Correlation of Structure and Function in Chromosomes

In a chromosome, regions with special structures
and functions can be differentiated. The
centromere is the point of attachment of the
microtubuli of the mitotic spindle (kinetochore).
The telomeres at the ends contain no
genes and have a special structure.

Heterochromatin and euchromatin

In 1928 Emil Heitz observed that certain parts
of the chromosomes of a moss (Pellia epiphylla)
remain thickened and deeply stained during interphase,
as chromosomes otherwise do only
during mitosis. He named these structures heterochromatin,
as opposed to euchromatin,
which becomes invisible during late telophase
and subsequent interphase. Functionally, heterochromatin
is defined as a region in which few
or no active genes lie and in which repetitive
DNA sequences occur. When active genes become
located close to the heterochromatin,
they usually become inactivated

Characteristic regions of a chromosome

The centromere and telomeres contain repetitive
DNA sequences. They are evolutionarily
conserved because they are important for chromosome
stability. The segments located between
the telomeres and centromere consist of
trypsin-sensitive light and trypsin-resistant
dark G-bands. The light G-band areas of DNA
form loops in which the protein-coding genes
lie. The DNA loops are bound to a protein matrix
at special attachment sites.

Model of a chromosome segment in interphase

A three-dimensional model of a chromosome
segment shows that the constitutive heterochromatin
(C-band) in the centromere region is
very tightly wound. In the light G-bands, the
euchromatin is relatively loosely packed, and in
the dark G-bands, somewhat more tightly
packed. (With kind permission of the author,
from Manuelidis, 1990, copyright 1990 by the
AAAS).

Constitutive heterochromatin (C-bands) in the centromeric region

The constitutive heterochromatin in the
centromeric region can be specifically stained
(C bands). The distal half of the long arm of the Y
chromosome is also C-band positive. The
centromeric heterochromatin in chromosomes
1, 9, and 16 and in the long arm of the Y chromosome
in humans is polymorphic. The lengths of
the heterochromatic segments in one or more
of these regions may vary among different individuals.

Functional attributes of the euchromatin regions

The light and dark G-bands differ in functional
respects. An average G-band contains around
1.5 megabases

Special Structure at the Ends of a Chromosome: the Telomere

Unlike the circular chromosomes of bacteria,
bacteriophages, plasmids, and mitochondrial
DNA, the chromosomes of eukaryotes are linear.
Each end is ”sealed” by a specialized region, the
telomere. Telomeres stabilize chromosomes at
both ends.