Brutlag, D.L. (1977). In The Molecular Biology of the Mammalian Genetic Apparatus, New York: North-Holland Publishing. pp. 43-50.
The highly repeated DNA sequences of eukaryotes can usually be
isolated as satellites in CsCI buoyant gradients. The tandem
arrangement of short repeated sequences gives rise to the distinctive
physical properties of satellites. These DNAs are usually restricted
to centromeric heterochromatin and, as might be expected, are neither
transcribed nor translated. The function of centric heterochromatin
is difficult to assess because, like satellite DNA itself, it is so
variable between closely related species. Centric heterochromatin in
Drosophila was thought to be genetically inert due to the lack of
genes in this region and the viability of large heterochromatic
deletions. However, such deletions in the sex chromosomes do result
in improper disjunction in the first meiotic division and cause a
marked decrease in fertility. This genetic evidence indicates that
centric heterochromatin and perhaps satellite DNAs are involved in
normal chromosome propagation in meiosis. We have, therefore, been
studying the arrangement of satellite DNA in the heterochromatin in
Drosophila in order to provide a molecular basis for such a proposed
function.
We have found that the bulk of the heterochromatic DNA of Drosophila
consists of four different satellite species. These DNAs contain
short nucleotide sequences tandemly repeated in arrays over 1,000,000
base pairs long. Several of these satellites. although appearing
homogeneous by many physical criteria, contain more than one distinct
DNA with different repeating sequences, The sequences in one
satellite are so similar that the different DNAs cannot be separated
by classical procedures. I will conclude this paper by summarizing
the progress we have made in separating these components by cloning
individual molecules in hybrid bacterial plasmids.
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