Meiosis is a special type of cell division of germ cells in sexually-reproducing organisms used to produce 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 female will fuse to create a cell with two copies of each chromosome again, the zygote.
Tetrahymena, a unicellular eukaryote, is a genus of free-living ciliates that can also switch from commensalistic to pathogenic modes of survival. They are common in freshwater ponds. Tetrahymena species used as model organisms in biomedical research are T. thermophila and T. pyriformis.
Prophase (from the Greek πρό, "before" and φάσις, "stage") 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 passed on from the parents to the offspring. Most recombination is naturally occurring.
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.
A germ cell is any biological cell that gives rise to the gametes of an organism that reproduces sexually. In many animals, the germ cells originate in the primitive streak and migrate via the gut of an embryo to the developing gonads. There, they undergo meiosis, followed by cellular differentiation into mature gametes, either eggs or sperm. Unlike animals, plants do not have germ cells designated in early development. Instead, germ cells can arise from somatic cells in the adult, such as the floral meristem of flowering plants.
Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. There are three forms of nondisjunction: failure of a pair of homologous chromosomes to separate in meiosis I, failure of sister chromatids to separate during meiosis II, and failure of sister chromatids to separate during mitosis. Nondisjunction results in daughter cells with abnormal chromosome numbers (aneuploidy).
An oocyte, oöcyte, ovocyte, or rarely ocyte, is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell (PGC), which then undergoes mitosis, forming oogonia. During oogenesis, the oogonia become primary oocytes. An oocyte is a form of genetic material that can be collected for cryoconservation.
Oogenesis, ovogenesis, or oögenesis is the differentiation of the ovum into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated in the embryonic stage.
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. Autosomes undergo synapsis during meiosis, and are held together by a protein complex along the whole length of the chromosomes called the synaptonemal complex. Sex chromosomes also undergo synapsis; however, the synaptonemal protein complex that holds the homologous chromosomes together is only present at one end of each sex chromosome.
A chromomere, also known as an idiomere, is one of the serially aligned beads or granules of a eukaryotic chromosome, resulting from local coiling of a continuous DNA thread. Chromeres are regions of chromatin that have been compacted through localized contraction. In areas of chromatin with the absence of transcription, condensing of DNA and protein complexes will result in the formation of chromomeres. It is visible on a chromosome during the prophase of meiosis and mitosis. Giant banded (Polytene) chromosomes resulting from the replication of the chromosomes and the synapsis of homologs without cell division is a process called endomitosis. These chromosomes consist of more than 1000 copies of the same chromatid that are aligned and produce alternating dark and light bands when stained. The dark bands are the chromomere.
Coprinopsis cinerea is a species of mushroom in the family Psathyrellaceae. Commonly known as the gray shag, it is edible, but must be used promptly after collecting.
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.
Meiotic recombination protein REC8 homolog is a protein that in humans is encoded by the REC8 gene.
HORMA domain-containing protein 1 (HORMAD1) also known as cancer/testis antigen 46 (CT46) is a protein that in humans is encoded by the HORMAD1 gene.
The meiotic recombination checkpoint monitors meiotic recombination during meiosis, and blocks the entry into metaphase I if recombination is not efficiently processed.
The leptotene stage, also known as the leptonema, is the first of five substages of prophase I in meiosis. The term leptonema derives from Greek words meaning "thin threads". A cell destined to become a gamete enters the leptotene stage after its chromosomes are duplicated during interphase. During the leptotene stage those duplicated chromosomes—each consisting of two sister chromatids—condense from diffuse chromatin into long, thin strands that are more visible within the nucleoplasm. The next stage of prophase I in meiosis is the zygotene stage.
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-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.
Resumption of meiosis occurs as a part of oocyte meiosis after meiotic arrest has occurred. In females, meiosis of an oocyte begins during embryogenesis and will be completed after puberty. A primordial follicle will arrest, allowing the follicle to grow in size and mature. Resumption of meiosis will resume following an ovulatory surge (ovulation) of luteinising hormone (LH).
