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The Nucleus (What are the key structures in the nucleus? (nuclear envelope…
The Nucleus
What are the key structures in the nucleus?
The nucleolus
most obvious structure
not bound by membrane
huge aggregate of macromolecules
site for processing of ribosomal RNAs and their assembly into ribosomes
Cajal bodies and interchromatin granules
cajal bodies
not bound by membrane
composed of selected RNA/ protein molecules
histone RNA processing
modification of snRNAs for transport
methylation of snRNAs
interchromatin granules
nuclear domains enriched in pre-mRNA splicing factors
located in interchromatin regions of the nucleoplasm
very dynamic structures
consitutents exchange continuously with nucleoplasm
nuclear envelope
defines nuclear compartment
encloses DNA
responsible for compartmentalisation
membranes
inner membrane
surrounded by outer one
contains many proteins that act as binding sites for chromosomes and for nuclear lamina
one continuous structure
distinct protein composition for each membrane related to its function
outer membrane
continuous with the endoplasmic reticulum
studded with ribosomes that are engaged in protein synthesis
penetrated by numerous nuclear pores
export of mRNA
import of proteins
Nuclear Matrix or scaffold
dense network of intermediate filaments and proteins located throughout the nucleus
includes the nuclear lamina which lies close to inner surface of nuclear envelope
provides structural support for the nucleus
regulates important cellular events including DNA replication and cell division
What is the nucleus?
information store of the cell
most prominent organelle in eukaryotic cell
function of compartmentalisation
contains genetic material
allows the many proteins that act on DNA to be concentrated
keeps nuclear and cytosolic enzymes separate
How is the transport across the nuclear envelope achieved? The NPCs
can be 2000-4000 NPCs in nuclear envelope of typical mammalian cell
comprised of around 30 different proteins called nucleoporins
arranged in eightfold rotational symmetry
3 different groups of nucleoporins
transmembrane ring proteins
span nuclear envelope and anchor NPC to envelope
scaffold nucleoporins
form layered ring structures
channel proteins
which line nuclear pore
trafic
transport of up to 1000 macromolecules per second
transport molecules in both directions at the same time
NPCs contain aqueous passages through which small water soluble molecules can passively diffuse
how are proteins directed through the nuclear envelope?
Nuclear Localisation Signals
Ran-GEF
Ran-GAP
cargo molecule
What is the role of the nucleolus?
ribosome producing factory
size of nucleolus reflects number of ribosomes that it is producing
can occupy 25% of nuclear volume if cell is producing large amounts of protein
What is the nuclear scaffold/matrix and how is it related to chromosomes?
network of intermediate filament proteins found throughout the nucleoplasm
nuclear lamina
2D mesh which lines and strengthens ineer surface of nuclear membrane
made of intermediate filament proteins called lamins
creates tough layer provides support to nuclear membrane
readily disassembled and reassembled during cell division
connects inner surface of nuclear membrane with the chromatin
clusters domains of silent chromatin to the nuclear envelope