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 (nuclear division), the chromosomes are condensed and called metaphasic chromosomes. This is the only natural context 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 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 chromosomes to be the carriers of genes.
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3 Chromosomes in different species
5 Chromosomal aberrations
- 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.
Figure 2: Chromosome.
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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.
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 .