|
Synizesis (biology)
Synizesis refers to a phenomenon sometimes observed in one of the subphases of meiosis. This phenomenon, sometimes referred to as a "synizetic knot", and contrasted with the chromosome "bouquet" more typically observed, is characterized by the localization of the meiotic chromosomes in a tight clump on one side of the nucleus. The term synizesis seems to have been coined by Clarence Erwin McClung Clarence Erwin McClung (April 5, 1870 – January 17, 1946) was an eminent American zoologist and prairie pioneer cytologist who discovered the role of chromosomes in sex-determination. Graduating pharmacy at the University of Kansas in 1892, a ... in 1905. The synizetic knot (Synizesis) was later found to be a technical artifact induced by the feature of strong acidic fixatives used during that time (e.g., Flemming's strong fixative) to precipitate the thread-like delicate chromosomes of the Leptotene stage of first meiotic prophase into a dark staining knot. References Cellular ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Meiosis
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. 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. ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Chromosomes
A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most important of these proteins are the histones. Aided by chaperone proteins, the histones bind to and condense the DNA molecule to maintain its integrity. These eukaryotic chromosomes display a complex three-dimensional structure that has a significant role in transcriptional regulation. Normally, chromosomes are visible under a light microscope only during the metaphase of cell division, where all chromosomes are aligned in the center of the cell in their condensed form. Before this stage occurs, each chromosome is duplicated ( S phase), and the two copies are joined by a centromere—resulting in either an X-shaped structure if the centromere is located equatorially, or a two-armed structure if the centromere is located distally; the joined ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Clarence Erwin McClung
Clarence Erwin McClung (April 5, 1870 – January 17, 1946) was an eminent American zoologist and prairie pioneer cytologist who discovered the role of chromosomes in sex-determination. Graduating pharmacy at the University of Kansas in 1892, after a year of teaching he entered the college as a graduate student (A.B.,1896, A.M.,1898, and Ph.D.,1902.) He studied one summer with W.M. Wheeler at the University of Chicago and at his suggestion, studied the spermatogenesis of Xiphidium fasciatum, a long-horned grasshopper; launching his scientific career in the research of chromosomes. McClung took advantage of the great abundance of grasshoppers in Kansas ''"to make them pivotal for cytological study and to found a school of grasshopper cytologists''." Later, the use of Brachystola magna ("Lubber" grasshopper) for cytological studies would spread to laboratories throughout the world, as germ-cells in the testes were very large and easily studied. Development of the “McClung Mode ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Walther Flemming
Walther Flemming (21 April 1843 – 4 August 1905) was a German biologist and a founder of cytogenetics. He was born in Sachsenberg (now part of Schwerin) as the fifth child and only son of the psychiatrist Carl Friedrich Flemming (1799–1880) and his second wife, Auguste Winter. He graduated from the ''Gymnasium der Residenzstadt'', where one of his colleagues and lifelong friends was writer Heinrich Seidel. Career Flemming trained in medicine at the University of Prague, graduating in 1868. Afterwards, he served in 1870–71 as a military physician in the Franco-Prussian War. From 1873 to 1876 he worked as a teacher at the University of Prague. In 1876 he accepted a post as a professor of anatomy at the University of Kiel. He became the director of the Anatomical Institute and stayed there until his death. With the use of aniline dyes he was able to find a structure which strongly absorbed basophilic dyes, which he named chromatin. He identified that chromatin was corr ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Leptotene Stage
The leptotene stage, also known as leptonema, is the first of five substages of prophase I during meiosis, the specialized cell division that reduces the chromosome number by half to produce haploid gametes in sexually reproducing organisms. Terminology 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. Chromosome condensation During the leptotene stage, the duplicated chromosomes - each consisting of two sister chromatids - condense from diffuse chromatin into long, thin strands that are more visible within the nucleoplasm (nucleus contents). The chromosomes become visible as thin threadlike structures known as leptonema under a light microscope. Each chromosome consists of two identical sister chromatids held together by cohesin proteins along the entire length, connected at the centromere region. As the chromosomes condense, the nuclear ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
