From Greek: chromos - color; soma - body
A chromosome is a long, continuous strand of desoxyribonucleic acid (DNA), which wraps around a multitude of histons (nuclear proteins) as double helix and becomes visible during the nuclear division of a eukaryotic microorganism cell as a compact form with multiple spirals together with other chromosomes, and contains many genes.
In both haploid and diploid organisms, at the beginning of a nuclear division (Mitosis), every chromosome comprises two chromatids that are linked to each other (two-chromatid chromosome) at the centromere. After a successful nuclear division, the chromosome consists of only one chromatid (single chromatid) and exists as a one-chromatid chromosome, and doubles again after some time to become a two-chromatid chromosome.
During mitosis (nuclear division) the chromatin strands shorten themselves to become the so-called metaphase chromosomes (two-chromatid chromosomes). The thread-like material of the DNA linked to the histons is wound up several times, whereupon the compact shape of the chromosome occurs. The chromosomes are visible under a microscope only in this spiral state. If the nuclear division doesnâ€™t take place, the chromosomes exist in eukaryotes as longer DNA strands in the cell nucleus in an â€œunwoundâ€? (uncoiled) state. The DNA is repeatedly wound in greater distances around packages of 8 histons (structural proteins), so that with many histons it resembles a pearl necklace. In this state, the chromosomes are termed chromatins. The DNA is capable of transcription, regulation and duplication (replication) only in this unwound, non-spiral state.
Prokaryotes do not have any histons or cell nuclei; the major part of their genetic material is present in the form of an individual ring-shaped closed DNA strand, which lies relatively without any order in the plasma of the bacteria cell. Sometimes it is also called "bacterial chromosome," even though it does not have much in common with eukaryotic chromosomes, so that the term is not generally recommendable.
There are two types of chromatin:
Heterochromatin can be subdivided into two subtypes:
Chromosomes are highly structured. The genes with similar functions lie often side by side, but not in linear chromatin strands. The chromosomes possess a primary constricted region, the centromere. The chromosome is divided into two mostly different long arms (branches) by this. The short arm of a chromosome can be extended by a satellite in satellite chromosomes (SAT-chromosomes). The DNA section in this area encodes for the ribosomal RNA. The ends of the chromosomes are termed telomeres. In every cell division, the DNA gets shorter and shorter at the teleomeres. Therefore, they play an important role in the aging process.
The number of chromosomes are the same in most cases within a species (table 1). Asexually reproducing organisms have one chromosome set, which is equal in all body cells. Sexually reproducing animal and plant types have often somatic cells (normal body cells) with a diploid (double) chromosome set [2n] (i.e. each chromosome from both parents) and germ cells (gametes), which are haploid and contain only one chromosome of each chromosome type. However in many species polyploidchromosome sets [xn] can occur. When two suitable (haploid) gametes fuse (fertilization) together, there develops a cell with a diploid chromosome set, the zygote. This can develop into a diploid organism through a mitotic cell division. There are also cases in which the zygote, often after a more or less long respite, undergoes a meiosis (reduction division) without undergoing a previous mitotic division. The haploid oranisms develop from the four haploid cells that emerge from the meiosis. Apart from this, there is also the possibility that organisms of the same type in their lifetime change to both haploid as well as diploid alternately. This occurs in mosses and ferns. (Full particulars on this can be found under the subject index alternation of generations.) A meiosis occurs later in diploid organisms, if the germ cells develop in the gonads of the sexually mature animal. During such a meiosis, the chromatids can replace opposite homologous two-chromatid chromosome parts (crossing-over or crossover). Thereby, genetically new composite chromosomes develop, which differ from those of the parent organisms.
In order to determine the number of the (diploid) chromosomes of an organism, they are arrested in vitro with colchicine (spindle poison) during the metaphase, so that they are not drawn to the cell poles. The cells can then be stained (the term chromosome is derived from their color), photographed and arranged in a karyotype (also termed karyogram and idiogram).
Like many other sexually reproducing species, the humans also have gonosomes (special sex chromosomes in contrast to autosomes for the body functions). These are XX in women and XY in men. In women, one of the X chromosomes is inactivated and appears under the microscope as Barr bodies or sex chromatins.
A defect in the chromosomal division or during crossing-over can lead to severe diseases. These can be classified into two groups, which are respectively assigned with some examples.
Note: An illustration of all human chromosomes and the diseases associated with them can be found here.
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