. This includes Fertilisation, recombination and meiosis. These processes help to increase genetic diversity in offspring.
Certain organisms reproduce asexually via processes such as apomixis, parthenogenesis, autogamy, and cloning. (2025). 9783540670933, Springer. ISBN 9783540670933Lowe, Andrew; Harris, Stephen; Ashton, Paul (1 April 2000). Ecological Genetics: Design, Analysis, and Application. John Wiley & Sons. . Apomixis and Parthenogenesis both refer to the development of an embryo without fertilization. The former typically occurs in plants seeds, while the latter tends to be seen in nematodes, as well as certain species of reptiles, birds, and fish. Autogamy is a term used to describe self pollination in plants. Cloning is a technique used to creation of genetically identical cells or organisms.
In sexually reproducing organisms, cells that are not in the germline are called . According to this definition, , recombinations and other genetic changes in the germline may be passed to offspring, but changes in a somatic cell will not be.C.Michael Hogan. 2010. Mutation. ed. E.Monosson and C.J.Cleveland. Encyclopedia of Earth. National Council for Science and the Environment. Washington DC This need not apply to somatically reproducing organisms, such as some Sponge and many plants. For example, many varieties of citrus, plants in the Rosaceae and some in the Asteraceae, such as Taraxacum, produce seeds apomictically when somatic diploid cells displace the ovule or early embryo.
In an earlier stage of genetic thinking, there was a clear distinction between germline and somatic cells. For example, August Weismann proposed and pointed out, a germline cell is immortal in the sense that it is part of a lineage that has reproduced indefinitely since the beginning of life and, barring accident, could continue doing so indefinitely. However, it is now known in some detail that this distinction between somatic and germ cells is partly artificial and depends on particular circumstances and internal cellular mechanisms such as telomeres and controls such as the selective application of telomerase in germ cells, stem cells and the like.Watt, F. M. and B. L. M. Hogan. 2000 Out of Eden: Stem Cells and Their Niches Science 287:1427-1430.
Not all multicellular organisms differentiate into somatic and germ lines, but in the absence of specialised technical human intervention practically all but the simplest multicellular structures do so. In such organisms somatic cells tend to be practically totipotent, and for over a century sponge cells have been known to reassemble into new sponges after having been separated by forcing them through a sieve.
Germline can refer to a lineage of cells spanning many generations of individuals—for example, the germline that links any living individual to the hypothetical last universal common ancestor, from which all plants and animals common descent.
The mutation frequencies for cells in different stages of gametogenesis are about 5 to 10-fold lower than in both for spermatogenesis and oogenesis. The lower frequencies of mutation in germline cells compared to somatic cells appears to be due to more efficient DNA repair of DNA damages, particularly homologous recombinational repair, during germline meiosis.Bernstein H, Byerly HC, Hopf FA, Michod RE. Genetic damage, mutation, and the evolution of sex. Science. 1985 Sep 20;229(4719):1277-81. doi: 10.1126/science.3898363. PMID 3898363 Among humans, about five percent of live-born offspring have a genetic disorder, and of these, about 20% are due to newly arisen germline mutations.
In the mouse, by days 6.25 to 7.25 after fertilization of an egg by a sperm, cells in the embryo are set aside as primordial germ cells (PGCs). These PGCs will later give rise to germline sperm cells or egg cells. At this point the PGCs have high typical levels of methylation. Then primordial germ cells of the mouse undergo genome-wide DNA demethylation, followed by subsequent new methylation to reset the epigenome in order to form an egg or sperm.
In the mouse, PGCs undergo DNA demethylation in two phases. The first phase, starting at about embryonic day 8.5, occurs during PGC proliferation and migration, and it results in genome-wide loss of methylation, involving almost all genomic sequences. This loss of methylation occurs through passive demethylation due to repression of the major components of the methylation machinery. The second phase occurs during embryonic days 9.5 to 13.5 and causes demethylation of most remaining specific loci, including germline-specific and meiosis-specific genes. This second phase of demethylation is mediated by the TET enzymes TET1 and TET2, which carry out the first step in demethylation by converting 5-mC to 5-hydroxymethylcytosine (5-hmC) during embryonic days 9.5 to 10.5. This is likely followed by replication-dependent dilution during embryonic days 11.5 to 13.5. At embryonic day 13.5, PGC genomes display the lowest level of global DNA methylation of all cells in the life cycle.
In the mouse, the great majority of differentially expressed genes in PGCs from embryonic day 9.5 to 13.5, when most genes are demethylated, are upregulated in both male and female PGCs.
Following erasure of DNA methylation marks in mouse PGCs, male and female germ cells undergo new methylation at different time points during gametogenesis. While undergoing mitotic expansion in the developing gonad, the male germline starts the re-methylation process by embryonic day 14.5. The sperm-specific methylation pattern is maintained during mitotic expansion. DNA methylation levels in primary oocytes before birth remain low, and re-methylation occurs after birth in the oocyte growth phase.
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