Centromere Location and Chromosome Separation

Centromere Location and Chromosome Separation A centromere is a region on a chromosome that joins sister chromatids. Sister chromatids are double-stranded, replicated chromosomes that form during cell division. The primary function of the centromere is to serve as a place of attachment for spindle fibers  during cell division. The spindle apparatus elongates cells and separates chromosomes  to ensure that each new daughter cell has the correct number of chromosomes at the completion of mitosis and meiosis. The DNA in the centromere region of a chromosome is composed of tightly packed chromatin known as heterochromatin. Heterochromatin is very condensed and is therefore not transcribed. Due to its heterochromatin composition, the centromere region stains more darkly with dyes than the other regions of a chromosome. Key Takeaways Centromeres are regions on a chromosome that join sister chromatids whose primary function is for the attachment of spindle fibers in cell division.While centromeres are typically located in the central area of a chromosome, they can also be located near the mid-region or at a number of different positions on the chromosome.Specialized zones on centromeres called kinetochores attach the chromosomes to spindle fibers in prophase in mitosis.Kinetochores have protein complexes that generate kinetochore fibers. These fibers help to orient and separate chromosomes during cell division.In meiosis, in metaphase I, the centromeres of homologous chromosomes are oriented toward opposite cell poles while in meiosis II, spindle fibers extending from both cell poles attach to sister chromatids at their centromeres. Centromere Location A centromere is not always located in the central area of a chromosome. A chromosome is comprised of a short arm region (p arm) and a long arm region (q arm) that are connected by a centromere region. Centromeres may be located near the mid-region of a chromosome or at a number of positions along the chromosome. ​ Metacentric centromeres are located near the chromosome center.Submetacentric centromeres are non-centrally located so that one arm is longer than the other.Acrocentric centromeres are located near the end of a chromosome.Telocentric centromeres are found at the end or telomere region of a chromosome. The position of the centromere is readily observable in a human karyotype of homologous chromosomes. Chromosome 1 is an example of a metacentric centromere, chromosome 5 is an example of a submetacentric centromere, and chromosome 13 is an example of an acrocentric centromere. Chromosome Segregation in Mitosis Prior to the start of mitosis, the cell enters a stage known as interphase where it replicates its DNA in preparation for cell division. Sister chromatids are formed that are joined at their centromeres.In prophase of mitosis, specialized regions on centromeres called kinetochores attach chromosomes to spindle polar fibers. Kinetochores are composed of a number of protein complexes that generate kinetochore fibers, which attach to spindle fibers. These fibers help to manipulate and separate chromosomes during cell division.During metaphase, chromosomes are held at the metaphase plate by the equal forces of the polar fibers pushing on the centromeres.During anaphase, paired centromeres in each distinct chromosome begin to move apart as daughter chromosomes are pulled centromere first toward opposite ends of the cell.During telophase, newly formed nuclei enclose separated daughter chromosomes. After cytokinesis (division of the cytoplasm), two distinct daughter cells are formed. Chromosome Segregation in Meiosis In meiosis, a cell goes through two stages of the dividing process. These stages are meiosis I and meiosis II. During metaphase I, the centromeres of homologous chromosomes are oriented toward opposite cell poles. This means that homologous chromosomes will attach at their centromere regions to spindle fibers extending from only one of the two cell poles.When spindle fibers shorten during anaphase I, homologous chromosomes are pulled toward opposite cell poles but sister chromatids remain together.In meiosis II, spindle fibers extending from both cell poles attach to sister chromatids at their centromeres. Sister chromatids are separated in anaphase II when spindle fibers pull them toward opposite poles. Meiosis results in the division, separation, and distribution of chromosomes among four new daughter cells. Each cell is haploid, containing only half the number of chromosomes as the original cell. Centromere Anomalies Centromeres play an important role by participating in the separation process for chromosomes. Their structure however, can make them possible sites for chromosome rearrangements. Keeping the integrity of centromeres intact is thus an important job for the cell. Centromere anomalies have been linked to various diseases like cancer.

All About Zero

All About Zero All About Zero All About Zero By Mark Nichol Zero is the basis of a small set of terms and idiomatic phrases, which are listed and defined below. Zero derives, through French and Italian, from the Latin term zephirum, which in turn stems, as do the other mathematical terms algebra and algorithm, from Arabic: Sifr means â€Å"cipher† (and is the origin of that word as well). Sifr, in turn, comes from Sanskrit. Absolute zero (quantified as 273.15 degrees below zero Celsius) is the temperature at which matter stops moving, while ground zero is the origin point of a phenomenon. (Originally, it referred to the blast site of a thermonuclear explosion.) Zero hour is the time at which something is scheduled to begin. Zero gravity and zero visibility refer to a near, not absolute, absence of the qualities referred to in the phrases, and patient zero is the first person to contract a disease in an outbreak. The colloquial expression â€Å"From zero to hero† denotes a change in state from anonymity or a lack of distinction or popularity to fame, from the sense of zero as meaning â€Å"an undistinguished or worthless person.† (One can also, unfortunately, transition in the other direction as well.) To zero in is to focus on something or to come closer to it; the expression stems from the idea of adjusting a setting on a device or instrument to zero but originally applied to shooting a firearm. To zero out is unrelated- it means either to reset something, such as timer, or to cut off funding or reduce a quantity. Zero tolerance is the concept of absolute adherence to a rule; the phrase has entered mainstream discourse in references to zero tolerance for weapons or illegal drugs in a given area or jurisdiction. A zero-sum game, meanwhile, is a situation in which a defeated competitor or participant loses as much as the victor wins. (The sum of the gains and losses is zero, hence the name.) Want to improve your English in five minutes a day? Get a subscription and start receiving our writing tips and exercises daily! Keep learning! Browse the Expressions category, check our popular posts, or choose a related post below:How to Format a US Business LetterRules for Capitalization in Titles50 Synonyms for â€Å"Villain†