Biology Reference
In-Depth Information
the nuclear scaffold in order to facilitate their transcription. MARs do not appear
to share extensive sequence homology but often comprise 200 bp of AT-rich
DNA, for example the sequence AATATTTTT in the murine immunoglobulin
κ
gene locus (Cockerill and Garrard, 1986). A number of MAR-binding proteins are
known to bind to MARs including the attachment region-binding protein, ARBP
and histone H1.
MARs appear to be preferentially associated with topoisomerase II cleavage
sites (reviewed by Laemmli
et al
., 1992) and share sequence homology with bind-
ing sites for homeobox proteins (Boulikas, 1992). Topoisomerase II plays a role in
the segregation of daughter chromosomes after DNA replication and also in
chromosome condensation; it binds preferentially to MARs (Adachi
et al
., 1989).
Vertebrate topoisomerase II cleavage sites also occur in association with MARs
and manifest a consensus sequence, A/G N T/C N N C N N G T/C N G G/T T N
T/C N T/C (Spitzner and Muller, 1988).
MARs also appear to be preferentially associated with enhancer-type elements.
Indeed, MARs stimulate heterologous gene expression in reporter gene experiments.
A
cis
-acting regulatory element 3
-globin (
HBG1
) gene, known to be
associated with the nuclear matrix, has been shown to bind specifically to an AT-rich
binding protein (SATB1) that binds to MARs (Cunningham
et al
., 1994).
to the human
A
Origins of DNA replication.
Chromosomal DNA replication initiates at specific
points (
origins
) and proceeds outward bidirectionally from specific loci. Although
a number of putative origins of replication have been identified in mammalian
species (reviewed by Coverley and Laskey, 1994; De Pamphilis, 1993), data are
still sparse. One of the best characterized is that found in human between the
-
(
HBD
) and
-globin (
HBB
) genes on chromosome 11 (Kitsberg
et al
., 1993a).
This replication origin is bidirectional and functional regardless of the transcrip-
tional state of the
-globin gene. From the study of six putative origins of replica-
tion (including one in the human c-
myc
oncogene), Dobbs
et al
. (1994) claimed to
have derived a consensus sequence, albeit a fairly redundant one: A/T A A/T T T
A/G/T A/G/T A/T A/T A/T A/G/T A/C/T A/T G A/T A/C/T A/C A A/T T T.
However, there are probably several different classes of replication origins which
possess different sequence characteristics (Boulikas, 1996).
Replication origins are often associated with CpG-rich regions (Delgado
et al
.,
1998; Rein
et al
., 1997; Tasheva and Roufa, 1995) and may sometimes be located in
the vicinity of matrix attachment regions (Section 1.1.1,
Matrix attachment regions
)
(Lagarkova
et al
., 1998). In the human genome, the units of DNA replication range
in size from 50 kb to 600 kb and are often clustered (Hand, 1978). For instance,
there are at least six such
replicons
within the human dystrophin (
DMD
; Xp21) gene
(Verbovaia and Razin, 1997). The gene-rich R bands replicate early in S phase
whilst the G bands replicate late. Housekeeping genes invariably replicate early
whilst tissue-specific genes can be early or late replicating (some replicate earlier
when transcriptionally active) (Goldman
et al
., 1984; Hatton
et al
., 1988).
Nontranscribed genes on the X chromosome also replicate late (Torchia
et al
., 1994).
1.1.2 Gene organization and transcriptional regulation
Gene structure and regulation.
The coding portion of the human genome,
roughly 5% of the total DNA complement, probably contains some 70 000
