Biology Reference
In-Depth Information
melanogaster
has four pairs of chromosomes; chromosomes 2 and 3 are large
with central centromeres, and chromosome 4 is the shortest. Females have two
X chromosomes, whereas males have an X and a Y chromosome. The Y chromo-
some is largely heterochromatic, containing only a few genes.
Because polytene salivary gland chromosomes of
Drosophila
are large and
have a well-defined morphology, specific genes can be localized by a procedure
called
in situ
hybridization. Radiolabeled DNA or RNA
probes
can be added to
salivary gland cells that have been squashed on glass slides. (A probe is a mole-
cule labeled with radioactive isotopes, or another tag, that is used to identify or
isolate a specific gene, gene product, or protein.) The labeled probes will anneal
to the homologous DNA by base pairing after the chromosomal DNA is dena-
tured (the DNA strands are separated). After washing off any excess probe, the
position of the specific gene can be localized to a specific band or interband
region of a specific chromosome by the presence of radioactive grains on an
X-ray film. Genes, identified by a particular phenotype, also can be localized to
specific sites in polytene salivary gland chromosomes if the mutation is associ-
ated with duplications, deletions, inversions, translocations, or other chromo-
somal abnormalities that can be detected by abnormal banding patterns under
the light microscope.
Polytene chromosomes are thought to represent a special case of the more gen-
eral phenomenon of endopolyploidy (
White 1973
). In
polyploidy
, an increase in
chromosome
number
occurs within the nuclei of certain tissues without a break-
down in the nuclear membrane. Thus, chromosome duplication takes place and
the chromosomes separate after replication (unlike the situation in polyteny), but
no cell division occurs. Many insect cells have 4n, 8n, 16n, and so on, numbers of
chromosome sets.
3.15 Chromosomal Puffing
At particular stages in development of many Diptera, some of the genes in sali-
vary gland polytene chromosomes undergo swelling or
pufing
. This puffing is
correlated with gene activity. Puffing involves an unraveling of the DNA in a
region of the chromosome
≈
1 to 10 bands in length. The patterns of puffing dif-
fer in different instars in
D. melanogaster
, indicating different genes are active
in different instars. Puffing is controlled by the hormone ecdysterone, heat
shock, and other environmental conditions. The largest puffs contain genes cod-
ing for proteins that are produced in very large amounts in the salivary gland,
such as the salivary gland secretions and silk. Puffs are associated with extensive
transcription of DNA (
Figure 3.5
).
