http://en.wikipedia.org/wiki/Chromosome
Chromosome, the
microscopic threadlike part of the cell that
carries hereditary information
in the form of genes. A defining
feature of any chromosome is its compactness. For instance, the 46 chromosomes
found in human cells have a combined length of 200 nm (1 nm = 10− 9 metre);
A chromosome is
a structure of DNA, protein, and RNA found in cells. It is a single piece of coiled DNA containing
manygenes, regulatory elements and other nucleotide sequences. Chromosomes also
contain DNA-bound proteins, which serve to package the DNA and control its
functions. Chromosomal DNA encodes most or all of an organism's genetic
information; some species also contain plasmids or other extrachromosomal genetic elements.
Every eukaryotic species has a
characteristic number of chromosomes (chromosome
number). In species that reproduce asexually, the chromosome
number is the same in all the cells of the organism.
Among sexually reproducing organisms, the number of chromosomes in the body
(somatic) cells is diploid (2n; a pair of each chromosome), twice the haploid (1n) number found in the sex cells, or gametes. The haploid
number is produced during meiosis. During fertilization, two gametes
combine to produce a zygote, a single cell
with a diploid set of chromosomes.
The DNA molecule may be
circular or linear, and can be composed of 100,000 to over 3,750,000,000
nucleotides in a long chain. Typically, eukaryotic cells (cells with nuclei) have large linear chromosomes and prokaryotic cells (cells without defined nuclei) have smaller circular chromosomes.
Chromosomes in humans can
be divided into two types: autosomes and sex chromosomes. Certain genetic traits are linked to a person's
sex and are passed on through the sex chromosomes. The autosomes contain the
rest of the genetic hereditary information. All act in the same way during cell
division. Human cells have 23 pairs of chromosomes (22 pairs of autosomes and
one pair of sex chromosomes), giving a total of 46 per cell. In addition to
these, human cells have many hundreds of copies of the mitochondrial genome.
Sexually reproducing species have somatic cells (body cells), which are diploid [2n] having two sets of chromosomes
(23 pairs in humans with one set of 23 chromosomes from each parent), one set
from the mother and one from the father. Gametes, reproductive
cells, are haploid [n]: They have one set of chromosomes.
Gametes are produced by meiosis of a diploid germ linecell.
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.
Some
animal and plant species are polyploid [Xn]: They have more than two sets of homologous chromosomes. Plants important
in agriculture 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 cultivars 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.
Prokaryotes
Prokaryote species generally have one copy of each major
chromosome, but most cells can easily survive with multiple copies.For example, Buchnera,
a symbiont of aphids has multiple copies of its chromosome,
ranging from 10–400 copies per cell.However, in some large bacteria, such as Epulopiscium
fishelsoni up to
100,000 copies of the chromosome can be present.
Diagram of a replicated
and condensedmetaphase eukaryotic
chromosome. (1)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.
Organization of DNA in a
eukaryotic cell.
Human chromosomes during metaphase
Karyogram of a human male
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