The cell nucleus contains most of the cell's
genetic material, organized as multiple long
linear DNA molecules in complex with a large
variety of proteins, such as histones, to form
chromosomes. The genes within these
chromosomes are the cell's nuclear genome.
The function of the nucleus is to maintain the
integrity of these genes and to control the
activities of the cell by regulating gene
expression — the nucleus is therefore the
control center of the cell. The main structures
making up the nucleus are the nuclear
envelope, a double membrane that encloses
the entire organelle and separates its contents
from the cellular cytoplasm, and the nuclear
lamina, a meshwork within the nucleus that
adds mechanical support, much like the
cytoskeleton supports the cell as a whole.
Because the nuclear membrane is
impermeable to most molecules, nuclear pores
are required to allow movement of molecules
across the envelope. These pores cross both
of the membranes, providing a channel that
allows free movement of small molecules and
ions. The movement of larger molecules such
as proteins is carefully controlled, and
requires active transport regulated by carrier
proteins. Nuclear transport is crucial to cell
function, as movement through the pores is
required for both gene expression and
chromosomal maintenance. Although the
interior of the nucleus does not contain any
membrane-bound subcompartments, its
contents are not uniform, and a number of
subnuclear bodies exist, made up of unique
proteins, RNA molecules, and particular parts
of the chromosomes. The best known of these
is the nucleolus, which is mainly involved in
the assembly of ribosomes. After being
produced in the nucleolus, ribosomes are
exported to the cytoplasm where they
translate mRNA during the synthesis of
proteins.
Author: Mishii
