2009 Group 1 Project

From CellBiology

Meiosis

Introduction

The term meiosis is derived from the Greek word meaning diminution or, to lessen, as it essentially involves the halving of the number of chromosomes contained within the daughter cells that result from the process. Meiosis is a specialized form of cell division which generates mature haploid gametes, or sex cells, with exactly half the number of chromosomes and genetically distinct from the diploid primordial germ cells from which they are produced. It is the starting point of sexual reproduction and, the immense genetic diversity in all multi-cellular eukaryotic organisms [1]. Random variation, competition, natural selection and the evolution of the species start with meiosis. Charles Darwin and Gregor Mendel were neither aware of DNA, the universal genetic material, by-which inheritance is conferred, nor of the mechanisms of genetic re-assortment, but, they were aware of the phenotypic variation of form and function that conferred advantage and disadvantage within and between the species [2].


References

  1. Essential Cell Biology (2nd ed.) Alberts, etal. 2003 p663
  2. Editorial, Nature Cell Biology 2009,11, p111

History of Discovery of Meiosis

  • 1876 German biologist Oskar Hertwig: Observed the fusion of egg and sperm in the transparent Sea Urchin Egg concluding that the nuclei of the two cells contributed to the inherited traits received by the offspring.
  • 1883 by Belgian zoologist Eduard Van Beneden: Described the haploid nature of germ cells of the Ascaris worm; also discovered and named chromatin, the complex of DNA, RNA and protein that make up a chromosome.
  • 1890 German biologist August Weismann: The fact that Meiosis required two rounds of cell division; also revealed that the somatic cells of the body do not contribute to inheritance a task restricted to germ cells.
  • 1901 German cytologist Theodor Boveri:a fertilized egg must contain a certain number of chromosomes to develop normally and that each chromosome contributes different qualities to the offspring.
  • 1930's: The complexity of the behaviour of chromosomes and the essential events of Meiosis were not revealed until the 1930’s. British biologist, Cyril Dean Darlington clarified the essential events, occurring during meiosis and Thomas Hunt Morgan, awarded the Nobel Prize in 1933, revealed that genes were transmitted on chromosomes and they were the carriers of inherited genetic information.
  • 1990's onward: Genetic and molecular studies have revealed many of the key meiosis specific proteins: those which act to regulate the process, for example MPF (Maturation Promoting Factor) and CSF (Cytostatic factor); those which cause chromosomes to align side to side such as the recently discovered protein complex Augmin [1]; and, those which facilitate recombination such as the four known central element proteins of the Synaptonemal, Complex Proteins 1-4.-[2]


References

  1. J Cell Biol. 2009 Mar 23;184(6):777-84. Epub 2009 Mar 16
  2. PLoS Genet. 2009 Feb;5(2):e1000393. Epub 2009 Feb 27

Hand drawing of meiosis:hand drawing

The Process of Meiosis I

illustration of key meiotic events

During meiosis the number of chromosomes is exactly halved in each of the four daughter cells produced by a single primordial germ cell. The four daughter cells are genetically distinct from each other and from the parent cell. Meiosis involves one round of DNA replication and two successive rounds of cell division, Meiosis I and Meiosis II. These are further subdivided into a number of distinct steps: Prophase, Metaphase, Telophase and Anaphase for each meiotic division. These phases, to some degree mirror the steps of Mitosis. DNA replication in preparatory phase is the same in both processes: cell size increases, the chromosomes condense, the nuclear envelope breaks down, the Golgi apparatus and the ER re-organize, the cytoskeleton reforms into specialized structure that will facilitate cell division. When the DNA of the chromosomes replicates a pair of sister chromatids, linked along their length by Cohesins, is formed for each of the 23 maternally and 23 paternally derived chromosomes, a total of 92 chromosomes[1].


However, unlike mitosis, where each of the phases plays a more balanced role in the entirety of the process, the prophase of Meiosis I is, to a large degree, the most important phase. It is the most complicated and time consuming of the meiotic phases accounting for up to 90% of the total time taken by the entire process and it is the phase in which the two most significant and distinct events take place: homologous pairing and genetic re-assortment [2].


Prophase Substages: Leptotene; Zygotene; Pachytene; Diplotene; Diakinesis. Immunolabeling of Spread Spermatocytes with Anti-SYCP3 (Red) and Anti-SYCP1 (Green) Antibodies (A–E) and Anti-SYCP3 (Red) and Anti- Centromere (Blue) Antibodies (F)

Prophase I is sub-divided into five sequential steps: leptotene (thin thread), zygotene (yolked thread), pachytene (thick thread), diplotene (double thread) and diakinesis which are defined in relation to the synapsis of the homologous chromosomes. The Leptotene stage is when the homolog begins to pair and condense but before tight synapsis occurs. At the zygotene stage, tight side-by side pairing, or synapsis occurs and the synaptonemal complex starts to form. The Pachytene stage occurs when synapsis is complete, where there is genetic recombination and further condensation of the chromosomes occurs. Diplotene is the stage in which the chiasmata appear as a result of the chromosomes homologues separation leaving only a residual of the complete synapsis remains at points of crossover. Diakenesis is the stage where the chromosomes are further condensed and the chiasmata are moving toward towards the end of the chromosomes[3].


Link: Protiens in Meiosis: synaptonemal complex

References

  1. The Cell - A Molecular Approach 2nd ed., Cooper, Geoffrey M., Sunderland (MA): Sinauer Associates, Inc., 2000. ch 20 p3
  2. Essential Cell Biology (2nd ed.) Alberts, etal. 2003 p670
  3. The Cell - A Molecular Approach 2nd ed., Cooper, Geoffrey M., Sunderland (MA): Sinauer Associates, Inc., 2000. ch 20 p9


Homologous Pairing

To generate four haploid gametes containing either a maternal or a paternal copy of each gene, and not both, sister chromatids for each of the 46 chromosomes pair up with their homolog, (same chromosome but from the other parent) forming a bivalent, where they are physically connected or synapsed along their length while a protenaceous structure, the synaptonemal complex, is formed between them. How this initial homology recognition of same chromosome occurs is yet to be understood but, there is some evidence to suggest that interaction between the SYCE2 gene and the repair protein RAD51 may promote recognition and then synapse at points of recombination [1]. It may also be mediated by complementary base-pair interactions [2] or as is the case in some species of plants, Chromatin re-modelling at the on-set of meiosis may allow recognition and synapsis to occur [3].


References

  1. PLoS Genet. 2009 Feb;5(2):e1000393. Epub 2009 Feb 27
  2. The Cell - A Molecular Approach 2nd ed., Cooper, Geoffrey M., Sunderland (MA): Sinauer Associates, Inc., 2000.ch 20 p2.
  3. Proc Natl Acad Sci U S A. 2008 Apr 22;105(16):6075-80. Epub 2008 Apr 15


Genetic Recombination

At the Zygotene stage, once synapse is complete, an extensive process of genetic recombination, or crossover, occurs between maternal and paternal homolog within the bivalent [1].One of the key proteins involved in regulating and initiating the process of recombination is Spo11 Protein. The bivalent form is held together by a synaptonemal complex and is maintained during the long meiotic prophase, lasting years in the case of the formation of the human egg cell. The synaptonemal complex, required for successful crossover/ recombination events and correct disjunction, consists of two parallel lateral elements, elongated central core and many transverse filaments. The homologous chromosomes are aligned around the synaptonemal complex forming the characteristic axis-loop structure of meiotic chromosomes. Interactions between the synaptonemal complex proteins and the cohesins responsible for the bonding of the sister chromatids act together to produce the architecture required for recombination between non-sister chromatids at properly segregated intervals[2]. Recent genetic studies in mice have shown that where genes encoding certain protein elements of the synaptonemal complex are absent, a high incidence of aneuploidy and miscarriage occur.


link: Proteins in Meiosis: Spo11 Protein


Crossover can occur at almost any point along a chromosome, generating a new combination of maternal and paternal genes on a chromosome[3]. At the diplotene stage, a number of chiasmata occur on the bivalent corresponding to the points where the non-sister chromatids crossover from each homologous pair and have joined[4]. Once prophase ends and recombination is complete during prophase I diakinesis, metaphase begins and the chromosomes align along the equator. During anaphase and telophase the duplicated homologs are pulled apart to the opposite sides of the spindle and one double stranded chromosome of each homologous pair is distributed to each daughter cell[5]. The centromere does not replicate because, at this point, the chromosomes remains double stranded. Cytokinesis, or cell division then occurs, producing two haploid cells each of which contains 46 chromosomes.


References

  1. Essential Cell Biology (2nd ed.) Alberts, etal. 2003 p664
  2. Chromosoma. 2006 Jun;115 (3):235-40. Epub 2006 Mar 4
  3. Essential Cell Biology (2nd ed.) Alberts, etal. 2003, p667
  4. Essential Cell Biology (2nd ed.) Alberts, etal. 2003 p664
  5. Human Embryology (3rd ed.) Larsen, William J 2001 p7


The Process of Meiosis II

The second maturation division begins shortly after the first meiotic division. It kicks in immediately after cytokinesis at the end of Telophase I, before the chromosomes have completely decondensed. In many ways, the second division is very similar to the process of mitosis and meiosis I but in contrast to the first, meiosis II begins without further replication of the chromosomes.

Illustration of meiosis II cell division.

The process of meiosis II consists of four stages similar to meiosis I: Prophase II, Metaphase II, Anaphase II, and Telophase II.

In prophase II, the chromatins shorten and thicken and condense into the 23 double-stranded chromosomes, which consist of two sister chromatids joined together by a centromere. Both the nucleoli and the nuclear envelop breaks down at this stage, while the spindle apparatus forms as the centrioles duplicate, which then separate into two centrosomes and move towards the polar regions.

In Metaphase II, the meiotic spindle apparatus, which has been formed by gamma-tubulin, in both of the daughter cells have been completed. Each chromosomes then arrange themselves on the metaphase plate, which has been rotated and become perpendicular to the previous plate. (Cooper, 2000) Chromosomes become aligned on the spindle, with the kinetochores of the sister chromatids facing and being attached to the microtubule fibers from the opposite poles.

Subsequently, the chromosomal centromeres are then cleaved and being replicated, separating the two sister chromatids as they are pulled apart by the microtubules that are attached to their kinetochores. (Cooper, 2000)This is when anaphase II occurs. The two chromatids then segregate to opposing poles on the spindle, and become two single-stranded chromosomes, one of which is distributed to each of the daughter nuclei. (Larsen, 2001)

Telophase II ends the whole meiotic process with the uncoiling and lengthening of the chromosomes, the disappearance of the spindle apparatus, and the reformation of the nuclear envelope around each set of the chromosomes. Cytokinesis then takes place, giving rise to a total of four daughter cells, each with a haploid set of chromosomes.

During the first meiotic division, two secondary spermatocytes are produced in the male and a secondary oocyte and a first polar body are produced in the female. During the second meiotic cell division two definitive spermatocytes or spermatids are produced in the male. In the female however, the second meiotic cell division is quite unequal, producing one large definitive oocyte and another rather diminutive polar body. The first polar body may then at the same time undergo a second meiotic division to produce a second and third polar body. (Larsen, 2001)


Link:

References

1. (2007, June). Phschool- The biology place. Retrieved May 13, 2009, from Pearson Education Web site: http://www.phschool.com/science/biology_place/biocoach/index.html

2. Cooper, G. M. (2000). The cell, a molecular approach. Sunderland, Missachusetts: ASM Press

3. Larsen, W. J. (2001). Human embryology. United States of America: Churchil Livingstone.

Differences Between Male and Female meiosis

In humans as in most other mammals, there are some fundamental differences between male and female meiosis. Apart from the obvious differences - male meiosis generates sperm in the testes, female meiosis produces eggs in the ovaries – there are three major differences:

  • In males, a primordial spermatogonium produces four spermatids, whereas in females, a primordial oogonium produces one definitive oocyte and three apparently redundant polar bodies.
  • Male germ cell meiosis begins in puberty and continues throughout life. Female germ cell meiosis begins in fetal life, occurring between 12 weeks – 5 months, is arrested in early prophase I, continues again at puberty but is arrested once more in the second meiotic metaphase and is completed only if and when fertilization occurs.
  • Eggs carry only the x, or female sex chromosome, sperm carry either an x or a y chromosome.


Mechanisms underlying many of the differences not yet fully understood. Female germ cells begin meiosis during embryogenesis and arrest at prophase I and metaphase II. Male germ cells undergo mitotic G1/G0 arrest at the same stage of development and commence meiosis at puberty. The Stra8 (Stimulated by Retinoic Acid Gene 8) expression appears to trigger meiosis in both the sexes but at different stages of development. Retinoic acid signaling is required for Stra8 expression and sex specific regulation of the retinoic acid signaling pathway plays an important role in initiating meiosis in female embryonic germ cells and delaying it in males. Two factors appear to delay the process male meiosis until puberty are: Cytochrome p450, in mediating retinoic acid signaling, prevents expression of Stra8 [1]; and, expression of Sry, the master regulator gene of male sex determination, appears to block meiosis in embryonic germ cells by sequestering them within the spermatic cords[2].


The first arrest phase in female meiosis at prophase I appears, at least in part, to be mediated by Protein Kinase A activity, which inhibits Cdk1. Meiotic resumption requires Maturation-Promoting Factor (MPF) which is comprised of Cyclin dependent kinase-1 (Cdk1) and regulatory subunit cyclin B. Protein Kinase A appears to inactivate MPF [3]. Metaphase II arrest is mediated by the activity of cytostatic factor (CSF) proteins Emi1 and Emi2 which inhibit the anaphase promoting complex/cyclosome (APC/C) via the Mos-MAPK pathway [4].


Link: Comparative diagram of male and female meiosis - Nature Reviews Genetics

References

  1. Koubova et al Proc Natl Acad Sci USA, 2006 Feb 21, 2006
  2. Yao et al Novataris Found Symp 2002; 244:187-98
  3. Pirino et al Cell Cycle, 2009 Feb 15:8(4):665-70
  4. Wu and Kornbluth Journal of Cell Science, 2008 Nov 1; 121(pt21):3509-14

Differences between Meiosis and Mitosis

Z Mitosis Meiosis.jpg

There have been a number of differences identified between meiosis and mitosis. First, mitosis occurs in somatic cells whereas meiosis occurs in reproductive cells. Second, mitosis process involves one time cell division and produces two cells. In addition, the resultant two cells, also known as daughter cells carry exactly the same genetic information as the original cell. On contary, meiosis involves two times cell division processes and produces four new daughter cells. Furthermore, each of the newly produced daughter cells contain half of the number of choromosomes of the original cell. In mitosis, dividing cells can either be diploid or haploid. However, in meiosis, dividing cells can only be diploid. As well as the above, in mitosis, cytokinesis occurs once. However, in meiosis, cytokinesis may take place once, or twice. In mitosis, it is preceded by a S phase in which the amount of DNA is duplicated. However, in meiosis, only meiosis 1 is preceded by a S phase. In mitosis, there is no exchange of DNA between chromosomes. However, in meiosis, there is at least one DNA exchange or genetic recombination per homologous pair of chromosomes. In mitosis, the centromeres split during anaphase. However, in meiosis, the centromeres split during anaphase 2, not anaphase 1.

links: Differences between Meiosis and Mitosis

Similarities between Meiosis and Mitosis

There are a number of similiarities existed between meosis and mitosis. Firstly, both meiosis and mitosis have not introduced new gene combinations when each new cell splits. Secondly, each new cell in both meiosis and mitosis shares the same number of chromosomes. Thirdly, both meiosis and mitosis are involved in DNA replication. Lastly, both meiosis and mitosis utilise cell cycle in terms of cellular phases. Such as interphase, prophase, metaphase, anaphase and telophase.

Regulation of Meiosis

Meiosis is a biological process that is restricted to sexually reproducing germ line cells. Similar to other cell cycle such as mitosis, it requires complex network of regulatory proteins that governs the progression of the cell to various stages of meiosis, which enhance the fidelity of meiotic division. As meiosis is restricted to germ line cells, it is likely that the cell cycle control is unique and different from somatic cells. The exploration of regulation of meiosis has great value in enhancing understanding of the origin of human infertility, and may provide new directions for contraception.

Cyclins: Key Regulator of Meiosis

llustrates interaction between cyclin and cyclin-dependent kinase (Cdk). Cyclin: shown in green, Cdk: shown in red


Major class of proteins involved in controlling meiotic cycles can be simplified into: regulatory subunits: cyclins, and its catalytic partner: cyclin-dependent kinases (CDKs), in which its genes are highly conserved in amongst all eukaryotic species. In a simplistic model, cyclins are critical regulatory subunits of CDKs, generating enzymatically active heterodimeric complex. The cyclin-CDK complex phosphorylates and activates proteins that promote germ cells to undergo meiosis and ultimately, gives rise to specialised cell cycle that does not exist in somatic cells. The catalytic activity of cyclin-CDK complex is regulated and modulated by other complex network of proteins such as activators and inhibitors, and it is further regulated in the level of transcription and translation.

Several classes of cyclins have been identified in mammalian cells, from A to I and T. Whilst there are still much research involved in understanding the role of cyclin-CDK complexes in controlling steps in meiosis, cyclin B is currently well known, and have demonstrated its pivotal role in meiosis, and there are some less well known cyclins namely: cyclin A, D, and E.

Cyclin A

Cyclin A has now been identified for over ten years and has been identified in to two distinct subclasses: cyclin A1, and Cyclin A2. However, in comparison to cyclin B, cyclin A have not been clear to understanding largely by the fact that no homologous gene is present in yeasts.

  • Cyclin A1: this cyclin has been shown to be male gonad specific and restricted to male germ line cells. Cyclin A1 is expressed specifically in pachytene and diplotene stage of meiosis, which suggests its role in regulating the process between G2 and M phase. Functional study of cyclin A1 has revealed that mammal lacking cyclin A1 gene (Ccna1) had reduced MFP kinase activity suggesting cyclin A1 is involved in activating the MFP complex [1], which is considered as universal pre-meiotic initiator. In addition, ability for cyclin A1 to bind to Rb family proteins, transcription factor E2F-1, and the p21 family proteins further highlights its role in regulating meiotic cell cycle [2]. Phenotypic analysis of knock-out Ccna1 gene have shown incomplete desynapsis between pachytene and diplotene, which is followed by apoptosis [3].
  • Cyclin A2: In contrast to cyclin A1, which is expressed specifically in male germ line cells, cyclin A2 is expressed abundantly in both mitotically dividing cells and meiotically dividing cells [4]. Unsurprisingly, cyclin A2 is expressed in between G1 and S phase, and it is expressed in pre-leptotene stage in meiosis. Cyclin A2 is able to bind to both CDK1 and CDK2, and its association with CDKs have shown to phosphorylate rb-related protein p107, further illustrating its regulatory role [5]. Functional study of cyclin A2 by mutation is difficult as this results in early embryonic lethality, due to its role in mitotic phase as well as meiotic phase [6]. Owing to these restrictions, much less is known about cyclin A2. Further study of its function should be done by cell-specific targeted loss of function.

Cyclin B

Cyclin B forms a heterodimeric structure with cyclin-dependent kinase 1 (CDK1), which act as a universal pre-meiotic initiator known as maturation promoting factor (MPF). Multiple subclass of cyclin B can associate with CDK1, which gives rise to specificity in phosphorylating meiosis specific proteins.

  • Cyclin B5, 6 (clb5, clb6): These cyclins associate with CDK1, and have found to be essential in initiation of premeiotic S-phase. In addition, these proteins play key role in homologous recombination during Pachytene stage of meiosis [7].
  • Cyclin B1, 3, 4: These cyclins have more general role in meiotic cell cycle, and they promote progression through meiotic division [8]. Its function was determined by deletion of the clb1, 3, 4 genes and absence of any two combination of the gene lead the cell to progress through only one meiotic division.
  • Cyclin B2: Whilst clb2 is a major mitotic gene, it is not expressed in meiosis.

Cyclin D

Multiple members of cyclin D family has been identified, Cyclin D1, 2, and 3. Cyclin D family play pivotal role in mitosis, especially between G1 and S-phase, and this is further highlighted by early embryonic lethality when all three subclass of cyclin D1 gene were knocked out. Cyclin D2 and 3 were expressed in various stages of spermatogenesis; however, gene knock-out of cyclin D family did not show essential function in germ line cells.

Cyclin E

There are two subclasses of cyclin E family, cyclin E1 and 2. Phenotypic observation of the mouse with Cyclin E genes knocked-out has shown loss of cell integrity of germ line cells and stages of meiosis terminated randomly [9]. This study has concluded that cyclin E does play role in regulation of meiosis, which is observed by abnormality in meiosis in knock-out mouse. However, little is known in regards to specific function of cyclin E.


Although there seems to be compelling evidences for cyclins playing role in meiosis, some of the evidences are derived from phenotypic observation rather than observation of the exact molecular mechanism and pathways. This short coming restricts us to develop more concrete understanding of the role of cyclins in regulating meiosis. Furthermore, many of the functional studies of cyclins have been related to yeasts. Although yeasts are considered as a good model organism to understand higher eukaryotic species, homologous genes or proteins for higher eukaryotic species do not always exist. As with cyclin A, no homologous proteins exist for yeast, making it challenging to study. Above challenges and limitations must be surmounted in order to have greater flexibility in manipulating the process of meiosis, which will lead to greater clinical and practical application.


References

  1. Liu D, Liao C, Wolgemuth DJ (2000) A role for cyclin A1 in the activation of MPF and G2-M transition during meiosis of male germ cells in mice. developmental biology 224(2): 388-400.
  2. Joshi AR, Jobanputra V, Lele KM, Wolgemuth DJ (2009) Distinct properties of cyclin-dependent kinase complexes containing cyclin A1 and cyclin A2. Biochemical and biophysical research communications 378(3): 595-599.
  3. Salazar G, Liu D, Liao C, Batkiewicz L, Arbing R, Chung SS, Lele K, DJ., W (2003 Oct 15;66(8):1571-9) apoptosis in male germ cells in response to cyclin a1-deficiency and cell cycle arrest. Biochemical pharmacology 66(8): 1571-1579.
  4. Ravnik SE, Wolgemuth DJ (1996 Jan 10;173(1):69-78) The developmentally restricted pattern of expression in the male germ line of a murine cyclin A, cyclin A2, suggests roles in both mitotic and meiotic cell cycles. Developmental biology 173(1): 69-78.
  5. Salazar G, Liu D, Liao C, Batkiewicz L, Arbing R, Chung SS, Lele K, DJ, W (2003) Apoptosis in male germ cells in response to cyclin A1-deficiency and cell cycle arrest. Biochemical pharmacology 66(8): 1571-1579.
  6. Murphy M, Stinnakre MG, Senamaud-Beaufort C, Winston NJ, Sweeney C, Kubelka M, Carrington M, Bréchot C, Sobczak-Thépot J (1997) Delayed early embryonic lethality following disruption of the murine cyclin A2 gene. Nature genetics 15(1): 83-86.
  7. Henderson KA, Kee K, Maleki S, Santini PA, S, K (2006) Cyclin-dependent kinase directly regulates initiation of meiotic recombination. Cell 125(7): 1321-1332.
  8. Dahmann C, Futcher B (1995) Specialization of B-type cyclins for mitosis or meiosis in S. cerevisiae. 140(3): 957-963.
  9. Geng Y, Yu Q, Sicinska E, Das M, Schneider JE, Bhattacharya S, Rideout WM, Bronson RT, Gardner H, Sicinski P (2003 Aug 22;114(4):431-43) Cyclin E ablation in the mouse. Cell 114(4): 431-443.

Errors in Meiosis - Chromosomal abnormalities

During meiotic divisions, abnormalities in chromosome number may occur. In normal conditions, two members of a homologous chromosome pair would separate during first meiotic cell division, so that each daughter cell receives one component of each pair. However, sometimes this normal separation procedure does not occur, which is known as “nondisjunction”, (Sadler, 1995)and that both members of the pair would move into the one daughter cell. This error could occur during either first or second meiotic cell division. Consequently, one daughter cell would then receive 24 chromosomes, and the other daughter cell would receive 22 instead of the normal 23 chromosomes. During fertilization, a normal gamete with 23 chromosomes would then fuse with a gamete containing 24 or 22 chromosomes. As a result, the individual will be having either 47 chromosomes, which is a condition known as “trisomy”, or 45 chromosomes, which is “monosomy”.


Reference

Sadler, T. W. (1995). Medical Embryology. United States of America: Williams & Wilkins.

Trisomy

Trisomy, or Down Syndrome; Notice the three copies of chromosome 21.

The occurrence of chromosomal abnormalities increases dramatically as a woman reaches the age of 35 or over, (Sadler, 1995)and that cases of monosomy and trisomy would occur more often as well and may involve the sex chromosomes or autosomes. For example, Down syndrome is a chromosomal disorder characterized by the presence of an extra copy of the 21st chromosome. It can be a result of a nondisjunction event during meiotic cell division, which happens when a gamete is produced with an extra copy of chromosome 21, with the gamete containing 24 chromosomes itself. When fuses with a normal gamete, the embryo thus have 47 chromosomes, with three copies of chromosome 21. This condition is known as Trisomy 21, and is the cause of approximately 95% of observed Down syndromes, with 80% of the cases caused by nondisjunction of the maternal gamete and the remainder due to nondisjunction of the paternal gamete.

Down syndrome can also be caused by conditions known as “translocations”, which happens when chromosomes maybe broken, and pieces of one chromosome may become attached to another. (Sadler, 1995)Translocation can be balanced or unbalanced: When it is balanced, in which case breakage and reunion occur between the two chromosomes, but no critical genetic material is lost, the individual is fine. However, when it is unbalanced, which happens when part of one chromosome is lost and a mutated phenotype is produced, conditions like Down syndrome would occur. (Sadler, 1995)In this case, unbalanced translocations between the long arms of chromosomes 14 and 21 during first or second meiotic divisions took place.


References

Sadler, T. W. (1995). Medical Embryology. United States of America: Williams & Wilkins.

Current Research

Although the basic mechanisms of the process of meiosis cell division have been known for decades, there are still a lot of recent research to be done on this area and chromosomal abnormalities is one of them.

Advances in genetic manipulation technologies for various model organisms have increased our knowledge of meiotic chromosome segregation, errors which contribute to chromosomal aneuploidy. It is already known that in human trisomy 21, three general characteristics are acknowledged: (1) in most cases the extra chromosome 21 is of maternal origin; (2) most cases are derived from a non-disjunction event in meiosis I; and (3) the frequency of these errors increases with maternal age. Studies with mice have shown that oocytes from older mice display a decrease in number of chiasmata, which might predispose the germ line cells to non-disjunction in meiosis I. Polymorphic markers were used in studies to show supportive findings that the number of recombination events in chromosome 21 is reduced in human trisomy 21 cases. Extensive studies have also revealed that sites of recombination have a distal location bias in cases of trisomy 21 and that more detailed analysis has revealed that such distal recombination is a risk in young mothers but that recombination pattern is not atypical in trisomy 21 offspring of an older mother. Further studies have also shown support to the hypothesis that the degradation of the cohesin complex contributes to reduced number of chiasmata that will lead to an age-dependent increase of non-disjunction events in meiosis I. (Kurahashi, 2009)

Other research on the details of the basic mechanism of the meiotic cell division process has also been done, such as the segregation of homologous maternal and paternal centromeres to opposite poles during meiosis I, how that is depended on post-replicative crossing over between homologous non-sister chromatids, which creates chiasmata and therefore bivalent chromosomes. Experiments with fission and budding yeasts were done to research on proteins such as cohesion Rec8, how its proteolytic cleavage causes the destruction of sister chromatid cohesion, thus resolving chiasmata and thereby triggering the first meiotic division. A newly discovered protein known as shugoshin (Sgo1/MEI-S332) is also being studied as to how Its inactivation causes loss of centromeric cohesin at anaphase I and random segregation of sister centromeres at the second meiotic division. (Riedel, 2006)


Link: Proteins in Meiosis- Cohesion Rec8

References

Kurahashi, H. (2009).Recent advance in our understanding of the molecular nature of chromosomal abnormalities. Journal of Human Genetics . 54, 253-260.

Riedel, C. G. (2006).Protein phosphatase 2A protects centromeric sister chromatid cohesion during meiosis I. Nature. 441, 53.

Group Reflection

Based on the work that we have done through week 3 to week 10 period, we have made reflections to our group project meiosis. Initially, we have learned that meiosis is a process which involved in process of specialised cell division that generates mature male and female gametes. And once the DNA replication and two successive cell divisions of meiosis is complete, as a result of cell divisions, four non-identical haploid daughter cells are produced. Then, we have learned that meiosis process involved two major subdivisions such as meiosis 1 and meiosis 2. In addition, we have also learnt that meiosis is imperative to sexual reproduction, as it provides immense genetic diversity in all multi-cellular eukaryotic organisms. According to these findings and characteristics of meiosis, we have divided our page into a number of sections, such as introduction, structure and function, dynamic process, current research, timeline, glossary and references. These particular sections have assisted our group narrowing target areas of meiosis, and made our research more specific. There were a number of changes made to our page during initial period of research. These included condensing paragraphs, adding pictures of meiosis and checking references. As a consequence of editing and research, we have learned that our materials become concise and easy to read as well as narrowing down related areas of meiosis.

In terms of our research, we have learned that there are errors in meiosis just like every other biological mechanism. These errors exhibit in the form of chromosomal abnormalities. It involves separation of homologous chromosome pair absence, this has resulted both members of the pair move into one daughter cell. An example of this is known as "Trisomy", which involve an individual having 47 chromosomes. Furthermore, a discussion section regarding to differences bewteen male and female meiosis has been made, as we have reliased it would add depth to our understanding of meiosis.

Our final version of the project has been structured into the following order. First, introduction. Then, history of discovery of meiosis. Thirdly, meiosis 1 which contains homologous pairing and genetic recombination. Fourthly, meiosis 2 process. Followed by the differences between male and female meiosis. Regulation of meiosis. Errors in meiosis. Lastly, current research of meiosis, glossary and references. This final version has covered the functional aspect of meiosis, as well as its future research direction in related fields.

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Glossary

Anaphase: Stages of meiosis 1 & 2, during which the two sets of choromosomes separate and move away from each other. Composed of anaphase A (choromosomes move toward the two spindle poles) and anaphase B (spindle poles move apart);

Bivalent: A duplicated chromosome paired with its homologous duplicated chromosome at the beginning of meiosis;

Centromere: A centromere is a region of DNA found in the middle of a chromosome where two identical sister chromatids come in contact;

Chiasmata: x-shaped connection visible between paired homologous chromosomes in division 1 of meiosis, and which represents a site of crossing over;

Chromosome: Long threadlike structure composed of DNA and associated proteins that carries part or all of the genetic informaiton of an organism. Especially evident in plant and animal cells undergoing mitosis or meiosis;

Cytokinesis: Division of the cytoplasm of a animal cell into two;

Cytoskeleton: System of protein filaments in the cytoplasm of a eukaryotic cell that gives the cell shape and the capacity for directed movement. Its most abundant components are actin filaments, microtubules, and intermediate filaments;

Cyclin: Protein that periodically rises and falls in concentration in step with the eukaryotic cell cycle. Cyclins activate specific protein kinases and thereby help control progression from one stage of the cell cycle to the next;

Cyclin-dependent protein Kinase(Cdk): Protein kinase that has to be complexed with a cyclin protein in order to act. Different Cdk-cyclin complexes trigger different steps in the cell-division cycle by phosphorylating specific target proteins;

Cyclin-CDK complex: A combination of cyclin and cyclin dependent protein kinase;

Sister chromotids crossing-over: see Genetic Recombination;

Diploid: A cell or organism containing two sets of homologous chromosomes;

DNA: double-stranded polynucleotide formed two separate chains of covalently linked deoxyribonucleotide units;

DNA replication: It is a fundamental process occurring in all living organisms to copy their DNA

G0 phase: It is a period in the cell cycle where cells exist in a quiescent state;

G1 phase: Gap 1 phase of the eukaryotic cell cycle, between the end of cytokinesis and the start of DNA synthesis;

G2 phase: Gap 2 phase of the eukaryotic cell cycle, between the end of DNA synthesis and the beginning of mitosis;

Gamete: cell type in a diploid organism that carries only one set of chromosomes and is specialised for sexual reproduction. A sperm or egg;

Genetic recombination: It is the process by which a strand of genetic material, usually DNA, is broken and then joined to a different DNA molecule;

Haploid: A cell with only one set of chromosomes;

Homolog: A homologous chromosome that has a close evolutionary relationship to another;

Interphase: Long period of the cell cycle between one mitosis and the next. Includes G1 phase, S phase, and G2 phase.

M phase: Peroid of the eukaryotic cell cycle during which the nucleus and cytoplasm divide;

Meiosis: It is a process of reductional division in which the number of chromosomes per cell is halved;

Metaphase: Stage of meiosis at which chromosomes are firmly attached to the mitotic spindle at its equator but have not yet segregated toward opposite poles;

Monosomy: Monosomy is a form of aneuploidy with the presence of only one chromosome from a pair;

Nuclear envelope: Double membrane surronding the nucleus. Consists of outer and inner membranes perforated by nuclear pores;

Phenotype: A phenotype is any observable characteristic or trait of an organism;

Prophase: First stage of meiosis during which the chromosomes are condensed but not yet attached to a mitotic spindle;

Primordial germ cells: see Germ cell;

S phase: Period during a eukaryotic cell cycle in which DNA is synthesized;

Sister chromatids: One copy of a chromosome formed by DNA replication that is still joined at the centromere to the other copy;

Spindle apparatus: It is the structure that separates the chromosomes into the daughter cells during cell division;

Telophase: Fianl stage of mitosis in which the two sets of separated chromosomes decondense and become enclosed by nuclear envelopes;

Trisomy: A trisomy is a genetic abnormality in which there are three copies, instead of the normal two, of a particular chromosome.

2009 Group Projects

Group 1 Meiosis | Group 2 Cell Death - Apoptosis | Group 3 Cell Division | Group 4 Trk Receptors | Group 5 The Cell Cycle | Group 6 Golgi Apparatus | Group 7 Mitochondria | Group 8 Cell Death - Necrosis | Group 9 Nucleus | Group 10 Cell Shape