384
Cell Nucleus Biogenesis, Structure and Function
catalyzed by a number of small nuclear
RNA-protein complexes (snRNPs). The
RNAs involved are called U1, U2, U4,
U5, and U6 (U3 is involved in rRNA
processing). U1 snRNP binds to the 5
0
splice site and U2 snRNP the intronic
branch acceptor site. A U4/U6.U5 com-
plex then assembles so that U6 snRNP
replaces U1 at the 5
0
splice site. U2 and
U6 RNAs then pair to form the catalytic
site and U5 pairs with the end of the
5’exon. Splicing proceeds through two
transesteriFcation reactions that form
Frst a lariat at the internal acceptor
site and Fnally the spliced product.
Many proteins – such as the SR pro-
teins (see 3.5.4) – are involved in this
splicing reaction and one consequence
of the splicing process is the forma-
tion of a mature RNA–protein complex
that is required for mRNA export to
the cytoplasm.
3. The 3
0
end of the mRNA is gener-
ated by a processing event that adds
a poly(A) tail (typically of 150–200
residues) to the nascent transcript. This
serves a structural role and is impor-
tant for RNA stability. Two elements
within RNA determine the location of
the poly(A) tail. The polyadenylation
signal, AAUAAA, binds the cleavage
poly(A) speciFcity factor (CPS±) protein
and a downstream element, UGUGUG,
binds the cleavage stimulatory factor
(Cst±). These two proteins interact with
and position the cleavage factor proteins
(C±I and C±II) and poly(A) polymerase
(PAP), which together generate cleaved
RNA with a poly(A) tail.
2.2
DNA Replication
The vast majority of cells in multicellu-
lar eukaryotes contain the same genetic
information that is descended from an
original diploid cell – the fertilized egg. It
is self-evident, that if cells are to maintain
their genetic integrity the proliferation pro-
cess must be tightly controlled so that cell
division can only proceed once the genetic
material – DNA – of the mother cell is fully
duplicated. This process of DNA replica-
tion is clearly central to the activities of cell
proliferation and is the target for a wide
variety of cellular controls and checkpoints
(quality controls).
Prokaryotes such as the bacterium
Es-
cherichia coli
have a circular genome and
initiate DNA synthesis at a single site,
the replication origin,
OriC
. The molec-
u
l
a
rd
e
t
a
i
l
so
fth
i
sp
r
o
c
e
s
sa
r
ekn
own
.
Quite clearly, the size and complexity of
eukaryotic genomes demand that many
more initiation events occur. In human
cells, origins of DNA replication are
about 150 kbp apart, which means that
an average chromosome will have
1000.
The molecular mechanisms that deFne
replication origins involve the association
of a protein complex with DNA. This
complex includes a multi-subunit origin
recognition complex (ORC1–6), and an
MCM 2 to 7 complex that restricts ini-
tiation to one event/origin in each cell
cycle, and proteins that are targets for
the cell cycle machinery that activates
the replication process. Once an origin
is activated the synthetic machinery is
recruited to the origin and replication
ensues.
±or most eukaryotes, it is not clear how
the origins of DNA replication are de-
Fned. The yeast
Saccharomyces cerevisiae
is an exception. In this organism, replica-
tion origins can be deFned as autonomous
replicating sequence (ARS) elements that
are about 200 bp in length and have con-
sensus sequence motifs – called
the A and
Be
l
em
en
t
s
– that act as DNA unwinding
previous page 1058 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online next page 1060 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online Home Toggle text on/off