A chromosome (of the Greek chroma , color and summoned , body, element) is a microscopic element made up of Molécule S of DNA. In the cell S eucaryotes, the chromosomes are in the core where they take the shape either of a stick, or of a hank, according to whether they are condensed or not. In the procaryotic cell S , the chromosomes are in the Cytoplasme, in an area called nucleoid. (see bacterial Structure).
The details given here correspond to the chromosomes of organizations eucaryotes. There exist several distinctions to be made compared to the bacterial chromosomes (Procaryote).
With appeared, the word chromosome is used to describe its contents in genetic term of Information. The chromosomes carry the Gène S, carriers data genetic transmitted of the cells mothers to the cells girls at the time of the Mitose or the Méiose (sexuée reproduction).
The chromosomes are usually represented per pairs, in parallel with their homologous. They are often illustrated in their duplicated form condensed and (in Métaphase of the Mitose).
The chromosomes are of variable number following each species. The mankind counts 46 of them: 23 pairs, of which 22 are homologous chromosomes ( autosomes ), the last pair corresponding to the two sexual chromosomes (Gonosome S). The human Chromosomes are numbered of 1 with 22, longest with shortest, and the two sexual chromosomes are named X and Y. This rule is not the same one for all the organizations.
The whole of the chromosomes is represented on a Caryotype, or chart of chromosomes.
After their Réplication during the Interphase of the cellular cycle, the chromosomes are composed of two identical copies of Chromatide S, attached enters on the level of a Centromère. Each Chromatide is made of a molecule of DNA (nucléofilament) associated with proteins Histone S - assemblies in Nucléosome S - and proteins not histones. In optical microscopy, one distinguishes on the chromosomes from the condensed areas, formed of Hétérochromatine, and the décondensées areas, formed of Euchromatine. The expressed genes are located mainly on the level of the euchromatine. Let us note that the télomères and the centromère are repetitive sequences, and that the centromère do not code genetic information, it is about DNA satellites not coding.
The Mitose transmits the totality of the chromosomes to the cells girls, while the Méiose transmits only half of genetic inheritance to the cells girls, which allows the increase in the diversity of genetic inheritance by the genetic phenomenon of Recombinaison.
Chromosomes of the procaryotes
The Bacterium S and the Archaea have a single circular chromosome in general. However, at some species, like the bacterium Borrelia burgdorferi (reponsable of the disease of Lyme), the chromosome is linear. There exist also some bacteria which have two chromosomes. The DNA of the bacteria also exists in the form of Plasmide S. They are genetic elements nonchromosomal. The distinction between plasmides and chromosomes is vague, although their size and their importance are generally taken into account. The bacterial chromosomes initiate the replication and thus allow the manufacture of drugs.
Chromosomes at the eucaryotes
The eucaryotes have multiple linear chromosomes contained in the cellular core. Each chromosome has its clean Centromère, with one or two arms being projected from this one. At the time of the mitosis and meiosis, the centromère allows the assembly of the Kinétochore which binds the chromosomes to the Microtubule S, thus allowing their displacements and their distribution between the two cells girls. The end of the chromosomes are special structures called Télomère S . These extremities shorten with each replication because the DNA polymerase needs a starter to begin the replication. An enzyme, the télomérase, makes it possible in certain cases to restore the length of the télomères. The replication of the DNA starts at various places of the chromosome.
Description of the chromosomes of the cells eucaryotes
The chromosomes eucaryotes can be distinguished according to the position from their centromère: One speaks about chromosome metacentric when it has a centromère in central position (median position) what him gives arms (or chromatides) length about equal. A chromosome submetacentric is a chromosome whose centromère is almost in central position; the chromatides of this chromosome present arms unequal length (a small arm named “p” and a long arm “L”). If the centromère is closer to the one of both télomères (or ends chromosomal), the chromosome is known as acrocentric . Lastly, a telocentric chromosome presents a centromère is very close to the one of its ends (télomères). In the event of loss of the centromère (anomaly), the chromosome is known as acentric . Other anomalies can cause the appearance of a chromosome having two centromères named dicentric chromosome . This one is unstable and can break (at the time of the meiosis) in various segments which are distributed randomly in each cell girls.
Chromosomes of the various species
Species all cuts the same number off chromosomes (Table 1). Asexually reproducing species cuts one set off chromosomes, which is the same in all body concealments. Sexually reproducing species cuts Somatic concealment S (body concealments), which are Diploid (they cuts two sets off chromosomes, one from the mother, one from the father) gold Polyploid (more than two sets off chromosomes), and Gamete S (reproductive concealments) which are Haploid (they cuts only one set off chromosomes). Gametes are produced by Meiosis off has diploid Germ line concealment. During meiosis, the matching chromosomes off father and mother edge exchange small shares off themselves (Crossover), and thus create new chromosomes that are not inherited solely from either relative. When has male and has female gamete merge (Fertilization), has new diploid organism is formed. -->
Chromosomes at the plants
The plants have they-also a very variable number of chromosomes. They can be Haploïde S, Diploïde S or Polyploïde S. At the Phanérogame S, the plant having the greatest number of chromosomes is the black Mûrier with 2n=308 chromosomes. The Ophioglossum count they 1260. A model plant, Arabidopsis thaliana, is often used by the researchers for better including/understanding the importance of the exchanges of chromosomes during the Méiose.
Chromatin
See also: Chromatin
Two types of Chromatine can be distinguished:
- the Euchromatine, which consists of active DNA, for example expressed out of protein.
- the Hétérochromatine, which consists of mainly inactive DNA. It seems to be used for structural ends during the chromosomal phases. The hétérochromatine can in its turn be subdivided in two types:
- the constitutive hétérochromatine , which is never expressed. It is located around the centromère and consists in general of repetitive sequences.
- the optional hétérochromatine , which is sometimes expressed.
Centromere . Length Microtubule S are attached At the centromere and two opposite ends off the concealment. During mitosis, the microtubules sweater the chromatids apart, so that each daughter concealment inherits one set off chromatids. Ounce the concealments cuts divided, the chromatids are uncoiled and edge function again ace chromatin. In off splashes to their appearance, chromosomes are highly structured (fig. 2). For example, constrained with similar functions are often kept closed together in the nucleus, even yew they are far apart one the chromosome. The shorts ARM off has chromosome edge Be extended by has Satellite ribosomal chromosome that contains codes for RNA. -->
Chromosomic chart
To determine the number (diploïde) of chomosomes of an organization, cells can be blocked in Métaphase In vitro (in a Tube tests) with Colchicine. These cells are then dyed (the name " chromosome" refers to the fact that one can color them), photographed and arranged in a Caryotype (an ordered whole of chromosomes) also named " caryogramme". As much of species with reproduction sexuée, the human ones have Gonosome S (sexual chromosomes, in opposition to the Autosome S for the body functions). It are XX in the females and XY in the males. In the females, one of two X chromosomes is inactive and can be seen under the microscope like a Corpuscule of Barr.
Human chromosome
Deteriorations of the chromosomes
Deteriorations of the chromosomes are:- the chromosomal breaks, like the Délétion or the inversion ;
- the exchanges of fragments enter chromosomes: Translocation and Insertion ;
- the Duplication .
Examples
- Syndrome of the cry of the cat, due to a Délétion of part of the arm court of chromosome 5.
- Syndrome of Wolf-Hirschhorn, due to a délétion partial of the arm court of chromosome 4.
- Syndrome of Down or Trisomy 21.
- Syndrome of Edward, trisomy of chromosome 18: the most frequent trisomy after the syndrome of Down.
- Syndrome of Patau, or syndrome-D or trisomy 13.
- Syndrome of Jacobsen, or disorder of final délétion 11q (very rare). More details on the http://www.11q.org site.
- Syndrome of Klinefelter (XXY).
- Syndrome of Turner (X instead of XX or XY).
- Syndrome Y
- Syndrome triples X
Complete listing
Detailed graphic posting of all the human chromosomes and the diseases annotated at the good place (English '' '').
External bonds
- shown chromosomes
- Glossary of the biotechnology of FAO
Sources
Simple: Chromosome
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