Biology Reference
In-Depth Information
If the rearranged chromosome lacks a centromere, it is
acentric
. A cell con-
taining the acentric fragment will be unlikely to transmit this fragment to its
daughter cells during meiosis or mitosis, resulting in loss of significant amounts
of genetic information, which is usually lethal. Chromosomes that end up with
two centromeres (
dicentric
) also are unstable, leading to breaks in the chro-
mosomes if the centromeres are distributed to opposite poles during meiosis
or mitosis. This instability results in breakage and loss of genetic information,
which is often lethal.
In some cases, two different chromosomes are broken, and the repair results
in chromosomes that contain fragments from two different chromosomes.
These changes are called
translocations
. If the broken chromosomes are
repaired but the broken piece is inserted in the wrong orientation, it is called
an
inversion
. Sometimes, small portions of a chromosome are deleted (dele-
tion), and sometimes a small portion of the chromosome receives a novel piece
(insertion).
3.14 Polyteny
In a normal chromosome replication cycle (mitosis), chromosomes condense, rep-
licate, separate, and segregate to daughter cells. In
polytene
cells,
≥
10 DNA rep-
lication cycles may occur but the daughter chromosomes remain in an extended
state and
do not separate
; such cells become larger and do not divide. The
daughter DNA strands stay paired, with homologous regions aligned, and this
gives rise to a banding pattern along the length of the chromosome under the
light microscope even during interphase. In some cases, the maternal and pater-
nal homologous chromosomes may synapse, resulting in an apparently haploid
(n) number of giant chromosomes.
Polyteny is particularly common in larval salivary glands of Diptera, especially
in flies from the Drosophilidae, Chironomidae, Cecidomyiidae, and Sciaridae, but
it also occurs in the midgut and fat body in these insects. Polyteny also occurs in
Collembola. The number of rounds of DNA strand replication varies from tissue to
tissue, with the largest number found in the salivary glands, where there may be
as many as 1000-2000 chromatids per giant chromosome.
In
Drosophila
salivary gland chromosomes, the euchromatin regions contain
genes, whereas the heterochromatic regions primarily contain repetitive DNA
sequences including centromeres and telomeres (
Leach et al. 2000
). The band-
ing patterns make it easy to identify specific sites on
Drosophila
salivary chro-
mosomes (
Figure 3.5
). Approximately 5000 chromosome bands have been
identified in
D. melanogaster
, providing a detailed cytological map.
Drosophila
