<<Up     Contents

Chromosome

Redirected from Chromosomes

A chromosome 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 is called chromatin. During mitosis (nuclear division), the chromosomes are condensed and called metaphasic[?] chromosomes. This is the only state in which DNA is 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 discovered by Karl von Naegli[?] in 1842. In 1910, Thomas Hunt Morgan proved chromosomes to be the carriers of genes.

Chromosomes in eukaryotes

Chromatin chromosome.png
Figure 1: Different stages of DNA condensation.
(1) Single DNA strand.
(2) Chromatin strand (DNA with histones).
(3) Condensed chromatin during interphase with centromere.
(4) Condensed chromatin during prophase.
(5) Chromosome during metaphase[?].

Two types of chromatin can be distinguished:

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 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 well structured (Fig. 2). For example, genes with similar functions are often close together in the chromosome, but not in the linear DNA. The short arm of a chromosome can be extended by a satellite chromosome[?] that contains codes for ribosomal RNA.

Figure 2: Chromosome.
  1. Chromatid. One of the two identical parts of the chromosome.
  2. Centromere. The point where the two chromatids touch, and where the microtubules attach.
  3. Short arm.
  4. Long arm.
Chromosome.png

Species

# of chromosomes

Species

# of chromosomes
Fruit fly 8 Human 46
Rye 14 Ape 48
Guinea Pig 16 Sheep 54
Dove 16 Horse 66
edible snail 24 Chicken 78
Earthworm 32 Carp 104
Pig 40 Butterflies ~380
Wheat 42 Fern ~1200

Table 1: Examples of chromosome numbers (diploid).

Within a species, the number of chromosomes is the same (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 germ line cells[?] (reproductive cells) which are haploid [n] (they have only one set of chromosomes). When a male and a female germ line cell merge (fertilization), the (now diploid) cell undergoes meiosis (maturation of the fertilized egg). 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.

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.

Human karyogram.png
Figure 3 : Karyogram of a human female.
(copyright 1995 Department of Pathology, University of Washington, Cytogenetics Gallery. Reproduced with permission.)
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] (http://www.ornl.gov/hgmis/posters/chromosome/).

See also:

wikipedia.org dumped 2003-03-17 with terodump