Biology Reference
In-Depth Information
heterochromatinized. The original assumption was that the transgenes became
inactive because they had been inserted into a heterochromatic site (
Henikoff
2000
). Thus, understanding the mechanism(s) by which heterochromatin forms is
essential in improving the function of transgenes inserted into genetically modi-
fied insects developed for pest-management programs.
Heterochromatin is hypothesized to serve as a defense mechanism after para-
sitic DNA invades genomes (
Henikoff 2000
). In
Drosophila miranda
, heterochro-
matin forms at clusters of retrotransposons (a type of transposable element)
that have recently invaded the genome. The transformation of chromosome
regions into heterochromatin might prevent these invasive elements from func-
tioning and causing damage to the genome.
Many functional genes do occur within heterochromatic regions in
Drosophila
.
For example, the Y chromosome of
D. melanogaster
is heterochromatic, yet car-
ries some genes that are required for male fertility (
Gatti and Pimpinelli 1992
).
3.10 Centromeres
Most chromosomes possess a centromere. The centromere is important in the
organization of the developing spindle before mitosis or meiosis and the separa-
tion of the daughter chromosomes at anaphase. Chromosome fragments lacking
centromeres,
acentric fragments
, do not get transmitted to daughter cells in an
orderly manner, and the genetic information contained on them is eventually
lost, which can be lethal.
Some species do not have localized centromeres; rather, the whole chro-
mosome seems to have centromeric properties (
holocentric chromosomes
).
If holocentric chromosomes are fragmented, each portion can attach to the
spindle, and these fragments are not lost at mitosis. Holocentric chromosomes
are found in the orders Hemiptera, Mallophaga, Anoplura, and Lepidoptera
(
White 1973
). Centromeres are difficult to study (
Tyler-Smith and Floridia 2000,
Henikoff et al. 2001
).
Analysis of a centromere in a
Drosophila
minichromosome indicated that the
essential core of the centromere is a 220-kb region containing complex DNA. In
addition, another 200 kb of DNA on either side is essential to centromeric func-
tion and contains highly repeated sequences (
Murphy and Karpen 1995
).
Analysis of a centromere from a standard chromosome confirmed that the
Drosophila
centromere spans 420kb, more than 85% of which consists of two
highly repeated satellite DNAs with the sequences AATAT and AAGAG. The
remainder of the centromere consists of interspersed transposable elements, as
