Biomedical Engineering Reference
In-Depth Information
G
C
C
U
Extended
chromatin
(300 nm)
Condensed
chromatin
(700 nm)
DNA double helix (2 nm)
Nucleosomes (11 nm)
Packed nucleosomes (30 nm)
7. 7
Packaging of genetic material into chromosomes. (Image created by
Edward Rodriguez, St Peter's University, Art Department, Graphic Arts,
Jersey City, NJ.)
have different amounts of structural genes and the various types of non-
informational repeated sequences that make up the vast majority of the
chromosome's DNA content. It is important to remember that one meta-
phase chromosome band can contain 5-10 million DNA base pairs, which
contains numerous informational gene segments. This is why even small
alterations to chromosome structure can affect many genes and have a dra-
matic effect on the organism's phenotype.
The majority of the chromatin within a chromosome is called
euchro-
matin
and contains the bulk of the coding regions (genes). In chromosome
preparations that are trypsin G-banded (Fig. 7.8), the chromosome is seen
as having alternating dark and light bands. The light G bands represent the
euchromatin, while the dark G bands represent the
heterochromatin
, which
contains relatively fewer genes than that seen in the light G bands. The het-
erochromatin is usually characterized as one of two types.
Constitutive heterochromatin
is found in the long arm of the Y chromo-
some, in the centromeres of all the chromosomes, and in the satellites of the
acrocentric chromosomes. Chromosome-specifi c DNA sequences are found
in the centromeres, which are highly repetitive elements with no known
function. However, these sequences are useful markers for chromosome
