Biology Reference
In-Depth Information
well as a nonrepetitive segment of AT-rich DNA (
Sun et al. 1997
). Both the repet-
itive and nonrepetitive sequences contribute to the centromere function.
The chromosomal region adjacent to the
Drosophila
centromere contains very
long blocks of highly repetitive DNA in which simple sequences are repeated
thousands of times (satellite DNA). There may be several different satellite DNA
types in a given species; for example, three satellites are found near centromeric
DNA of
Drosophila virilis
. One satellite type has a 5
′
-ACAAACT-3
′
repeat, the sec-
ond satellite type is 5
′
-ATAAACT-3
′
, and the third satellite type is 5
′
-ACAAATT-3
′
.
These satellite DNAs apparently are not transcribed and they may bind proteins
essential for centromere function. Likewise, for the Australian sheep blowfly
Lucilia cuprina
, several subfamilies of satellite DNA are present in the centro-
meric regions of the chromosomes, as well as in the sex chromosomes (
Perkins
et al. 1992
).
Although each insect species normally has several types of satellite DNA in the
centromere, two parasitic wasps,
Diadromus pulchellus
and
Eupelmus vuilleti
,
have only one type (
Bigot et al. 1990
). In these two species, satellite DNA con-
stitutes 15 and 25% of the genome, respectively. Likewise,
≈
50% of the genome
of the tenebrionid
Tenebrio molitor
, consists of only one type of satellite DNA,
and it is distributed evenly over the centromeric regions (
Plohl et al. 1992
). In
the tenebrionid
Palorus ratzeburgii
,
≈
31% of the genome is a single type of
satellite DNA (
Ugarkovic et al. 1992
).
3.11 Telomeres
The ends of the chromosomes have distinct structures called
telomeres
.
Telomeres have two important functions: 1) maintain the length of chromo-
somes despite the inability of DNA polymerase to replicate linear DNA ends
completely, and 2) distinguish natural chromosome ends from double-stranded
breaks in DNA. The latter function, known as “capping,” is important because
DNA damaged by double-stranded breaks is attacked by repair and degradative
enzymes. Telomeres also associate with one another, with the nuclear matrix,
and with the nuclear envelope, an association that could be important in main-
taining organization within the nucleus and for meiotic chromosome pairing
(
Mason and Biessmann 1995
).
Molecular analyses indicate telomeres consist of a series of repeated nucleo-
tides and proteins (
Blackburn 1991, Wagner et al. 1993, Zakian 1989, de Lange
2001
). As described in Chapter 1, DNA replication conventionally occurs only in
the 5
′
to 3
′
direction and cannot be initiated without a primer, which is usu-
ally RNA. After primer removal, gaps would remain at the 5
′
ends of new DNA
