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biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...
and
genetics Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinian friar wor ...
, the germline is the population of a multicellular organism's cells that pass on their genetic material to the progeny (
offspring In biology, offspring are the young creation of living organisms, produced either by a single organism or, in the case of sexual reproduction, two organisms. Collective offspring may be known as a brood or progeny in a more general way. This ca ...
). In other words, they are the cells that form the egg, sperm and the fertilised egg. They are usually differentiated to perform this function and segregated in a specific place away from other bodily cells. As a rule, this passing-on happens via a process of
sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote th ...
; typically it is a process that includes systematic changes to the genetic material, changes that arise during recombination,
meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately r ...
and
fertilization Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...
for example. However, there are many exceptions across multicellular organisms, including processes and concepts such as various forms of apomixis,
autogamy Autogamy, or self-fertilization, refers to the fusion of two gametes that come from one individual. Autogamy is predominantly observed in the form of self-pollination, a reproductive mechanism employed by many flowering plants. However, species o ...
,
automixis ''Automixis'' is the fusion of (typically haploid) nuclei or gametes derived from the same individual. The term covers several reproductive mechanisms, some of which are parthenogenetic. Diploidy might be restored by the doubling of the chromoso ...
,
cloning Cloning is the process of producing individual organisms with identical or virtually identical DNA, either by natural or artificial means. In nature, some organisms produce clones through asexual reproduction. In the field of biotechnology, c ...
or
parthenogenesis Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction in which growth and developmen ...
. The cells of the germline are called
germ cell Germ or germs may refer to: Science * Germ (microorganism), an informal word for a pathogen * Germ cell, cell that gives rise to the gametes of an organism that reproduces sexually * Germ layer, a primary layer of cells that forms during emb ...
s. For example,
gamete A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...
s such as a sperm and an egg are germ cells. So are the cells that divide to produce gametes, called
gametocyte A gametocyte is a eukaryotic germ cell that divides by mitosis into other gametocytes or by meiosis into gametids during gametogenesis. Male gametocytes are called ''spermatocytes'', and female gametocytes are called ''oocytes''. Development ...
s, the cells that produce those, called gametogonia, and all the way back to the
zygote A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicell ...
, the cell from which an individual develops. In sexually reproducing organisms, cells that are not in the germline are called somatic cells. According to this view,
mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA replication, DNA or viral repl ...
s, recombinations and other genetic changes in the germline may be passed to offspring, but a change in a somatic cell will not be. This need not apply to somatically reproducing organisms, such as some Porifera and many plants. For example, many varieties of
citrus ''Citrus'' is a genus of flowering trees and shrubs in the rue family, Rutaceae. Plants in the genus produce citrus fruits, including important crops such as oranges, lemons, grapefruits, pomelos, and limes. The genus ''Citrus'' is native to ...
, plants in the Rosaceae and some in the
Asteraceae The family Asteraceae, alternatively Compositae, consists of over 32,000 known species of flowering plants in over 1,900 genera within the order Asterales. Commonly referred to as the aster, daisy, composite, or sunflower family, Compositae w ...
, such as ''
Taraxacum ''Taraxacum'' () is a large genus of flowering plants in the family Asteraceae, which consists of species commonly known as dandelions. The scientific and hobby study of the genus is known as taraxacology. The genus is native to Eurasia and Nor ...
'' 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 August Friedrich Leopold Weismann FRS (For), HonFRSE, LLD (17 January 18345 November 1914) was a German evolutionary biologist. Fellow German Ernst Mayr ranked him as the second most notable evolutionary theorist of the 19th century, after Cha ...
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 A telomere (; ) is a region of repetitive nucleotide sequences associated with specialized proteins at the ends of linear chromosomes. Although there are different architectures, telomeres, in a broad sense, are a widespread genetic feature mos ...
and controls such as the selective application of
telomerase Telomerase, also called terminal transferase, is a ribonucleoprotein that adds a species-dependent telomere repeat sequence to the 3' end of telomeres. A telomere is a region of repetitive sequences at each end of the chromosomes of most euka ...
in germ cells,
stem cells In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type of ...
and the like. 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 Pluripotency: These are the cells that can generate into any of the three Germ layers which imply Endodermal, Mesodermal, and Ectodermal cells except tissues like the placenta. According to Latin terms, Pluripotentia means the ability for many thin ...
, 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 The last universal common ancestor (LUCA) is the most recent population from which all organisms now living on Earth share common descent—the most recent common ancestor of all current life on Earth. This includes all cellular organisms; th ...
, from which all plants and animals descend.


Evolution

Plants and basal metazoans such as sponges (Porifera) and corals (Anthozoa) do not sequester a distinct germline, generating gametes from multipotent stem cell lineages that also give rise to ordinary somatic tissues. It is therefore likely that germline sequestration first evolved in complex animals with sophisticated body plans, i.e. bilaterians. There are several theories on the origin of the strict germline-soma distinction. Setting aside an isolated germ cell population early in embryogenesis might promote cooperation between the somatic cells of a complex multicellular organism. Another recent theory suggests that early germline sequestration evolved to limit the accumulation of deleterious mutations in mitochondrial genes in complex organisms with high energy requirements and fast mitochondrial mutation rates.


DNA damage, mutation and repair

Reactive oxygen species In chemistry, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (). Examples of ROS include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen. The reduction of molecular oxygen () p ...
(ROS) are produced as byproducts of metabolism. In germline cells, ROS are likely a significant cause of
DNA damage DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA d ...
s that, upon
DNA replication In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all living organisms acting as the most essential part for biological inheritanc ...
, lead to
mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA replication, DNA or viral repl ...
s.
8-Oxoguanine 8-Oxoguanine (8-hydroxyguanine, 8-oxo-Gua, or OH8Gua) is one of the most common DNA lesions resulting from reactive oxygen species modifying guanine, and can result in a mismatched pairing with adenine resulting in G to T and C to A substitutions ...
, an oxidized derivative of
guanine Guanine () ( symbol G or Gua) is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine (uracil in RNA). In DNA, guanine is paired with cytosine. The guanine nucleoside is c ...
, is produced by spontaneous oxidation in the germline cells of mice, and during the cell's DNA replication cause GC to TA
transversion Transversion, in molecular biology, refers to a point mutation in DNA in which a single (two ring) purine ( A or G) is changed for a (one ring) pyrimidine ( T or C), or vice versa. A transversion can be spontaneous, or it can be caused by i ...
mutations. Such mutations occur throughout the mouse
chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
s as well as during different stages of
gametogenesis Gametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes. Depending on the biological life cycle of the organism, gametogenesis occurs by meiotic di ...
. The mutation frequencies for cells in different stages of gametogenesis are about 5 to 10-fold lower than in somatic cells 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 DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA da ...
of DNA damages, particularly
homologous recombination Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in cellular organisms but may ...
al repair, during germline
meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately r ...
.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.


Epigenetic alterations

Epigenetic alterations of DNA include modifications that affect gene expression, but are not caused by changes in the sequence of bases in DNA. A well-studied example of such an alteration is the methylation of DNA cytosine to form 5-methylcytosine. This usually occurs in the DNA sequence CpG, changing the DNA at the
CpG site The CpG sites or CG sites are regions of DNA where a cytosine nucleotide is followed by a guanine nucleotide in the linear sequence of bases along its 5' → 3' direction. CpG sites occur with high frequency in genomic regions called CpG isl ...
from CpG to 5-mCpG. Methylation of cytosines in CpG sites in promoter regions of genes can reduce or silence gene expression. About 28 million CpG dinucleotides occur in the human genome, and about 24 million CpG sites in the mouse genome (which is 86% as large as the human genome). In most tissues of mammals, on average, 70% to 80% of CpG cytosines are methylated (forming 5-mCpG). 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 Demethylation is the chemical process resulting in the removal of a methyl group (CH3) from a molecule. A common way of demethylation is the replacement of a methyl group by a hydrogen atom, resulting in a net loss of one carbon and two hydrogen at ...
, followed by subsequent new methylation to reset the
epigenome An epigenome consists of a record of the chemical changes to the DNA and histone proteins of an organism; these changes can be passed down to an organism's offspring via transgenerational stranded epigenetic inheritance. Changes to 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 The TET enzymes are a family of ten-eleven translocation (TET) methylcytosine dioxygenases. They are instrumental in DNA demethylation. 5-Methylcytosine (see first Figure) is a methylated form of the DNA base cytosine (C) that often regulates ge ...
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.


See also

*
August Weismann August Friedrich Leopold Weismann FRS (For), HonFRSE, LLD (17 January 18345 November 1914) was a German evolutionary biologist. Fellow German Ernst Mayr ranked him as the second most notable evolutionary theorist of the 19th century, after Cha ...
*
Epigenetics In biology, epigenetics is the study of stable phenotypic changes (known as ''marks'') that do not involve alterations in the DNA sequence. The Greek prefix '' epi-'' ( "over, outside of, around") in ''epigenetics'' implies features that are ...
* Germ line development *
Germinal choice technology Germinal may refer to: *Germinal (French Republican Calendar), the seventh month of the calendar, approximately March 21 - April 19 Émile Zola * ''Germinal'' (novel), an 1885 novel by Émile Zola ** ''Germinal'' (1913 film), a French silent film ...
*
Weismann barrier The Weismann barrier, proposed by August Weismann, is the strict distinction between the "immortal" germ cell lineages producing gametes and "disposable" somatic cells in animals (but not plants), in contrast to Charles Darwin's proposed pangenesi ...


References

{{Authority control Developmental biology *