Meiosis
In 1876, German biologist Oscar Hertwig observed the fusion of sperm and egg in sea urchin eggs. He noted that a single nucleus formed from two distinct nuclei during this process. This observation marked the first recorded description of meiosis. A decade later, Belgian zoologist Edouard Van Beneden examined Ascaris roundworm eggs. He described chromosomes at a level never seen before. In 1890, August Weismann proposed that two cell divisions were necessary to maintain chromosome numbers across generations. The term meiosis entered biology in 1905 through J.B. Farmer and J.E.S. Moore. They used the spelling Maiosis based on Greek roots meaning lessening. Koernicke changed the spelling to meiosis in 1905. Pantel and De Sinety followed suit in 1906. Thomas Hunt Morgan detected crossovers in fruit flies Drosophila melanogaster by 1911. His work established that genetic traits travel on chromosomes.
Meiosis begins with DNA replication during S phase of the cell cycle. Each chromosome duplicates into two identical sister chromatids held together by cohesin. Meiotic prophase follows immediately after replication. Homologous chromosomes pair up and exchange genetic information through recombination. Crossovers create physical links called chiasmata between homologs. These links help direct chromosome segregation during meiosis I. Two haploid cells form after the first division. Sister chromatids separate during meiosis II without further DNA replication. Four daughter cells result from this process. Human oocytes arrest at prophase I before birth. This suspended state is known as dictyotene or dictyate. It lasts until puberty or later when ovulation occurs. In mice, prophase I lasts thirteen out of fourteen days. Approximately 300 double strand breaks occur per meiosis in mice. The enzyme SPO11 initiates recombination by creating these breaks. RAD51 and DMC1 proteins coat single stranded DNA filaments to invade homologous chromosomes. Inter-axis bridges form within a distance of roughly 400 nanometers in mice. Chromosomes align along an equatorial plane during metaphase I. Cohesin holds sister chromatids together until anaphase I.
Independent assortment creates random distribution of chromosomes into gametes. Each bivalent aligns independently along the metaphase plate. Orientation of one bivalent does not affect another. Crossing over exchanges genetic information between non-sister chromatids. At least one crossover forms per chromosome pair in most organisms. Gene conversion repairs some double strand breaks without forming crossovers. A subset of breaks results in physical exchange of chromosomal regions. Recombination rates differ between maternal and paternal DNA in humans. Maternal DNA recombines approximately forty-two times on average. Paternal DNA recombines about twenty-seven times on average. One million base pairs correspond to one centimorgan of recombination frequency. In silkworm Bombyx mori oocytes, crossing-over is completely absent. Synaptonemal complexes still form during pachytene stage. The absence of chiasmata allows achiasmate meiosis to proceed. Genetic diversity arises from new combinations of alleles. These variations provide material for natural selection to act upon. Some lineages like bdelloid rotifers lack meiotic sex entirely. They reproduce through parthenogenesis instead. Giardia intestinalis possesses core meiotic genes despite being a parasite. Evidence suggests facultative sex existed in eukaryotic common ancestors.
Meiosis appears fundamental to all eukaryotic organisms. It emerged early in eukaryotic evolution. Parasitic protozoa of genus Leishmania show sexual cycles consistent with meiosis. Amoeba lineages are anciently sexual according to phylogenetic analysis. Dacks and Rogers proposed that facultative sex was present in the common ancestor. Hydrogen peroxide treatment increases mating frequency in yeast Schizosaccharomyces pombe by four to eighteen fold. Heat shock induces meiotic sex in Volvox carteri green algae. Antioxidants can inhibit this heat-induced reproduction. Oxidative stress mediates DNA damage leading to increased recombination. Recombinational repair serves as an adaptation for genomic integrity. Four copies of the genome exist in prophase I arrested oocytes. This redundancy allows efficient repair of double strand breaks. DNA repair capability determines fertility in female germ lines. Most monosomic and trisomic human embryos fail to develop. Trisomy 21 remains viable as Down syndrome. Patau syndrome involves chromosome thirteen. Edwards syndrome affects chromosome eighteen. Klinefelter syndrome adds extra X chromosomes to males. Turner syndrome lacks one X chromosome in females. Triple X syndrome adds an extra X to females. Jacobs syndrome adds a Y chromosome to males.
Maturation promoting factor MPF regulates meiosis progression. CDK1 and cyclin B levels correlate with oocyte competence. Cyclin B synthesis peaks at end of meiosis I. Ubiquitin-dependent pathways degrade cyclin B during anaphase I. Natriuretic peptide type C from mural granulosa cells initiates arrest. cGMP halts meiosis by inhibiting phosphodiesterase 3A. Hypoxanthine acts as a purine inhibitor of oocyte meiosis. Luteinizing hormone spikes trigger resumption of meiotic arrest. Epidermal growth factor-like factors reduce cGMP transport through gap junctions. Follicle-stimulating hormone promotes expression of NPR2 receptors. WEE2 kinase activates nuclear processes via phosphorylation. CDC25B migrates to nucleus when unphosphorylated. MYT1 and WEE1 inhibit CDK1 activity at specific residues. Anaphase-promoting complex APC regulates cyclin degradation. MOS protein and MAPKK/MEK1 form cytostatic factor CSF. p90 RSK blocks entry into S-phase between divisions. Clb1 serves as main regulatory cyclin in budding yeast. IME1 transcription factor drives entry into meiotic S phase. Pat1-Mei2 system controls fission yeast regulation. Mei4 transcription factor activates cdc25 gene expression. Rad proteins and Mek1 kinases form checkpoint systems. Spo11 catalyzes double stranded breaks for recombination. Mre11, Sae2, and Exo1 assist breakage processes. Double Holliday junction pathways mediate homologous recombination.
Diplontic life cycles occur in humans and many animals. Organisms grow from diploid zygotes through mitosis. Germ-line stem cells undergo meiosis to produce gametes. Haplontic life cycles appear in fungi and protozoa. Single haploid cells proliferate to form organisms. Zygotes undergo immediate meiosis after formation. Haplodiplontic cycles alternate between haploid and diploid states. Diploid sporophytes produce spores via meiosis. Spores develop into multicellular haploid gametophytes. Gametophytes generate gametes without further meiosis. Land plants exhibit alternation of generations. Meiosis occurs in seminiferous tubules during spermatogenesis. Retinoic acid stimulates differentiation of spermatogonia. Sertoli cells synthesize retinoic acid postnatally. Nanos and DAZL adjust sensitivity to retinoic acid. Primordial germ cells migrate to ovaries in embryos. FSH secretion triggers folliculogenesis at puberty. One dominant follicle matures while others undergo atresia. Polar bodies eliminate excess cytoplasm in female meiosis. Three polar bodies typically result from oocyte division. Only one cell develops into mature ovum. Flagellates like parabasalids show rare one-divisional meiosis. Wood-feeding cockroach Cryptocercus hosts these organisms. Most eukaryotes utilize two-divisional meiosis with chiasmata.
Errors called nondisjunction cause abnormal chromosome numbers. Trisomy 21 produces Down syndrome phenotypes. Monosomy X results in Turner syndrome conditions. Maternal age increases probability of nondisjunction in oocytes. Loss of cohesin over time contributes to errors. Cohesin protects centromeres until anaphase I. Shugoshin prevents sister chromatid separation prematurely. Chromosome segregation fails when cohesion breaks down. Aneuploid gametes lead to developmental disabilities. Miscarriage rates rise significantly with chromosomal abnormalities. Human fetal oocytes form between three and four months gestation. Arrested dictyate stage persists for decades before ovulation. DNA repair capability determines fertility outcomes. Hydrogen peroxide induces oxidative stress leading to damage. Yeast studies show increased mating frequency under stress. Volvox carteri reproduces via heat shock induced sex. Antioxidants block this reproductive induction. Meiotic recombination redistributes genetic material across chromosomes. One million base pairs equal one centimorgan distance. Recombination frequencies vary by maternal or paternal origin. Female recombination averages forty-two events per cell. Male recombination averages twenty-seven events per cell. Most monosomic embryos fail viability checks. Trisomy thirteen causes Patau syndrome. Trisomy eighteen leads to Edwards syndrome. Extra Y chromosomes create Jacobs syndrome. Multiple X chromosomes result in Triple X syndrome.
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Common questions
When was meiosis first described by Oscar Hertwig?
Oscar Hertwig first described meiosis in 1876 when he observed the fusion of sperm and egg in sea urchin eggs. This observation marked the first recorded description of the process.
Who coined the term meiosis and when did it enter biology?
J.B. Farmer and J.E.S. Moore introduced the term meiosis into biology in 1905. Koernicke subsequently changed the spelling from Maiosis to meiosis later that same year.
How many double strand breaks occur per meiosis in mice?
Approximately 300 double strand breaks occur per meiosis in mice. The enzyme SPO11 initiates recombination by creating these specific breaks.
What is the average number of recombination events in human female DNA?
Maternal DNA recombines approximately forty-two times on average during human meiosis. Paternal DNA recombines about twenty-seven times on average.
Which syndrome results from trisomy 21?
Trisomy 21 produces Down syndrome phenotypes in humans. Most monosomic and trisomic human embryos fail to develop except for this condition.