Chromosome



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 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 discovered by
Karl Wilhelm von NŠgeli in 1842. In 1910, Thomas Hunt Morgan proved
chromosomes to be the carriers of genes.

Chromosomes in eukaryotes

     (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:

   * 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:
        o Constitutive heterochromatin, which is never expressed. It is
          located around the centromere and usually contains repetitive
          sequences.
        o 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. 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.

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

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.

The malfunction of either the chromosomal segregation or the crossover can
lead to severe diseases. They can be divided into two classes:

   * Chromosomal aberrations or Partial chromosomal dysplasia
     (malformation), which are usually the result of a defective crossover.
     Examples are:
        o 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.
        o Wolf-Hirschhorn syndrome, which is caused by partial deletion of
          the short arm of chromosome 4. It is is characterized by severe
          growth retardation and mental defect.what?
   * Missing or additional chromosomes, which are the result of an
     incomplete chromosomal segregation. Examples are:
        o 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.
        o 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.
        o 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.

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