Chromosome

A chromosome (in Greek chroma = colour and soma = body) is, minimally, a very long, continuous piece of DNA, which contains many genes, regulatory elements and other intervening nucleotide sequences. 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), and where this composite material is called chromatin. During mitosis (cell division), the chromosomes are condensed and called metaphasic chromosomes. This is the only natural context in which individual chromosomes are visible with an optical microscope. Prokaryotes do not possess histones or nuclei. In its relaxed state, the DNA can be accessed for transcription, regulation, and replication. Chromosomes were first observed by Karl Wilhelm von N䧥li in 1842 and their behavior later described in detail by Walther Flemming in 1882. In 1910, Thomas Hunt Morgan proved that chromosomes are the carriers of genes.

Contents

1 Chromosomes in eukaryotes 2 Chromosomes in bacteria 3 Chromatin 4 Chromosomes in different species 5 Karyotype 6 Human chromosome 7 Chromosomal aberrations

Chromosomes in eukaryotes

Eukaryotes possess multiple linear chromosomes contained in the cell's nucleus. Each chromosome has one centromere, with one or two arms projecting from the centromere. The ends of the chromosomes are special structures called telomeres. DNA replication begins at many different locations on the chromosome.

Chromosomes in bacteria

Bacterial chromosomes are often circular but sometimes linear. Some bacteria have one chromosome, while others have a few. Bacterial DNA also exists as plasmids. The distinction between plasmids and chromosomes is poorly defined, though size and necessity are generally taken into account. Bacterial chromosomes initiate replication and one origin of replication.

Chromatin

Two types of chromatin can be distinguished:

• Euchromatin, which consists of DNA that is active, e.g., expressed as protein.
• Heterochromatin, which consists of mostly inactive DNA. It seems to serve structural purposes during the chromosomal stages. Heterochromatin can be further distinguished into two types:
  • Constitutive heterochromatin, which is never expressed. It is located around the centromere and usually contains repetitive sequences.
  • Facultative heterochromatin, which is sometimes expressed.

In the 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 (condensed chromatin strands) become visible as a chromosome, linked at the centromere. Long microtubules are attached at the centromere and two opposite ends of the cell. 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 (Fig. 2). For example, genes with similar functions are often kept close together in the nucleus, even if they are far apart on the chromosome. The short arm of a chromosome can be extended by a satellite chromosome that contains codes for ribosomal RNA.

Chromosomes in different species

</table>
Normal members of a particular species all have the same number of chromosomes (Table 1). Asexually reproducing species have one set of chromosomes, which is the same in all body cells. Sexually reproducing species have somatic cells (body cells), which are diploid [2n] (they have two sets of chromosomes, one from the mother, one from the father) or polyploid [Xn] (more than two sets of chromosomes), and gametes (reproductive cells) which are haploid [n] (they have only one set of chromosomes). 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), 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.

Karyotype

To determine the (diploid) number of chromosomes of an organism, 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. Like many sexually reproducing species, humans have special gonosomes (sex chromosomes, in contrast to autosomes for body functions). 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.

Human chromosome

Table 1: Examples of chromosome numbers (diploid).

Species	# of chromosomes		<p align="left">Species	# of chromosomes
Fruit fly	8		Human	46
Rye	14		Ape	48
Guinea Pig	16		Sheep	54
Dove	16		Horse	64
edible Snail	24		Chicken	78
Earthworm	36		Carp	104
Pig	40		Butterflies	~380
Wheat	42		Fern	~1200
Mouse	40		Rat	42
Hare	46		Rabbit	44
Dog	78		Cat	38
Cow	60		Syrian hamster	44

Chromosome	Genes	Bases	Determined bases*
1	2968	245,203,898	218,712,898
2	2288	243,315,028	237,043,673
3	2032	199,411,731	193,607,218
4	1297	191,610,523	186,580,523
5	1643	180,967,295	177,524,972
6	1963	170,740,541	166,880,540
7	1443	158,431,299	154,546,299
8	1127	145,908,738	141,694,337
9	1299	134,505,819	166,880,540
10	1440	135,480,874	115,187,714
11	2093	134,978,784	130,709,420
12	1652	133,464,434	129,328,332
13	748	114,151,656	95,511,656
14	1098	105,311,216	87,191,216
15	1122	100,114,055	81,117,055
16	1098	89,995,999	79,890,791
17	1576	81,691,216	77,480,855
18	766	77,753,510	74,534,531
19	1454	63,790,860	55,780,860
20	927	63,644,868	59,424,990
21	303	46,976,537	33,924,742
22	288	49,476,972	34,352,051
X (sex chromosome)	1184	152,634,166	147,686,664
Y (sex chromosome)	231	50,961,097	22,761,097
unplaced various	?	25,263,157	25,062,835

* Human Genome Project goals called for determination of only the euchromatic portion of the genome. Telomeres, centromeres, and other heterochromatic regions have been left undetermined, as have a small number of unclonable gaps. [1] (http://www.ncbi.nlm.nih.gov/genome/seq/)

Chromosomal aberrations

Some chromosome abnormalities do not cause disease in carriers, such as translocations, or chromosomal inversions, although it may lead to a higher chance of having a child with an 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:

• Cri du chat syndrome, which is caused by the deletion of part of the short arm of chromosome 5. Victims make high-pitched cries that sound like a cat. They have wide-set eyes, a small head and jaw and are mentally retarded.
• Wolf-Hirschhorn syndrome, which is caused by partial deletion of the short arm of chromosome 4. It is characterized by severe growth retardation and mental defect.
• Down syndrome (extra chromosome 21). This is also known as mongolism or trisomy 21. Symptoms are decreased muscle tone, asymmetrical skull, slanting eyes and mental retardation.
• Edward's syndrome is the second most common trisomy after Down's Syndrome. It is a trisomy of chromosome 18. Symptoms include mental and motor retardation.
• Patau Syndrome, also called D-Syndrome or trisomy-13. Symptoms somewhat similar to those of trisomy-18.
• Jacobsen syndrome, also called the terminal 11q deletion disorder. A very rare disorder. More information at http://www.11q.org.
• Klinefelter's syndrome (XXY). Men with Klinefelter syndrome are usually sterile. They tend to have longer arms and legs and tend to be taller than their peers. Other common symptoms are fatigue, apathy, lack of emotion, and an increased tendency to develop psychiatric disorders.
• Turner syndrome (X instead of XX or XY). In Turner syndrome, female sexual characteristics are present but underdeveloped. People with Turner syndrome often have a short stature, low hairline, abnormal eye features and bone development and a "caved-in" appearance to the chest.
• XYY syndrome
• Triple-X syndrome
• small supernumerary marker chromosome