392
Cell Nucleus Biogenesis, Structure and Function
After a brief interphase, the second meiotic
division follows according to the pattern
described for a normal mitotic division.
The overall process generates cells with a
haploid DNA content and a small number
of novel chromosomes that have arisen
through recombination of chromosome
homologues that were initially derived
from the organisms’ male and female
predecessors.
3
Nuclear Structure
As the nuclear boundary might be as-
sumed to deFne the nucleus itself a
detailed understanding of the structure
and function of the nuclear periphery is
required to understand many features that
deFne how nuclei work. In doing this, it is
helpful to consider the nuclear periphery
as being composed of three components:
1. The nuclear envelope is a double-
membrane structure, with an outer
membrane that is contiguous with the
rough endoplasmic reticulum. Given
this continuity, it is not surprising that
areas of the outer nuclear membrane
can be seen to be studded with ri-
bosomes. The outer and inner lipid
bilayers of the double envelope have
common components but contain spe-
ciFc proteins that might be nuclear or
cytoplasmic in location.
2. Over most of the nuclear surface, the
two membranes of the nuclear envelope
are separated by a fluid Flled space
that separates the membranes by about
50 nm. Over a small proportion of the
nuclear surface, the inner and outer
membranes are fused together. This
fusion is seen to occur at structures
called
nuclear pores
. These pores are the
major routes of transit of any materials
that must pass between the nucleus and
the cytoplasm.
3. The nuclear face of the inner nuclear
envelope is associated with a peripheral
nuclear lamina. The lamina is com-
posed of intermediate Flaments of the
lamin A/C and B proteins and is be-
lieved to play a role in the regulation of
nuclear envelope structure and nuclear
shape. Proteins that bind to the nuclear
lamina also interact with chromatin so
that through these adaptor proteins the
lamina can also play a role in chro-
matin organization and the regulation
of chromatin function.
3.1
Nuclear Pore Structure
A proliferating human cell has a few thou-
sand pores and the number doubles in
concert with doubling of chromatin dur-
ing S-phase. Pores are disrupted during
mitosis and the pore proteins are dis-
persed into about 12 pore subassemblies.
Pores are rebuilt from these pore pro-
tein complexes during assembly of the
new nuclear membrane at the end of
mitosis.
In all eukaryotes, nuclear pores provide
the gateway between the nucleus and
cytoplasm. In mammalian cells, the pores
are a megacomplex of about 150 MDa
composed of roughly 50 different proteins
(Table 3); the yeast pore has some 30
recognized pore proteins and it may be
that some of the vertebrate proteins that
have been classiFed as pore components
are in fact transport factors. The general
structure of a nuclear pore is shown
in ±ig. 3. The basic features of pore
structure are:
1. The
major
structure
of
the
core
complexes is cylindrical with external
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