Meiosis is the special type of reductive cell division occurring only in the generation of the gametes or germ cells (oocyte and spermatozoa). Meiotic cell division reduces (halves) the chromosomal content. The overall process of germ cell development is called "gametogenesis" and includes not only meiosis but also the cellular changes, that occur differently in male and female gametes.
| Female gametogenesis
|| Male gametogenesis
Some Recent Findings
Bivalent separation into univalents precedes age-related meiosis I errors in oocytes
Karyotype of parthenogenetic blastocysts
- Review - The evolution of meiotic sex and its alternatives "Meiosis is an ancestral, highly conserved process in eukaryotic life cycles, and for all eukaryotes the shared component of sexual reproduction. The benefits and functions of meiosis, however, are still under discussion, especially considering the costs of meiotic sex."
- From Meiosis to Mitosis: The Astonishing Flexibility of Cell Division Mechanisms in Early Mammalian Development "The execution of female meiosis and the establishment of the zygote is arguably the most critical stage of mammalian development. The egg can be arrested in the prophase of meiosis I for decades, and when it is activated, the spindle is assembled de novo. This spindle must function with the highest of fidelity and yet its assembly is unusually achieved in the absence of conventional centrosomes and with minimal influence of chromatin. Moreover, its dramatic asymmetric positioning is achieved through remarkable properties of the actin cytoskeleton to ensure elimination of the polar bodies. The second meiotic arrest marks a uniquely prolonged metaphase eventually interrupted by egg activation at fertilization to complete meiosis and mark a period of preparation of the male and female pronuclear genomes not only for their entry into the mitotic cleavage divisions but also for the imminent prospect of their zygotic expression."
- Silencing of X-Linked MicroRNAs by Meiotic Sex Chromosome Inactivation  "During the pachytene stage of meiosis in male mammals, the X and Y chromosomes are transcriptionally silenced by Meiotic Sex Chromosome Inactivation (MSCI). MSCI is conserved in therian mammals and is essential for normal male fertility. ...Here we use RNA FISH to examine X-miRNA expression in the male germ line. We find that, like protein-coding X-genes, X-miRNAs are expressed prior to prophase I and are thereafter silenced during pachynema. X-miRNA silencing does not occur in mouse models with defective MSCI. Furthermore, X-miRNAs are expressed at pachynema when present as autosomally integrated transgenes. Thus, we conclude that silencing of X-miRNAs during pachynema in wild type males is MSCI-dependent. Importantly, misexpression of X-miRNAs during pachynema causes spermatogenic defects. We propose that MSCI represents a chromosomal mechanism by which X-miRNAs, and other potential X-encoded repressors, can be silenced, thereby regulating genes with critical late spermatogenic functions." Spermatozoa Development
- Bivalent separation into univalents precedes age-related meiosis I errors in oocytes "The frequency of chromosome segregation errors during meiosis I (MI) in oocytes increases with age. The two-hit model suggests that errors are caused by the combination of a first hit that creates susceptible crossover configurations and a second hit comprising an age-related reduction in chromosome cohesion. This model predicts an age-related increase in univalents, but direct evidence of this phenomenon as a major cause of segregation errors has been lacking. Here, we provide the first live analysis of single chromosomes undergoing segregation errors during MI in the oocytes of naturally aged mice. Chromosome tracking reveals that 80% of the errors are preceded by bivalent separation into univalents. The set of the univalents is biased towards balanced and unbalanced predivision of sister chromatids during MI. Moreover, we find univalents predisposed to predivision in human oocytes. This study defines premature bivalent separation into univalents as the primary defect responsible for age-related aneuploidy."
- Premature dyad separation in meiosis II is the major segregation error with maternal age in mouse oocytes "Changes consistent with chromosome cohesion deterioration were found with age, including increased interkinetochore distance and loss of the centromeric protector of cohesion SGO2 in metaphase II arrested (metII) eggs, as well as a rise in the number of weakly attached bivalents in meiosis I (MI) and lagging chromosomes at anaphase I. However, there were no MI errors in congression or biorientation. Instead, premature separation of dyads in meiosis II was the major segregation defect in aged eggs and these were associated with very low levels of SGO2. These data show that although considerable cohesion loss occurs during MI, its consequences are observed during meiosis II, when centromeric cohesion is needed to maintain dyad integrity."
- Chromosomes in the Porcine First Polar Body Possess Competence of Second Meiotic Division within Enucleated MII Stage Oocytes "These results demonstrate that chromosomes in PB1 can participate in normal pre-implantation embryonic development when injected into enucleated MII stage oocytes, and that tetraploid PA blastocysts are produced (although at a low proportion) when PB1 chromosomes are injected into intact MII stage oocytes."
| More recent papers
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Search term: Meiosis
Divya Pattabiraman, Baptiste Roelens, Alexander Woglar, Anne M Villeneuve Meiotic recombination modulates the structure and dynamics of the synaptonemal complex during C. elegans meiosis. PLoS Genet.: 2017, 13(3);e1006670 PubMed 28339470
Ines Teichert, Tim Dahlmann, Ulrich Kück, Minou Nowrousian RNA editing during sexual development occurs in distantly related filamentous ascomycetes. Genome Biol Evol: 2017; PubMed 28338982
Duncan A Taylor, Oliva Handt, Damien Abarno, John Buckleton Likelihood ratio formulae for disputed parentage when the product of conception is trisomic. Int. J. Legal Med.: 2017; PubMed 28337598
Corinne Grey, Julie A J Clément, Jérôme Buard, Benjamin Leblanc, Ivo Gut, Marta Gut, Laurent Duret, Bernard de Massy In vivo binding of PRDM9 reveals interactions with noncanonical genomic sites. Genome Res.: 2017; PubMed 28336543
Min-Su Lee, Jeong Hwan Joo, Keunpil Kim Roles of Budding Yeast Hrr25 in Recombination and Sporulation. J. Microbiol. Biotechnol.: 2017; PubMed 28335590
| Older recent papers
- Bora regulates meiotic spindle assembly and cell cycle during mouse oocyte meiosis "Bora is the binding partner of Aurora A, which is required for its activation and phosphorylation of Polo like kinase 1 (Plk1), and is involved in the spindle assembly and progress of the cell cycle during mitosis. In this study, we examined the expression, localization, and function of Bora during mouse oocyte meiosis. The expression level of Bora was increased during oocyte meiotic maturation, with an elevated level at metaphase. Immunofluorescence analysis showed that Bora was concentrated as a dot shortly after germinal vesicle breakdown (GVBD), associating first with the surrounding chromosomes and then with the spindle throughout the oocyte meiotic maturation. Further experiments confirmed that Bora co-localized with α-tubulin at prometaphase/metaphase, but dissociated from α-tubulin at anaphase/telophase."
|| A mouse oocyte undergoing meiosis spindle migration followed by first polar body extrusion and MII spindle positioning.
- blue - Hoechst staining of chromosomes.
- green - UtrCH-GFP was used to label cortical changes during spindle migration.
The video shows that cytoplasmic streaming continues to the MII arrest stage to maintain the oocyte set of chromosomes/MII spindle in place close to the cortex. Frames are 11 min apart, and video length is 840 min. Bar, 20 µm.
|| A mouse spermatocyte undergoing meiosis prophase I. The chromosomal telomeres and synaptonemal complexes have been labelled to visualise chromosomal movement within this single nucleus.
Comparison of Meiosis/Mitosis
- After DNA replication 2 nuclear (and cell) divisions required to produce haploid gametes
- Each diploid cell in meiosis produces 4 haploid cells (sperm) 1 haploid cell (egg)
- Each diploid cell mitosis produces 2 diploid cells
Meiosis Germ cell division (haploid)
- Reductive division
- Generates haploid gametes (egg, sperm)
- Each genetically distinct from parent
- Genetic recombination (prophase 1)
- Exchanges portions of chromosomes maternal/paternal homologous pairs
- Independent assortment of paternal chromosomes (meiosis 1)
Homologous chromosomes pairing unique to meiosis
- Each chromosome duplicated and exists as attached sister chromatids before pairing occurs
- Genetic Recombination shown by chromosomes part red and part black
- chromosome pairing in meiosis involves crossing-over between homologous chromosomes
Meiosis I and II
- Meiosis I separates the pairs of homologous chromosomes, reduces the cell from diploid to haploid.
- Meiosis II separates each chromosome into two chromatids (chromosome behavior in meiosis II is like that of mitosis).
Figure 14.32. Comparison of meiosis and mitosis
- The homologous chromosomes pair and exchange DNA to form recombinant chromosomes.
- Note - in oocyte development, from birth until puberty oocytes are in "prophase I arrest" at diplotene stage. This is important for sustaining the ovarian oocyte pool and lutenizing hormone (LH) induces resumption of meiosis I.
| Mouse early meiotic prophase I stages
Prophase I is further divided into five stages (phases):
- leptotene phase, leptonema; Greek, leptotene = "thin threads"
- the duplicated paired chromosome homologs condense.
- zygotene phase, zygonema, Greek, zygotene = "paired threads"
- homologous chromosomes become closely associated (synapsis) to form pairs of chromosomes consisting of four chromatids (tetrads).
- the synaptonemal complex begins to form between the two sets of sister chromatids in each bivalent (the duplicated chromosome paired with its homologous duplicated chromosome).
- pachytene phase, pachynema; Greek, pachytene = "thick threads"
- crossing over between pairs of homologous chromosomes (meiotic recombination or synapsis) to form chiasmata (form between two nonsister chromatids at points where they have crossed over)
- synaptonemal complex is complete and can be stable for some time.
- Autosomal non-sister chromatids of homologous chromosomes can now extensively exchange segments in regions of homology.
- Only small regions of non-paired sex chromosomes interact
- Mutations that compromise meiotic recombination in male spermatocytes result in arrest and apoptosis at this stage.
Mouse meiosis pachytene
- diplotene phase, diplonema; Greek, diplonema = "two threads"
- homologous chromosomes begin to separate but remain attached by chiasmata.
- synaptonemal complex degrades and the chromosomes separate from one another a small amount giving this appearance.
- It is possible that some chromosome uncoiling may also occur allowing some gene transcription.
- In the developing human ovary, oocytes remain at the diplotene stage from fetal life through postnatal childhood, until puberty when the lutenizing hormone (LH) surges stimulate the resumption of meiosis.
- diakinesis phase; Greek, diakinesis = "moving through"
- homologous chromosomes continue to separate, and chiasmata move to the ends of the chromosomes.
- prophase I ends and chromosomes now recondense, transcription stops and the transition to metaphase occurs.
- Spindle apparatus formed, and chromosomes attached to spindle fibres by kinetochores.
- Homologous pairs of chromosomes (bivalents) arranged as a double row along the metaphase plate. The arrangement of the paired chromosomes with respect to the poles of the spindle apparatus is random along the metaphase plate. (This is a source of genetic variation through random assortment, as the paternal and maternal chromosomes in a homologous pair are similar but not identical. The number of possible arrangements is 2n, where n is the number of chromosomes in a haploid set. Human beings have 23 different chromosomes, so the number of possible combinations is 223, which is over 8 million.)
- The homologous chromosomes in each bivalent are separated and move to the opposite poles of the cell.
- The chromosomes become diffuse and the nuclear membrane reforms.
- Cellular cytoplasmic division to form two new cells, followed by Meiosis II.
- Note - in oocyte meiosis, the extrusion of the first polar body (1 PB) indicates completion of the first meiotic division.
- Chromosomes begin to condense, nuclear membrane breaks down and spindle forms.
- Spindle fibres attach to chromosomes, chromosomes align in cell centre.
- Chromosomes separate and move to the opposite poles of the cell.
- Chromosomes reach spindle pole ends and the nuclear membrane reforms.
Cellular cytoplasmic division to form new cells.
Meiosis Sex Differences
- Meiosis initiated once in a finite population of cells
- 1 gamete produced / meiosis
- Completion of meiosis delayed for months or years
- Meiosis arrested at 1st meiotic prophase and reinitiated in a smaller population of cells
- Differentiation of gamete occurs while diploid in first meiotic prophase
- All chromosomes exhibit equivalent transcription and recombination during meiotic prophase
- Meiosis initiated continuously in a mitotically dividing stem cell population
- 4 gametes produced / meiosis
- Meiosis completed in days or weeks
- Meiosis and differentiation proceed continuously without cell cycle arrest
- Differentiation of gamete occurs while haploid after meiosis ends
Sex chromosomes excluded from recombination and transcription during first meiotic prophase
In females, the total number of eggs ever to be produced are present in the newborn female.
- All eggs are arrested at an early stage of the first meiotic division as a primary oocyte (primordial follicle). Following purberty, during each menstrual cycle, pituitary gonadotrophin stimulates completion of meiosis 1 the day before ovulation.
- In meiosis 1, a diploid cell becomes 2 haploid (23 chromosomes) daughter cells, each chromosome has two chromatids. One cell becomes the secondary oocyte the other cell forms the first polar body.
- The secondary oocyte then commences meiosis 2 which arrests at metaphase and will not continue without fertilization.
- At fertilization meiosis 2 completes, forming a second polar body. Note that the first polar body may also undergo this process forming a third polar body.
Meiosis and Oogenesis
|Meiosis - divided into 3 temporally distinct phases.
- Prophase - after DNA replication, homologous chromosomes (shown in red and blue) undergo pairing, synapsis and recombination, and arrest at the diplotene (dictyate) stage.
- Dictyate arrest - oocytes remain in meiotic arrest until the female reaches maturity and the oocyte has completed an extensive period of growth following follicle formation.
- Divisions - luteinizing hormone (LH) surge that triggers ovulation also causes resumption and completion of the first meiotic division in the periovulatory oocyte. The ovulated egg is arrested at second meiotic metaphase, and anaphase onset and completion of meiosis II only occur if the egg is fertilized.
Oogenesis - complex involving 4 distinct phases.
- Commitment to meiosis and meiotic initiation - occurs at GA 8–10 weeks in humans.
- Follicle formation - occurs during the second trimester in humans.
- Oocyte growth - occurs in the sexually mature female under the control of paracrine and endocrine signals. Oocyte growth is thought to take approximately 85 days in humans and typically culminates in the ovulation of a single egg.
- Fertilization - of the ovulated egg results in the completion of the second meiotic division.
| Oocyte Meiotic Spindles
- Mammalian oocytes have no centrosomes, but still form spindles using many microtubule-organizing centres lacking centrioles. (cells dividing by mitosis have 2 centrosomes to help form the mitotic spindle) PMID 17693257
- Meiotic spindle is a microtubule structure but its relocation in the oocyte cytoplasm involves microfilaments, filamentous-actin structures nucleated by Formin-2 (Fmn2). PMID 19062278
First Polar Body
Human oocyte at metaphase II showing polar body at 12 o'clock position.
The breakdown of the germinal vesicle indicates a resumption of meiosis and the extrusion of the first polar body (1 PB) indicates completion of the first meiotic division in human oocytes. The polar body is a small cytoplasmic exclusion body formed to enclose the excess DNA formed during the oocyte (egg) meiosis and following sperm fertilization. There are 2-3 polar bodies derived from the oocyte present in the zygote, the number is dependent upon whether polar body 1 (the first polar body formed during meiosis 1) divides during meiosis 2. This exclusion body contains the excess DNA from the reductive division (the second and third polar bodies are formed from meiosis 2 at fertilization). These polar bodies do not contribute to the future genetic complement of the zygote, embryo or fetus.
Recent research in some species suggest that the space formed by the peripheral polar body (between the oocyte and the zona pellucia) can influence the site of spermatozoa fertilization.
Assisted reproductive techniques involving intracytoplasmic sperm injection (ICSI) have looked at the "quality" of the polar body and found that the morphology is related to mature oocyte viability and has the potential to predict oocyte fertilization rates and pregnancy achievement.
- Polar Body Electron Micrographs
First Polar Body cortical granules
First Polar Body chromosomes
First Polar Body microtubules
Second Polar Body chromatin
Second Polar Body cytoplasmic bridge
- Links: Category:Polar Body
Trisomy 21 female karyotype
Meiotic non-disjunction resulting in aneuploidy, most are embryonic lethal and not seen. The potential for genetic abnormalities increase with maternal age.
- Autosomal chromosome aneuploidy
- trisomy 21 - Down syndrome
- trisomy 18 - Edwards syndrome
- trisomy 13 - Patau syndrome
- Sex chromosome aneuploidy
- monosomy X - Turner's Syndrome
- trisomy X - Triple-X syndrome
- 47 XXY - Klinefelter's Syndrome
In males, sperm continues to be generated throughout life from a stem cell population in the testis. Spermatozoa maturation involves two processes meiosis and spermiogenesis
The above figure compares meiosis to the female (the polar bodies have been removed and labelling updated).
Adult Seminiferous tubule showing spermatozoa developmental stages
Seminiferous tubule cross-section and supporting cells
Human Spermatozoa Development
- Spermatogenesis process of spermatagonia mature into spermatazoa (sperm).
- Continuously throughout life occurs in the seminiferous tubules in the male gonad- testis (plural testes).
- At puberty spermatagonia activate and proliferate (mitosis).
- about 48 days from entering meiosis until morphologically mature spermatozoa
- about 64 days to complete spermatogenesis, depending reproduction time of spermatogonia
- follicle stimulating hormone (FSH) - stimulates the spermatogenic epithelium
- luteinizing-hormone (LH) - stimulates testosterone production by Leydig cells
|| Mature human spermatozoa
- 60 µm long, actively motile
- divided into 3 main regions (head, neck and tail)
- head - (flattened, 5 µm long by 3 µm wide) the nucleus and acrosome. Posterior part of nuclear membrane forms the basal plate.
- neck - (1 µm) attached to basal plate, transverse oriented centriole, contains nine segmented columns of fibrous material, continue as outer dense fibres in tail.
- tail - 3 parts a middle piece, principal piece and end piece
- middle piece - (5 µm long) axonema and dense fibres surrounded by mitochondria
- principal piece - (45 µm long) fibrous sheath interconnected by regularly spaced circumferential hoops
- end piece - (5 µm long) axonema surrounded by small amount of cytoplasm and plasma membrane
- Links: Spermatozoa Development
- In humans at puberty, hormonal and morphological changes occur within the gonad and other systems (secondary sex characteristics).
- Within the testis the immature Sertoli cells cease to proliferate and differentiate.
- Spermatogonium proliferate and spermatogenesis begins.
- It takes about 70 days for cells to mature from the diploid spermatogonium to a primary spermatocyte.
- This maturation occurs in waves along the seminiferous tubules.
- Links: Puberty
Azoospermia - Non-obstructive azoospermia (NOA) and Obstructive azoospermia (OA)
- release of spermatozoa and accessory gland secretions from the male genital tract (3.5 ml)
- 200-600 million sperm, by volume less than 10 % spermatozoa
- Accessory Gland secretions - 60 % seminal vesicle, 30 % prostate and 10 % bulbourethral
- Oligospermia - (Low Sperm Count) less than 20 million sperm after 72 hour abstinence from sex
- Azoospermia - (Absent Sperm) blockage of duct network
- Immotile Cilia Syndrome - lack of sperm motility
Meiosis in Other Species
- Sea urchin - oocytes complete meiosis before being shed.
- Starfish - oocytes only complete meiosis upon hormonal stimulation.
- Occurs when homologues fail to separate during meiotic division I or II
- For example trisomy 21 (Down Syndrome) caused by an extra copy of chromosome 21
- Links: trisomy 21 | Nondisjunction
- Philadelphia chromosome
- Chronic myelogenous leukemia
- Piece of Chr9 exchanged with Chr22 Generates truncated abl
Overstimulates cell production
- ↑ 1.0 1.1
Yogo Sakakibara, Shu Hashimoto, Yoshiharu Nakaoka, Anna Kouznetsova, Christer Höög, Tomoya S Kitajima Bivalent separation into univalents precedes age-related meiosis I errors in oocytes. Nat Commun: 2015, 6;7550 PubMed 26130582
| Nat Commun.]
- ↑ 2.0 2.1
Tao Lin, Yun Fei Diao, Jung Won Kang, Jae Eun Lee, Dong Kyo Kim, Dong Il Jin Chromosomes in the porcine first polar body possess competence of second meiotic division within enucleated MII stage oocytes. PLoS ONE: 2013, 8(12);e82766 PubMed 24312673
Ghader Mirzaghaderi, Elvira Hörandl The evolution of meiotic sex and its alternatives. Proc. Biol. Sci.: 2016, 283(1838); PubMed 27605505
L Bury, P A Coelho, D M Glover From Meiosis to Mitosis: The Astonishing Flexibility of Cell Division Mechanisms in Early Mammalian Development. Curr. Top. Dev. Biol.: 2016, 120;125-171 PubMed 27475851
Hélène Royo, Hervé Seitz, Elias ElInati, Antoine H F M Peters, Michael B Stadler, James M A Turner Silencing of X-Linked MicroRNAs by Meiotic Sex Chromosome Inactivation. PLoS Genet.: 2015, 11(10);e1005461 PubMed 26509798
Yan Yun, Simon I R Lane, Keith T Jones Premature dyad separation in meiosis II is the major segregation error with maternal age in mouse oocytes. Development: 2014, 141(1);199-208 PubMed 24346700
Rui Zhai, Yi-Feng Yuan, Yi Zhao, Xiao-Ming Liu, Yan-Hong Zhen, Fei-Fei Yang, Li Wang, Cheng-Zhu Huang, Jing Cao, Li-Jun Huo Bora regulates meiotic spindle assembly and cell cycle during mouse oocyte meiosis. Mol. Reprod. Dev.: 2013, 80(6);474-87 PubMed 23610072
- ↑ 8.0 8.1
Kexi Yi, Boris Rubinstein, Jay R Unruh, Fengli Guo, Brian D Slaughter, Rong Li Sequential actin-based pushing forces drive meiosis I chromosome migration and symmetry breaking in oocytes. J. Cell Biol.: 2013, 200(5);567-76 PubMed 23439682
| PMC3587830 | J Cell Biol.
Hiroki Shibuya, Akihiro Morimoto, Yoshinori Watanabe The Dissection of Meiotic Chromosome Movement in Mice Using an In Vivo Electroporation Technique. PLoS Genet.: 2014, 10(12);e1004821 PubMed 25502938
Manuela Pellegrini, Sara Di Siena, Giuseppina Claps, Silvia Di Cesare, Susanna Dolci, Pellegrino Rossi, Raffaele Geremia, Paola Grimaldi Microgravity promotes differentiation and meiotic entry of postnatal mouse male germ cells. PLoS ONE: 2010, 5(2);e9064 PubMed 20140225
| PLoS One
Sarai Pacheco, Marina Marcet-Ortega, Julian Lange, Maria Jasin, Scott Keeney, Ignasi Roig The ATM Signaling Cascade Promotes Recombination-Dependent Pachytene Arrest in Mouse Spermatocytes. PLoS Genet.: 2015, 11(3);e1005017 PubMed 25768017
Xiao-Ling Xu, Wei Ma, Yu-Bo Zhu, Chao Wang, Bing-Yuan Wang, Na An, Lei An, Yan Liu, Zhong-Hong Wu, Jian-Hui Tian The microtubule-associated protein ASPM regulates spindle assembly and meiotic progression in mouse oocytes. PLoS ONE: 2012, 7(11);e49303 PubMed 23152892
| PLoS ONE
So I Nagaoka, Terry J Hassold, Patricia A Hunt Human aneuploidy: mechanisms and new insights into an age-old problem. Nat. Rev. Genet.: 2012, 13(7);493-504 PubMed 22705668
| Nat Rev Genet.
T Ebner, C Yaman, M Moser, M Sommergruber, O Feichtinger, G Tews Prognostic value of first polar body morphology on fertilization rate and embryo quality in intracytoplasmic sperm injection. Hum. Reprod.: 2000, 15(2);427-30 PubMed 10655316
Johnny S Younis, Orit Radin, Ido Izhaki, Moshe Ben-Ami Does first polar body morphology predict oocyte performance during ICSI treatment? J. Assist. Reprod. Genet.: 2009, 26(11-12);561-7 PubMed 19960239
Gloria A Brar, Angelika Amon Emerging roles for centromeres in meiosis I chromosome segregation. Nat. Rev. Genet.: 2008, 9(12);899-910 PubMed 18981989
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Meiotic prophase I stages
Chromosome connections in meiosis
Kinetochore attachment to the spindle and chromosome cohesion in mitosis and meiosis
Human testis NANOG expression
Adult worm hermaphrodite gonad arm
Mouse oocyte microtubule-associated protein
| Cell Division Terms
| Meiosis | Mitosis
- anaphase - (Greek, ana = up, again) Mitosis term referring to the fourth stage, where the paired chromatids now separate and migrate to spindle poles. This is followed by telophase.
- anaphase A - Mitosis term referring to the part of anaphase during which the chromosomes move.
- anaphase B - Mitosis term referring to the part of anaphase during which the poles of the mitotic spindle move apart.
- aster - (Latin, aster = star) star-like object visible in most dividing eukaryotic cells contains the microtubule organizing center.
- astral microtubule - spindle apparatus microtubule (MT) originating from the centrosome which does not connect to a kinetochore. These microtubules only exist during mitosis, the other spindle types are polar and kinetochore microtubules.
- bouquet stage - meiosis term for when in prophase transition to the zygotene stage, the chromosome telomeres attachment to the inner nuclear envelope and form a cluster. This occurs before the onset of homologous pairing and synapsis. The name comes from the chromosomes resembling a "bouquet of flowers".
- diploid - (Greek, di = double + ploion = vessel) having two sets of chromosomes, the normal state for all cells other than the animal gametes that are haploid (a single set of chromosomes).
- diplotene stage- (diplotene phase, diplonema; Greek, diplonema = "two threads") meiotic stage seen during prophase I, the chromosomes separate from one another a small amount giving this appearance. In the developing human ovary, oocytes remain at the diplotene stage from fetal life through postnatal childhood, until puberty when the lutenizing hormone (LH) surges stimulate the resumption of meiosis. Prophase I, is divided into 5 stages (leptotene, zygotene, pachytene, diplotene, diakinesis) based upon changes associated with the synaptonemal complex structure that forms between two pairs of homologous chromosomes.
- haploid - (Greek, haploos = single) Having a single set of chromosomes as in mature germ/sex cells (oocyte, spermatozoa) following reductive cell division by meiosis. Normally cells are diploid, containing 2 sets of chromosomes. Ploidy refers to the number of sets of chromosomes in the nucleus of a cell.
- homologous chromosomes - meiosis term for the two matching (maternal and one paternal) chromosomes that align during meiosis I.
- kinetochore - the protein structure formed on chromatids where the spindle kinetochore microtubules attach during cell division.
- kinetochore microtubule - spindle apparatus microtubule (MT) that attaches to the chromosome kinetochore by its plus end, the other spindle types are astral and polar microtubules.
- kinesin - a microtubule (MT) motor protein that exists in many isoforms and most move towards the MT positive end. Different isoforms have different functions within the spindle apparatus. PMID 20109570
- meiosis - reductive cell division required to produce germ cells (oocyte, spermatozoa) and for sexual reproduction. Note that only spermatozoa complete meiosis before fertilisation. Chromosome number is reduced from diploid to haploid, during this process maternal and paternal genetic material are exchanged. All other non-germ cells in the body divide by mitosis. (More? Meiosis | Spermatozoa Development | Oocyte Development | Week 1)
- meiosis I - (MI) the first part of meiosis resulting in separation of homologous chromosomes, in humans producing two haploid cells (N chromosomes, 23), a reductional division.
- meiosis II - (MII) the second part of meiosis. In male human spermatogenesis, producing of four haploid cells (23 chromosomes, 1N) from the two haploid cells (23 chromosomes, 1N), each of the chromosomes consisting of two sister chromatids produced in meiosis I. In female human oogenesis, only a single haploid cell (23 chromosomes, 1N) is produced. Meiosis II: Prophase II - Metaphase II - Anaphase II - Telophase II.
- meiotic silencing of unsynapsed chromatin - (MSUC) an aneuploidy protective mechanism for subsequent generations, during meiosis where chromosomes are silenced that fail to pair with their homologous partners.
- merotelic kinetochore - cell division abnormality in chromosomal attachment that occurs when a single kinetochore is attached to microtubules arising from both spindle poles. Normal chromosomal attachment in early mitosis, is by only one of the two sister kinetochores attached to spindle microtubules (monotelic attachment) later sister kinetochores attach to microtubules arising from opposite spindle poles (amphitelic attachment).
- metaphase - mitosis term referring to the third stage where mitotic spindle kinetochore microtubules align chromosomes in one midpoint plane. Metaphase ends when sister kinetochores separate. Originally based on light microscopy of living cells and electron microscopy of fixed and stained cells. A light microscope analysis called a "metaphase spread" was originally used to detect chromosomal abnormalities in cells. Mitosis Phases: prophase - prometaphase - metaphase - anaphase - telophase
- metaphase spread - In mitosis using light microscope analysis originally used to detect chromosomal abnormalities in cells, as chromosomes are only visible during cell division.
- microfilament - (MF) cytoskeleton filament normally required for cytoplasmic intracellular transport, motility and cell shape. Named by the actin monomers assembling into the smallest in cross-section of the three filament systems (microtubules and intermediate filaments). This system is disassembled and reassembled as the contractile ring for cytokinesis (cytoplasm division) following cell division mitosis and meiosis.
- microtubule - (MT) cytoskeleton filament normally required for cytoplasmic intracellular transport and motility. Named by the tubulin monomers assembling into "tubes", and are the largest in cross-section of the three filament systems (microfilaments and intermediate filaments). This system is disassembled and reassembled as the spindle apparatus during cell division.
- mitosis - (M phase) The normal division of all cells, except germ cells, where chromosome number is maintained (diploid). In germ cell division (oocyte, spermatozoa) meiosis is a modified form of this division resulting in reduction in genetic content (haploid). Mitosis, division of the nucleus, is followed by cytokinesis the division of the cell cytoplasm and the cytoplasmic contents. cytokinesis overlaps with telophase.
- p - chromosome short arm (possibly French, petit) and used along with chromosome and band number to indicate genes located on this arm of the chromosome. The chromosome long arm is identified as q (possibly French, tall) chosen as next letter in alphabet after p. These chromosomal arms are only seen when the chromosome is folded for cell division.
- polar microtubule - spindle apparatus microtubule (MT) that can arise from either pole and overlap at the spindle midzone. This interdigitating structure consisting of antiparallel microtubules is responsible for pushing the poles of the spindle apart. The other spindle types are astral and kinetochore microtubules.
- prometaphase - (Greek, pro = before) mitosis term referring to the second stage, when the nuclear envelope breaks down into vesicles. Microtubules then extend from the centrosomes at the spindle poles (ends) and reach the chromosomes. This is followed by metaphase.
- pronuclear fusion - (Greek, pro = before) the process of the fusion of the two haploid nuclear structures (pronuclei) contributed from the spermatazoa and oocyte to form the first diploid nucleus cell. Can also be called "fusion of pronuclei".
- pronucleus - (Greek, pro = before; plural, pronuclei) the two haploid nuclei or nuclear structures containing the genetic material from the spermatozoa and the oocyte. These two haploid nuclei will fuse together to form the first diploid nucleus cell, the zygote. Therefore the nuclear structures that exist "before the nucleus", the plural term is pronuclei.
- prophase - (Greek, pro = before) - mitosis term referring to the first stage, when the diffusely stained chromatin resolves into discrete chromosomes, each consisting of two chromatids joined together at the centromere.
- prophase I - meiosis term refers to the first phase of meiosis I, which together with meiosis II results in the reductive cell division only occurring gametes. Prophase can be further divided into a number of stages: leptotene zygotene, pachytene, diplotene, diakinesis.
- q - chromosome long arm (possibly French, tall), the next letter in alphabet after p, and used along with chromosome and band number to indicate genes located on this arm of the chromosome. The chromosome short arm is identified as p (possibly French, petit). These chromosomal arms are only seen when the chromosome is folded for cell division.
- S phase - during interphase of cell cycle where DNA is duplicated prior to second growth period (G2 phase) that is followed by mitosis (M phase).
- synapsis - (syndesis) meiosis term for the pairing of two homologous chromosomes that occurs during prophase I.
- synaptonemal complex - meiosis term for a protein structure essential for synapsis of homologous chromosomes. (proteins SCP3 and SCP1).
- telomere - region found at each end of the chromosome and involved in cellular ageing and the capacity for division. The regions consist of repeated sequences protecting the ends of chromosomes and harbour DNA repair proteins. In the absence of the enzyme telomerase, these regions shorten during each cell division and becoming critically short, cell senescence occurs.
- telophase - mitosis term referring to the fifth stage, where the vesicles of the nuclear envelope reform around the daughter cells, the nucleoli reappear and the chromosomes unfold to allow gene expression to begin. This phase overlaps with cytokinesis, the division of the cell cytoplasm.
- telomerase - the enzyme that maintains the chromosome end length, the telomeres, involved in cellular ageing and the capacity for division. Absence of telomerase activity leads to the chromosome ends shorten during each cell division, becoming critically short and cell senescence then occurs.
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Cite this page: Hill, M.A. 2017 Embryology Cell Division - Meiosis. Retrieved March 27, 2017, from https://embryology.med.unsw.edu.au/embryology/index.php/Cell_Division_-_Meiosis
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