A chromosome is an organized structure of DNA and protein that is found in cells, with each chromosome being a very long, continuous, single piece of double-stranded DNA (a single DNA molecule) containing many genes, regulatory elements and other nucleotide sequences. The DNA, which carries a cell's genetic information, is normally packaged in the form of one or more of these large macromolecules called chromosomes. The word chromosome comes from the Greek χρώμα (color) and σώμα (body).
In the chromosomes of eukaryotes, the uncondensed DNA exists in a quasi-ordered structure inside the nucleus, where it wraps around histones (structural proteins, Fig. 1). This composite material (the complex of DNA and protein) is called chromatin. During mitosis (cell division), chromatin is condensed into chromosomes. This is the only natural context in which individual chromosomes are visible with an optical microscope.
Prokaryotes do not possess histones or nuclei.
The gain or loss of chromosome material can result in various inherited genetic disorders. In some cases, a failure of personal or societal responsibility can be a factor. For example, exposure to harmful chemicals or radiation, perhaps as a result of warfare or environmental pollution, can cause genetic damage in the germ cells of a parent and result in offspring with the genetic disorder. Illicit drug use, or infection with a pathogen through promiscuous sex can also lead to genetic damage. Even a prescribed drug, thalidomide, was discovered to correlate with birth defects when used during pregnancy.
Each chromosome has two arms, the shorter one called p arm (from the French petit, small) and the longer one q arm (q following p in the Latin alphabet). In its relaxed state, the DNA can be accessed for transcription, regulation, and replication.
Two types of chromatin can be distinguished:
In the very early stages of mitosis, the chromatin strands become more and more condensed. They cease to function as accessible genetic material and become a compact transport form. Eventually, the two matching chromatids become visible as a chromosome. (A chromatid is one-half of a replicated chromosome, being considered as a chromatid when attached at the centromere and prior to separation and becoming a daughter chromosome.)
A spindle composed of microtubules is formed. Microtubules are self-assembled from dimers of alpha and beta tubulin (a globular protein), and attach to chromosomes at specialized structures called the kinetochores, one of which is present on each sister chromatid. Sister chromatids are attached at an area called the centromere (not necessarily at the center of the chromosome). A special DNA base sequence in the region of the kinetochores provides, along with special proteins, longer-lasting attachment in this region.
During mitosis, the microtubules pull the chromatids apart, so that each daughter cell inherits one set of chromatids. Once the cells have divided, the chromatids are uncoiled and can function again as chromatin. In spite of their appearance, chromosomes are highly structured, which enables these giant DNA structures to be contained within a cell nucleus (Fig. 2).
Chromosomes were first observed in plant cells by Swiss botanist Karl Wilhelm von Nägeli (1817-1891) in 1842, and independently, in Ascaris worms, by the Belgian scientist Edouard Van Beneden (1846-1910). The use of basophilic aniline dyes was a fundamentally new technique for effectively staining the chromatin material inside the nucleus. Their behavior in animal (salamander) cells was later described in detail by German anatomist Walther Flemming (1843-1905), the discoverer of mitosis, in 1882. The name was invented later by another German anatomist, Heinrich von Waldeyer.
Bacterial chromosomes are usually circular, but are sometimes linear. Some bacteria have one chromosome, while others have a few. Bacterial DNA also exists as plasmids, which are circular pieces of DNA that can be transmitted between bacteria. Antibiotic resistance genes are often carried on plasmids and can thus spread between different bacteria. The distinction between plasmids and chromosomes is poorly defined, though size and necessity are generally taken into the account. Bacterial chromosomes have only one origin of replication.
When linear, bacterial chromosomes tend to be tethered to the plasma membrane of the bacteria. In molecular biology application, this allows for its isolation from plasmid DNA by centrifugation of lysed bacteria and pelleting of membranes (and the attached DNA).
Eukaryotes (cells with nuclei such as plants, yeast, and animals) possess multiple linear chromosomes contained in the cell's nucleus. Each chromosome has one centromere, with one or two arms projecting from the centromere.
|Fruit fly||8||Guinea Pig||16|
To determine the number of chromosomes of an organism (or number of homologous pairs), cells can be locked in metaphase in vitro (in a reaction vial) with colchicine. These cells are then stained (the name chromosome was given because of their ability to be stained), photographed, and arranged into a karyotype (an ordered set of chromosomes, Fig. 3), also called karyogram.
Gametes, reproductive cells, are haploid [n] and have one set of chromosomes. Sexually reproducing species have somatic cells, body cells, which are diploid (2n), having two sets of chromosomes, one from the mother and one from the father. Gametes are produced by meiosis of a diploid germ line cell. During meiosis, the matching chromosomes of father and mother can exchange small parts of themselves (crossover or recombination), and thus create new chromosomes that are not inherited solely from either parent. When a male and a female gamete merge (fertilization), a new diploid organism is formed.
Some animal and plant species are polyploid (Xn) and have more than two sets of chromosomes. Agriculturally important plants such as tobacco or wheat are often polyploid compared to their ancestral species. Wheat has a haploid number of seven chromosomes, still seen in some cultivarsm as well as the wild progenitors. The more common pasta and bread wheats are polyploid having 28 (tetraploid) and 42 (hexaploid) chromosomes compared to the 14 (diploid) chromosomes in the wild wheat. (Sakamur 1918).
In 1921, Theophilus Painter claimed, based on his observations, that human sex cells had 24 pairs of chromosomes, giving humans 48 chromosomes total. It wasn't until 1955 that the number of pairs was clearly shown to be 23.
Like many sexually reproducing species, humans have special gonosomes (sex chromosomes, in contrast to autosomes). These are XX in females and XY in males. In females, one of the two X chromosomes is inactive and can be seen under a microscope as Barr bodies.
|X (sex chromosome)||1184||152,634,166||147,686,664|
|Y (sex chromosome)||231||50,961,097||22,761,097|
Some chromosome abnormalities, such as translocations, or chromosomal inversions, do not cause disease in carriers, although they may lead to a higher chance of having a child with a chromosome disorder. Abnormal numbers of chromosomes or chromosome sets (aneuploidy) may be lethal or give rise to genetic disorders. Genetic counseling is offered for families that may carry a chromosome rearrangement.
The gain or loss of chromosome material can lead to a variety of genetic disorders. Examples include:
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