Chromosome: Encyclopedia II - Chromosome - Chromatin

Chromosome - 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 very 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 visible as 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 highly structured (Fig. 2). For example, genes with similar functions are often kept close together in the nucleus, even if they are far apart on the chromosome. The short arm of a chromosome can be extended by a satellite chromosome that contains codes for ribosomal RNA.

Adapted from the Wikipedia article "Chromatin", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki