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Chromosomes are very large DNA strands that contain many genes. During mitosis (cell division), the chromosomes are condensed and called metaphasic chromosomes. They are the only state in which DNA is visible with an optical microscope. In eukaryotes, the uncondensed DNA exists in a quasi-ordered structure inside the nucleus, where it is wrapped around histones (structural proteins, Fig. 1), and is called chromatin; prokaryotes do not possess histones. In its relaxed state, the DNA can be accessed for transcription, regulation, and duplication. 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
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:

  • 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:

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.

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.


# of chromosomes


# 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).

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). 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:

  • Chromosomal aberrations or Partial chromosomal dysplasia (malformation), which are usually the result of a defective crossover. Examples are:
    • Cat cry 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 is characterized by severe growth retardation and mental defect.
  • Missing or additional chromosomes, which are the result of an incomplete chromosomal segregation. 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 (XXX or 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, male and female characteristics are 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].

Old article
A chromosome is a contiguous piece of DNA.

Cellular DNA is normally found in an uncompacted state, called chromatin. During mitosis and meiosis, DNA is duplicated and packaged around protein complexes called histones for easier handling.

An image of the packaged chromosomes is called a karyotype.

A chromosome looks like the letter H. The two long pieces are called chromatids. The connection between the chromatids is called a centromere. The ends of the sides are called the telomeres.

When an organism has two copies of each chromosome (as do humans), it is called diploid. When only one copy of each chromosome is present, the organism is haploid.