Biology terminology

Centromere

(1) The constricted region joining the two sister chromatids that make up an X-shapedchromosome.

(2) The site where kinetochore is formed.

Supplement

Centromere is important particularly during mitosis. Aside from being the region where chromatids are held and kinetochore is formed, it also serves as the point of attachment for spindle fibers when the spindle fibers are pulling the chromosomes toward thecentrioles (situated on opposite poles in a cell) prior to cytokinesis.

When the centromere is not functioning properly, the chromatids do not align and separate properly, thus, resulting in the wrong number ofchromosomes in the daughter cells, and conditions such as Down syndrome.

Centrosome

Definition

noun, plural: centrosomes

(cell biology) The organelle located near the nucleus in the cytoplasm that divides and migrates to opposite poles of the cell during mitosis, and is involved in the formation ofmitotic spindle, assembly of microtubules, and regulation of cell cycle progression; the region pertaining to the organelle.

Supplement

It was discovered in 1888 by Theodor Boyeri describing it as a special organ of cell division.

Centrosomes in animals are the main microtubule organizing center (MTOC), and contain two orthogonally arranged centrioles surrounded by anamorphous mass of pericentriolar material.

Chromatids

Definition

noun, singular: chromatid

The two strands joined together by a single centromere, formed from the duplication of thechromosome during the early stages of cell division and then separate to becomeindividual chromosomes during the late stages of cell division.

Supplement

The term chromatid was proposed by Clarence Erwin McClung (1900) for each of the four threads making up a chromosome-pair during meiosis. It was later used also for mitosis.

Chromatids may be sister or non-sister chromatids.

When chromatids separate and move toward opposite poles of the cell, they are now referred to as daughter chromosomes.

Chromosome

Definition

noun, plural: chromosomes

A structure within the cell that bears the genetic material as a threadlike linear strand ofDNA bonded to various proteins in the nucleus of eukaryotic cells, or as a circular strandof DNA (or RNA in some viruses) in the cytoplasm of prokaryotes and in the mitochondrionand chloroplast of certain eukaryotes.

Supplement

In eukaryotes, the chromosomes appear as threadlike strand that condense into thickerstructures and aligns on the metaphase plate during mitosis. Humans normally have 23 pairs of chromosomes, each with a characteristic length and banding pattern. Chromosomes occur in pairs (in most somatic cells) since one member of each pair comes from the mother and the other from the father. In mostprokaryotes, the chromosome is usually a circular strand of DNA; hence, the entire genome is carried on only one chromosome. In viruses, the chromosome may appear as short linear or circular structure containing the DNA or RNA molecule often lacking any structural proteins.

tetrad  (ttrd)

1. A four-part structure that forms during prophase I of meiosis and consists of two homologous chromosomes, each composed of two identical chromatids. During prophase I of meiosis, one chromosome exchanges corresponding segments of genetic material with the other chromosome in the tetrad in the process called crossing over. See more at meiosis.

2. A group of four cells, as of spores or pollen grains, formed from a parent cell by meiosis. As part of the process of spermatogenesis, a spermatocyte divides into a tetrad of four spermatids, cells which go on to develop into sperm. See more at spermatogenesis.

chro·ma·tin  (krm-tn)

n.

A complex of nucleic acids and proteins, primarily histones, in the cell nucleus that stains readily with basic dyes and condenses to form chromosomes during cell division.

mi·cro·tu·bule  (mkr-tbyl, -ty-)

n.

Any of the proteinaceous cylindrical hollow structures that are distributed throughout the cytoplasm of eukaryotic cells, providing structural support and assisting in cellular locomotion and transport.

Meiosis (pronounced /maɪˈoʊsɨs/ ( listen)) is a special type of cell division necessary for sexual reproduction. In animals, meiosis produces gametes likesperm and egg cells, while in other organisms like fungi it generates spores. Meiosis begins with one diploid cell containing two copies of each chromosome—one from the organism's mother and one from its father—and produces four haploid cells containing one copy of each chromosome. Each of the resulting chromosomes in the gamete cells is a unique mixture of maternal and paternal DNA, ensuring that offspring are genetically distinct from either parent. This gives rise to genetic diversity in sexually reproducing populations, which enables them to adapt during the course of evolution.

Before meiosis, the cell's chromosomes are duplicated by a round of DNA replication. This leaves the maternal and paternal versions of each chromosome, called homologs, composed of two exact copies called sister chromatids and attached at the centromere region. In the beginning of meiosis, the maternal and paternal homologs pair to each other. Then they typically exchange parts by homologous recombination, leading to crossovers of DNA from the maternal version of the chromosome to the paternal version and vice versa. Spindle fibers bind to the centromeres of each pair of homologs and arrange the pairs at the spindle equator. Then the fibers pull the recombined homologs to opposite poles of the cell. As the chromosomes move away from the center, the cell divides into two daughter cells, each containing a haploid number of chromosomes composed of two chromatids. After the recombined maternal and paternal homologs have separated into the two daughter cells, a second round of cell division occurs. There meiosis ends as the two sister chromatids making up each homolog are separated and move into one of the four resulting gamete cells. Upon fertilization, for example when a sperm enters an egg cell, two gamete cells produced by meiosis fuse. The gamete from the mother and the gamete from the father each contribute half to the set of chromosomes that make up the new offsping's genome.

Meiosis uses many of the same mechanisms as mitosis, a type of cell division used by eukaryotes like plants and animals to split one cell into two identical daughter cells. In all plants, and in many protists, meiosis results in the formation of spores, haploid cells that can divide vegetatively without undergoing fertilization. Some eukaryotes, like Bdelloid rotifers, have lost the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis. Meiosis does not occur in archaea or bacteria, which reproduce via asexual processes such as binary fission.

Miosis (or myosis, from Ancient Greek μύειν, mūein, "to close the eyes") is constriction of thepupil of the eye to less than or equal to two millimeters.^[1] This is a normal response to an increase in light, but can also be associated with certain pathological conditions, microwaveradiation exposure, and certain drugs, especially opioids. Parasympathetic stimulation of the pupillary sphincter muscle decreases the pupillary aperature. This constriction is referred to as miosis.

The opposite, mydriasis, is the dilation of the pupil. Both mydriasis and miosis can be physiological.

Mitosis is the process by which a eukaryotic cell separates thechromosomes in its cell nucleus into two identical sets in two nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei,cytoplasm, organelles and cell membrane into two cells containing roughly equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic (M) phase of the cell cycle—the division of the mother cell into two daughter cells, genetically identical to each other and to their parent cell. This accounts for approximately 10% of the cell cycle.

Mitosis occurs only in eukaryotic cells and the process varies in different species. For example, animals undergo an "open" mitosis, where thenuclear envelope breaks down before the chromosomes separate, while fungi such as Aspergillus nidulans and Saccharomyces cerevisiae(yeast) undergo a "closed" mitosis, where chromosomes divide within an intact cell nucleus.^[1] Prokaryotic cells, which lack a nucleus, divide by a process called binary fission.

The process of mitosis is fast and highly complex. The sequence of events is divided into stages corresponding to the completion of one set of activities and the start of the next. These stages are interphase, prophase, prometaphase, metaphase, anaphase and telophase. During mitosis the pairs of chromatids condense and attach to fibers that pull the sister chromatids to opposite sides of the cell. The cell then divides in cytokinesis, to produce two identical daughter cells.^[2]

Because cytokinesis usually occurs in conjunction with mitosis, "mitosis" is often used interchangeably with "mitotic phase". However, there are many cells where mitosis and cytokinesis occur separately, forming single cells with multiple nuclei. This occurs most notably among the fungi and slime moulds, but is found in various different groups. Even in animals, cytokinesis and mitosis may occur independently, for instance during certain stages of fruit fly embryonic development.^[3] Errors in mitosis can either kill a cell through apoptosis or causemutations that may lead to cancer.