Chromosome

From Greek: chromos - color; soma - body
German: Chromosom

1 Definition

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.

The chromosomes were discovered in 1842 by Karl von Naegeli. In 1910, Thomas Hunt Morgan showed that the chromosomes are the carriers of 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.

2 The compact shape of the 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 doesn't take place, the chromosomes exist in eukaryotes as longer DNA strands in the cell nucleus in an "unwound" (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.

3 Chromosomes in eukaryotes

1. DNA double helix
2. Chromatin strand (DNA with histons)
3. Condensed chromatin during the interphase with centromere
4. Condensed chromatin during the prophase (consists of two chromatids)
5. Metaphase chromosome (two-chromatid chromosome)

4 Chromatin types

There are two types of chromatin:

• Euchromatin is the type that contains active DNA, i.e. it can be expressed in protein. The euchromatic section of the chromosome does not show any difference in its structure regardless of the degree of condensation of the chromosome.
• Heterochromatin mainly consists of inactive DNA. It apparently performs structural functions in the various degrees of condensation. The heterochromatic section of the chromosome shows the same degree of condensation in the interphase as well as in the metaphase. Heterochromatin can be subdivided into two subtypes:
  • Constitutive heterochromatin, which is not expressed. It is found in the area of the centromere and generally consists of repetitive DNA sequences.
  • Facultative heterochromatin, which is sometimes expressed.

5 Formation and transport of chromosomes during nuclear division (mitosis)

• Prophase: In the early stages of the mitosis, the chromatin strands condense increasingly. Thus they develop from an available source of genetic information into a no more readable compact form of transport.

In the course of this process, the two chromatids (condensed chromatin strands) that belong together by the connecting centromere form a two-chromatid chromosome.

• Metaphase: The chromosomes align along the equator of a cell.
• Anaphase: A spindle apparatus provides for the separation of the chromatids of every metaphase chromosome and its transport to the opposite poles of the cell. For this purpose, long microtubules fastens on to both the kinetochores of the centromere and the opposite ends (poles) of the cell. During mitosis, the microtubules pull apart both chromatids, so that each daughter cell contains one chromatid, i.e. a one-chromatid chromosome.
• Telophase: As soon as the cell is divided, the daughter chromatids decondense to become chromatin strands and are then again available as carriers of genetic information, and which double again later to become two-chromatid chromosomes.

6 Structure

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.

7 Chromosome number

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.

8 Karyotype

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).

9 Chromosomes in humans

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.

10 Genomic and chromosomal mutations due to defective nuclear division

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.

• Chromosomal aberrations or partial chromosomal dysplasia occurs normally as the result of a defective crossover.
  • Cri du chat syndrome is provoked by the deletion of the short arm of chromosome 5. The affected emit high-frequency sounds that reminds one of a cat's. They have widely set apart eyes, small head and jaws and are mentally retarded.
  • Wolf-Hirschhorn syndrome is provoked by the partial deletion of the short arm of chromosome 4. The affected suffer from severe mental and growth disorders.

• Missing or additional chromosomes as the result of an incomplete chromosomal segregation
  • Down syndrome or trisomy 21 (additional chromosome 21) was formerly also known as mongolism. Symptoms are retarded muscle development, asymmetric skull and mental retardation.
  • Klinefelter syndrome (XXY). Men with this syndrome are normally sterile, very large, have abnormally long arms and legs, a tendency to form breasts and reduced body hairs. Other symptoms are lack of emotions, fatigue, apathy and an increased tendency toward psychiatric disorders. A diminished intelligence, as often claimed, doesn't appear to be true.
  • Turner syndrome (X0). Women with this syndrome have underdeveloped female sexual characteristics. They have a small stature, deep hair line, an abnormal development of eyes and bones, and a funnel breast. The intelligence is pronounced like in XX women.

Note: An illustration of all human chromosomes and the diseases associated with them can be found here.

See also: Sex chromosomes, chromosome mutation, genome, and genomic mutation

11 Links

• Links to Research Ressources for each human chromosome
• National Human Genome Research Institute
• Chromosome Helpstation: For people with rare chromosome disorders

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