Biology Reference
In-Depth Information
so that chromosomes are distributed to the daughter cells in an orderly man-
ner during mitosis and meiosis, although some insect chromosomes have dif-
fuse centromeres along the entire chromosome. The centromeres of eukaryotic
chromosomes are complex regions that play a fundamental role in chromo-
some movement. Chromosomes always have telomeres at the ends; the teleo-
meres are specialized structures that help maintain the ends of chromosomes in
a stable state. In polygene tissues such as
Drosophila
salivary glands, banding
patterns are visible by light microscopy after staining. The bands staining lightly
are called euchromatin, and the bands staining darkly are called heterochroma-
tin. Euchromatin and heterochromatin represent active and less active regions,
respectively, during somatic interphase. In most insects, there are two copies of
each chromosome and hence two copies of each gene (2n
=
diploid complement),
although polyteny or gene amplification can increase the copy number of chro-
mosomes or genes. The reproductive cells (eggs, sperm) contain a single copy
(haploid complement
=
n) of chromosomes.
Chromosomes contain single-copy DNA (i.e., DNA present only once in the
genome that codes for a polypeptide), highly repetitive DNA, moderately repeti-
tive DNA, intergenic regions, centromeres, and telomeres. Intergenic regions
contain transcription and regulatory information. Many types of transposable ele-
ments have been identified in both the coding and noncoding regions of arthro-
pod chromosomes. Transposable elements are DNA sequences capable of moving
within and among chromosomes, and they may make copies of themselves as
they move. Transposable elements, or their defective derivatives, make up a sig-
nificant portion of the middle-repetitive DNA in insect chromosomes. Arthropods
also contain chromosomal DNA in the mitochondria that are located in the cyto-
plasm of the cell. Mitochondria are considered remnants of a microbial endo-
symbiont, and much of the original genetic information in this symbiont, turned
organelle, has been lost or transferred to the nuclear genome of its host.
3.2 Introduction
DNA in insects is organized into chromosomes, which are complex structures.
The number of chromosomes in insects varies, with the haploid number rang-
ing from 1 to 221. Insect chromosomes were among the first chromosomes to
be investigated, and studies of insect chromosomes have provided fundamen-
tal advances in genetics, including the initial proof that genes are on chromo-
somes and that spindle fibers exist in living cells and are not fixation artifacts
(
Ault 1996
).
The complete sequence of the
Drosophila
genome, only the third eukary-
otic genome to be sequenced after the yeast
Saccharomyces cerevisiae
and
