Chromosomes are microscopically visible (when stained) structures in a cell which appears during cell division, called mitosis. Between the cell divisions the chromosomes will be loosened and is visible as a diffuse network in the cell nucleus and the individual chromosomes can no longer be seen. A chromosome contains a long thread of DNA in which all or part of the genome is stored.
Since chromosomes carry a large piece of our genetics, it can be regarded as the most densely packed form of DNA.
Prokaryotes usually only have one chromosome, which often have a circular shape. The structure of this chromosome is simpler than the eukaryotes.
In eukaryotes, there is different types of chromosomes called plasmids, or circular DNA which are threads wrapped around proteins of various types so-called; histones. The precise form of how the DNA is wrapped in a cell varies during different phases. In mitosis the DNA is unwounded and it is at this stage the chromosomes can be distinguished under a microscope. DNA and proteins in its various forms of condensation, is usually called with a common name; chromatin. Each end of a chromosome-structure ends with a so-called telomere, and it is complemented with the so-called centromere, which also connects the two chromatids in the chromosome.
Chromosomes were observed for the first time by the Swiss biologist Karl Wilhelm von Nägeli in 1842. That the chromosomes were carriers of the genes was shown by the American Thomas H. Morgan who received the Nobel Prize in Medicine for this discovery in the year 1933.
Chromosomes in different species
In most species is the total genome divided into several chromosomes. In species with sexual reproduction are the normal somatic cells normally consisting of a double set of chromosomes, one set from each parent so-called; diploid cells. In some species, mainly plants, there are chromosomes in overlapping sets of so-called; polyploid species / cell. Gametes, sperm and egg cells contain only one set of chromosomes so-called; haploid cells.
We humans consist of 23 pair of chromosomes, which means 46 in total. This was discovered in 1956. 22 pairs of the chromosomes in humans are almost identical and are called; autosomes. The last chromosome pair is the sex chromosomes which consist of two X chromosomes in females and one X combined with one Y chromosome in males.
Deviations in the number of chromosomes in humans may lead to disorders, and one of the results of such a deviation is for instance, Down's Syndrome, during which a person have one additional chromosome 21 (also known as Trisomy 21).