Chromosome
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.
Chromosomes in eukaryotes
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.
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 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.
- Chromatid. One of the two identical parts of the chromosome.
- Centromere. The point where the two chromatids touch, and where the microtubules attach.
- Short arm.
- Long arm.
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Chromosomes in different species
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.
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.
Figure 3 : Karyogram of a human female.
(copyright 1995 Department of Pathology, University of Washington, Cytogenetics Gallery. Reproduced with permission.)
Chromosomal aberrations
The malfunction of either the chromosomal segregation or the crossover can
lead to severe diseases. They can be divided into two classes:
- Chromosomal aberration or Partial chromosomal dysplasia (malformation), which are usually the result of a defective crossover. Examples are:
- 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 sounds 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.
- Missing or additional chromosomes, called aneuploidy, which are the result of an incomplete chromosomal segregation, many of which are associated with cancer (see Duesberg hypothesis). Examples are:
- Down syndrome (extra chromosome 21). This is also know as mongolism or trisomy 21. Symptoms are decreased muscle tone, asymmetrical skull, slanting eyes and mental retardation.
- Klinefelters 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 lack of emotion, fatigue, apathy 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.
You can find a detailed graphical display of all human chromosomes and the diseases annotated at the correct spot at [1].
See also:
