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Meiosis 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 result in four cells with only one copy of each chromosome (haploid). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome. Later on, during fertilisation, the haploid cells produced by meiosis from a male and a female will fuse to create a cell with two copies of each chromosome again, the zygote.
Chromosomal crossover, or crossing over, is the exchange of genetic material during sexual reproduction between two homologous chromosomes' non-sister chromatids that results in recombinant chromosomes. It is one of the final phases of genetic recombination, which occurs in the pachytene stage of prophase I of meiosis during a process called synapsis. Synapsis begins before the synaptonemal complex develops and is not completed until near the end of prophase I. Crossover usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome.
Prophase is the first stage of cell division in both mitosis and meiosis. Beginning after interphase, DNA has already been replicated when the cell enters prophase. The main occurrences in prophase are the condensation of the chromatin reticulum and the disappearance of the nucleolus.
Genetic recombination is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryotes, genetic recombination during meiosis can lead to a novel set of genetic information that can be further passed on from parents to offspring. Most recombination occurs naturally and can be classified into two types: (1) interchromosomal recombination, occurring through independent assortment of alleles whose loci are on different but homologous chromosomes ; & (2) intrachromosomal recombination, occurring through crossing over.
A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during fertilization. Homologs have the same genes in the same loci where they provide points along each chromosome which enable a pair of chromosomes to align correctly with each other before separating during meiosis. This is the basis for Mendelian inheritance which characterizes inheritance patterns of genetic material from an organism to its offspring parent developmental cell at the given time and area.
The synaptonemal complex (SC) is a protein structure that forms between homologous chromosomes during meiosis and is thought to mediate synapsis and recombination during meiosis I in eukaryotes. It is currently thought that the SC functions primarily as a scaffold to allow interacting chromatids to complete their crossover activities.
Synapsis is the pairing of two chromosomes that occurs during meiosis. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover between them. Synapsis takes place during prophase I of meiosis. When homologous chromosomes synapse, their ends are first attached to the nuclear envelope. These end-membrane complexes then migrate, assisted by the extranuclear cytoskeleton, until matching ends have been paired. Then the intervening regions of the chromosome are brought together, and may be connected by a protein-RNA complex called the synaptonemal complex. During synapsis, autosomes are held together by the synaptonemal complex along their whole length, whereas for sex chromosomes, this only takes place at one end of each chromosome.
A bivalent is one pair of chromosomes in a tetrad. A tetrad is the association of a pair of homologous chromosomes physically held together by at least one DNA crossover. This physical attachment allows for alignment and segregation of the homologous chromosomes in the first meiotic division. In most organisms, each replicated chromosome elicits formation of DNA double-strand breaks during the leptotene phase. These breaks are repaired by homologous recombination, that uses the homologous chromosome as a template for repair. The search for the homologous target, helped by numerous proteins collectively referred as the synaptonemal complex, cause the two homologs to pair, between the leptotene and the pachytene phases of meiosis I.
Spo11 is a protein that in humans is encoded by the SPO11 gene. Spo11, in a complex with mTopVIB, creates double strand breaks to initiate meiotic recombination. Its active site contains a tyrosine which ligates and dissociates with DNA to promote break formation. One Spo11 protein is involved per strand of DNA, thus two Spo11 proteins are involved in each double stranded break event.
Chromosome segregation is the process in eukaryotes by which two sister chromatids formed as a consequence of DNA replication, or paired homologous chromosomes, separate from each other and migrate to opposite poles of the nucleus. This segregation process occurs during both mitosis and meiosis. Chromosome segregation also occurs in prokaryotes. However, in contrast to eukaryotic chromosome segregation, replication and segregation are not temporally separated. Instead segregation occurs progressively following replication.
TRIP13 is a mammalian gene that encodes the thyroid receptor-interacting protein 13. In budding yeast, the analog for TRIP13 is PCH2. TRIP13 is a member of the AAA+ ATPase family, a family known for mechanical forces derived from ATP hydrolase reactions. The TRIP13 gene has been shown to interact with a variety of proteins and implicated in a few diseases, notably interacting with the ligand binding domain of thyroid hormone receptors, and may play a role in early-stage non-small cell lung cancer. However, recent evidence implicates TRIP13 in various cell cycle phases, including meiosis G2/Prophase and during the Spindle Assembly checkpoint (SAC). Evidence shows regulation to occur through the HORMA domains, including Hop1, Rev7, and Mad2. Of note, Mad2's involvement in the SAC is shown to be affected by TRIP13 Due to TRIP13's role in cell cycle arrest and progression, it may present opportunity as a therapeutic candidate for cancers.
Meiotic recombination protein DMC1/LIM15 homolog is a protein that in humans is encoded by the DMC1 gene.
Synaptonemal complex protein 3 is a protein that in humans is encoded by the SYCP3 gene. It is a component of the synaptonemal complex formed between homologous chromosomes during the prophase of meiosis.
The meiotic recombination checkpoint monitors meiotic recombination during meiosis, and blocks the entry into metaphase I if recombination is not efficiently processed.
The origin and function of meiosis are currently not well understood scientifically, and would provide fundamental insight into the evolution of sexual reproduction in eukaryotes. There is no current consensus among biologists on the questions of how sex in eukaryotes arose in evolution, what basic function sexual reproduction serves, and why it is maintained, given the basic two-fold cost of sex. It is clear that it evolved over 1.2 billion years ago, and that almost all species which are descendants of the original sexually reproducing species are still sexual reproducers, including plants, fungi, and animals.
Structural maintenance of chromosomes protein 1B (SMC-1B) is a protein that in humans is encoded by the SMC1B gene. SMC proteins engage in chromosome organization and can be broken into 3 groups based on function which are cohesins, condensins, and DNA repair.SMC-1B belongs to a family of proteins required for chromatid cohesion and DNA recombination during meiosis and mitosis. SMC1ß protein appears to participate with other cohesins REC8, STAG3 and SMC3 in sister-chromatid cohesion throughout the whole meiotic process in human oocytes.
Stromal antigen 3 is a protein that in humans is encoded by the STAG3 gene. STAG3 protein is a component of a cohesin complex that regulates the separation of sister chromatids specifically during meiosis. STAG3 appears to be paramount in sister-chromatid cohesion throughout the meiotic process in human oocytes and spermatocytes.
Abby F. Dernburg is a Professor of Cell and Developmental Biology at the University of California, Berkeley, an Investigator of the Howard Hughes Medical Institute, and a Faculty Senior Scientist at Lawrence Berkeley National Laboratory.
Anne Villeneuve is an American geneticist. She is known for her work on the mechanisms governing chromosome inheritance during sexual reproduction. Her work focuses on meiosis, the process by which a diploid organism, having two sets of chromosomes, produces gametes with only one set of chromosomes. She is a Professor of Developmental Biology and of Genetics at Stanford University and a member of the National Academy of Sciences.
Ann Chester Chandley DSc, F.I.Biol., FRSE was an international cytogeneticist with the Medical Research Council unit which became the Human Genetics Unit at the University of Edinburgh. She became a Fellow of the Institute of Biology in recognition of her contribution and a Fellow of the Royal Society of Edinburgh.
References
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- ↑ "science.ca : Shirleen Roeder". www.science.ca. Retrieved 2021-12-30.
- ↑ Roeder, Glenna Shirleen (1978). Recombination, maturation and packaging of the bacteriophage T7 chromosome (Thesis). Toronto: [publisher not identified].
- 1 2 "Geneticist Shirleen Roeder named Eugene Higgins Professor". Yale Bulletin and Calendar. June 15, 2001. Retrieved 2021-12-30.
- ↑ "G Shirleen Roeder, Ph.D. | Molecular, Cellular and Developmental Biology". mcdb.yale.edu. Retrieved 2021-12-30.
- ↑ Sym, Mary; Engebrecht, JoAnne; Roeder, G. Shirleen (12 February 1993). "ZIP1 is a synaptonemal complex protein required for meiotic chromosome synapsis". Cell. 72 (3): 365–378. doi: 10.1016/0092-8674(93)90114-6 . ISSN 0092-8674. PMID 7916652. S2CID 6174855.
- 1 2 ""G Shirleen Roeder, Yale University"". National Academy of Sciences. Retrieved November 14, 2018.
- ↑ "NSF Award Search: Award # 8351607 - Presidential Young Investigator Award". www.nsf.gov. Retrieved 2021-12-30.
- ↑ ""G. Shirleen Roeder, PhD"". Howard Hughes Medical Institute. Retrieved November 14, 2018.
- ↑ Luna, Regina (September 1, 2010). ""Yale Scientists Awarded AAAS Fellowship"". Yale Scientific. Retrieved November 14, 2018.
- ↑ ""AAM Fellows G. Shirleen Roeder"". American Academy of Microbiology. Retrieved November 14, 2018.
- ↑ "78 scientists elected to the American Academy of Microbiology". EurekAlert!. Retrieved 2021-12-30.
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