The Chromosome reference article from the English Wikipedia on 24-Jul-2004
(provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Chromosome

See the real Africa
<strong>Figure 1:</strong> Chromosome. (1) <font color=Chromatid. One of the two identical parts of the chromosome after [[S phase" >
. (2) Centromere. The point where the two chromatids touch, and where the microtubules attach. (3) Short arm. (4) Long arm.]] A chromosome is, minimally, a very long, continuous piece of DNA, which contains many genes, regulatory elementss 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 Nägeli 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.

Table of contents
1 Chromosomes in eukaryotes
2 Chromosomes in bacteria
3 Chromatin
4 Chromosomes in different species
5 Karyogram
6 Chromosomal aberrations
7 External links

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 chromsomes 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:
<strong>Figure 2:</strong> Different levels of DNA condensation. (1) Single DNA strand. (2) Chromatin strand (<font color=DNA with histones). (3) Condensed chromatin during [[interphase" >
with centromere. (4) Condensed chromatin during prophase. (Two copies of the DNA molecule are now present) (5) Chromosome during metaphase.]]

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 1: Examples of chromosome numbers (diploid).

Species

# of chromosomes

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 32 Carp 104
Pig 40 Butterflies ~380
Wheat 42 Fern ~1200

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.

<strong>Figure 3</strong>: Karyogram of human femaleEnlarge

Figure 3: Karyogram of human female

Karyogram

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 karyogram (an ordered set of chromosomes, Fig. 3), also called karyotype. 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.

Chromosomal aberrations

The malfunction of either the chromosomal segregation or the crossover can lead to severe diseases. They can be divided into two classes: You can find a detailed graphical display of all human chromosomes and the diseases annotated at the correct spot at [1].

External links

See also: