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Movements, leaders, victories and the continuing fight for equality.
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Wilmington, North Carolina once had a thriving Black middle class and an elected interracial government. In 1898 white supremacists used violence to overthrow that government, kill Black residents and drive many others from the city.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
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.[5][6] 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.
Genetic exchange between two DNA molecules by homologous recombination can begin with a break in both strands of DNA—called a double-strand break—and recombination is started by an endonuclease enzyme that cuts the DNA molecule that "receives" the exchanged DNA. In meiosis the enzyme is SPO11, which is related to DNA topoisomerases. Topoisomerases change DNA by transiently breaking one or both strands, passing the unbroken DNA strand or strands through the break and repairing the break; the broken ends of the DNA are covalently linked to topoisomerase. SPO11 is similarly attached to the DNA when it forms double-strand breaks during meiosis.[7]
SPO11 is considered to play a predominant role in initiating meiotic recombination. However, recombination may also occur by alternative SPO11-independent mechanisms that can be studied experimentally using spo11 mutants.
In the budding yeast Saccharomyces cerevisiae, the meiotic defects in recombination and chromosome disjunction of spo11 mutants are alleviated by X-irradiation.[8] This finding indicates that X-ray induced DNA damages can initiate crossover recombination leading to proper disjunction independently of SPO11.
In the worm Caenorhabditis elegans, a homolog of spo11 is ordinarily employed in the initiation of meiotic recombination. However, radiation induced-breaks can also initiate recombination in mutants deleted for this spo11 homolog.[9]
Deamination of cytosine resulting in the dU:dG mismatch is one of the most common single-base-altering lesions in non-replicating DNA. Spo11 mutants of the fission yeast Schizosaccharomyces pombe and C. elegans undergo meiotic crossover recombination and proper chromosome segregation when dU:dG lesions are produced in their DNA.[10] This crossover recombination does not involve the formation of large numbers of double-strand breaks, but does require uracil DNA-glycosylase, an enzyme that removes uracil from the DNA phosphodiester backbone and initiates base excision repair. Thus, it was proposed that base excision repair of DNA damage such as a uracil base, an abasic site, or a single-strand nick is sufficient to initiate meiotic crossover recombination in S. pombe and C. elegans.[10]
In S. pombe, a mutant defective in the spo11 homolog Rec12 is deficient in meiotic recombination. However recombination can be restored to near normal levels by a deletion in rad2, a gene that encodes an endonuclease involved in Okazaki fragment processing (Farah et al., 2005). Both crossover and non-crossover recombination were increased but double-strand breaks were undetectable. On the basis of the biochemical properties of the rad2 deletion, it was proposed that meiotic recombination can be initiated by DNA lesions other than double-strand breaks, such as nicks and gaps which accumulate during premeiotic DNA replication when Okazaki fragment processing is deficient.[11]
The above findings indicate that DNA damages arising from a variety of sources can be repaired by meiotic recombination and that such a process can occur independently of SPO11.
The most recent common ancestor of the social amoeba genera Dictyostelium, Polysphondylium and Acytostelium, appears to have lacked the Spo11 gene.[12][13] Such an ancestor likely lived several hundred million years ago.[14] Dictyostelium discoideum and Polysphondylium pallidum are both capable of meiotic sexual reproduction (see D. discoideum sexual reproduction and P. pallidum sexual reproduction). Bloomfield[12] speculated that dormant cells in the soil might be exposed to many kinds of stress, such as desiccation or radiation, that could induce spontaneous DNA damage. Such damage would make the induction of double-strand breaks by Spo11 redundant for the initiation of recombination during meiosis, and thus explain its absence in this group.
Source: Wikipedia. Article content is retrieved live through the MediaWiki API.
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. Genetic exchange between two DNA molecules by homologous recombination can begin with a break in both strands of DNA—called a double-strand break—and recombination is started by an endonuclease enzyme that cuts the DNA molecule that "receives" the exchanged DNA. In meiosis the enzyme is SPO11, which is related to DNA topoisomerases. Topoisomerases change DNA by transiently breaking one or both strands, passing the unbroken DNA strand or strands through the break and repairing the break; the broken ends of the DNA are covalently linked to topoisomerase. SPO11 is similarly attached to the DNA when it forms double-strand breaks during meiosis.
A heteroduplex is a double-stranded (duplex) molecule of nucleic acid originated through the genetic recombination of single complementary strands derived from different sources, such as from different homologous chromosomes or even from different organisms. One such example is the heteroduplex DNA strand formed in hybridization processes, usually for biochemistry-based phylogenetic analyses. Another is the heteroduplexes formed when non-natural analogs of nucleic acids are used to bind with nucleic acids; these heteroduplexes result from performing antisense techniques using single-stranded peptide nucleic acid, 2'-O-methyl phosphorothioate or Morpholino oligos to bind with RNA. Meiotic recombination can be initiated by a double-strand break (DSB) in DNA. The 5’ ends of the break are degraded, leaving long 3’ overhangs of several hundred nucleotides (see Figure). One of these 3’ single stranded DNA segments then invades a homologous sequence on the homologous chromosome, forming an intermediate which can be repaired through different pathways resulting either in crossovers (CO) or noncrossovers (NCO) as illustrated in the Figure. By one pathway, a structure called a double Holliday junction (DHJ) is formed, leading to the exchange of DNA strands. By the other pathway, referred to as Synthesis-dependent strand annealing (SDSA), there is information exchange but not physical exchange. At various steps of these recombination processes, heteroduplex DNA (double-stranded DNA consisting of single strands from each of the two homologous chromosomes which may or may not be perfectly complementary) is formed. During meiosis non-crossover recombinants occur frequently and these appear to arise mainly by the SDSA pathway. Non-crossover recombination events occurring during meiosis likely reflect instances of repair of DNA double-strand damages or other types of DNA damages. When mismatches occur in heteroduplex DNA, the sequence of one strand can be repaired to bind the other strand with perfect complementarity. During mitosis, the major homologous recombination pathway for repairing DNA double-strand breaks appears to be the SDSA pathway (rather than the DSBR pathway). The SDSA pathway produces non-crossover recombinants (see Figure). In meiosis, the process of crossing-over occurs between non-sister chromatids, which results in new allelic combinations in the gametes. In crossing-over, a Spo11 enzyme makes staggered nicks in a pair of sister chromatid strands (in a tetrad organization of prophase). Subsequent enzymes trim back the 5' ends of the strand and a protein complex binds to the 3' single-stranded ends. Rad51 protein is recruited and binds in a protein complex to search for a complementary sequence analogous to double-strand-break repair. The filament searches for the homologous chromosome, strand invasion occurs where the new chromosome forms a D-loop over the bottom sister chromatid, then the ends are annealed. This process can yield double Holliday junctions that when cut in a transversal pattern by endonucleases form 2 heteroduplex strand products. Heteroduplex DNA is also a source of small RNAs (smRNAs), causing post-transcriptional gene silencing.
Meiosis ( ) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes: the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each 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 zygote, a cell with two copies of each chromosome. In mammals, the process of meiosis finishes only after fertilization so the product is a zygote. Errors in meiosis resulting in aneuploidy (an abnormal number of chromosomes) are the leading known cause of miscarriage and the most frequent genetic cause of developmental disabilities. In meiosis, DNA replication is followed by two rounds of cell division to produce four daughter cells, each with half the number of chromosomes as the original parent cell. The two meiotic divisions are known as meiosis I and meiosis II. Before meiosis begins, during S phase of the cell cycle, the DNA of each chromosome is replicated so that it consists of two identical sister chromatids, which remain held together through sister chromatid cohesion. This S-phase can be referred to as "premeiotic S-phase" or "meiotic S-phase". Immediately following DNA replication, meiotic cells enter a prolonged G2-like stage known as meiotic prophase. During this time, homologous chromosomes pair with each other and undergo genetic recombination, a programmed process in which DNA may be cut and then repaired, which allows them to exchange some of their genetic information. A subset of recombination events results in crossovers, which create physical links known as chiasmata (singular: chiasma, for the Greek letter Chi, Χ) between the homologous chromosomes. In most organisms, these links can help direct each pair of homologous chromosomes to segregate away from each other during meiosis II, resulting in two haploid cells that have half the number of chromosomes as the parent cell. During meiosis II, the cohesion between sister chromatids is released and they segregate from one another, as during mitosis. In some cases, all four of the meiotic products form gametes such as sperm, spores or pollen. In female animals, three of the four meiotic products are typically eliminated by extrusion into polar bodies, and only one cell develops to produce an ovum. Because the number of chromosomes is halved during meiosis, gametes can fuse (i.e., fertilization) to form a diploid zygote that contains two copies of each chromosome, one from each parent. Thus, alternating cycles of meiosis and fertilization enable sexual reproduction, with successive generations maintaining the same number of chromosomes. For example, diploid human cells contain 23 pairs of chromosomes including 1 pair of sex chromosomes (46 total), half of maternal origin and half of paternal origin. Meiosis produces haploid gametes (ova or sperm) that contain one set of 23 chromosomes. When two gametes (an egg and a sperm) fuse, the resulting zygote is once again diploid, with the mother and father each contributing 23 chromosomes. This same pattern, but not the same number of chromosomes, occurs in all organisms that utilize meiosis. Meiosis occurs in all sexually reproducing single-celled and multicellular organisms (which are all eukaryotes), including animals, plants, and fungi. It is an essential process for oogenesis and spermatogenesis.
Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there are two distinct types of cell division: a vegetative division (mitosis), producing daughter cells genetically identical to the parent cell, and a cell division that produces haploid gametes for sexual reproduction (meiosis), reducing the number of chromosomes from two of each type in the diploid parent cell to one of each type in the daughter cells. Mitosis is a part of the cell cycle, in which, replicated chromosomes are separated into two new nuclei. Cell division gives rise to genetically identical cells in which the total number of chromosomes is maintained. In general, mitosis (division of the nucleus) is preceded by the S stage of interphase (during which the DNA replication occurs) and is followed by telophase and cytokinesis; which divides the cytoplasm, organelles, and cell membrane of one cell into two new cells containing roughly equal shares of these cellular components. The different stages of mitosis all together define the M phase of an animal cell cycle—the division of the mother cell into two genetically identical daughter cells. To ensure proper progression through the cell cycle, DNA damage is detected and repaired at various cell cycle checkpoints. These checkpoints can halt progression through the cell cycle by inhibiting certain cyclin-CDK complexes. Meiosis undergoes two divisions resulting in four haploid daughter cells. Homologous chromosomes are separated in the first division of meiosis, such that each daughter cell has one copy of each chromosome. These chromosomes have already been replicated and have two sister chromatids which are then separated during the second division of meiosis. Both of these cell division cycles are used in the process of sexual reproduction at some point in their life cycle. Both are believed to be present in the last eukaryotic common ancestor. Prokaryotes (bacteria and archaea) usually undergo a vegetative cell division known as binary fission, where their genetic material is segregated equally into two daughter cells, but there are alternative manners of division, such as budding, that have been observed. All cell divisions, regardless of organism, are preceded by a single round of DNA replication. For simple unicellular microorganisms such as the amoeba, one cell division is equivalent to reproduction – an entire new organism is created. On a larger scale, mitotic cell division can create progeny from multicellular organisms, such as plants that grow from cuttings. Mitotic cell division enables sexually reproducing organisms to develop from the one-celled zygote, which itself is produced by fusion of two gametes, each having been produced by meiotic cell division. After growth from the zygote to the adult, cell division by mitosis allows for continual construction and repair of the organism. The human body experiences about 10 quadrillion cell divisions in a lifetime. The primary concern of cell division is the maintenance of the original cell's genome. Before division can occur, the genomic information that is stored in chromosomes must be replicated, and the duplicated genome must be cleanly divided between progeny cells. A great deal of cellular infrastructure is involved in ensuring consistency of genomic information among generations.
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →Joe Louis.
