Nucleus Structure and Function
Ch12 (475-485), Ch11 (428-430)
nucleus is where DNA is stored
in the form of chromatin
heterochromatin - tightly bound cannot be transcribed
euchromatin - unwound ready to be transcribed
structure of nucleus and how it allows things to happen
nuclear envelope: two membranes. inner and outer.
many nuclear pores
stability inside, maintained by the nuclear matrix
nucleoplasm: liquidy stuff in the nucleus
nucleolus: darker staining region inside the nucleus; where ribosome are made
- chromatin: storage form for DNA
- outer membrane is continuous with the ER membrane
inner membrane is continuous with the outer-membrane
gap between the inner and outer called the inter-membrane space
Lamina: intricate layer of filaments (made of intermediate filaments)
lamina is on the inside of the inner membrane: strong layer, rigid, structure and
Integral proteins in the membrane: attached so that they go in the inter-
in order for the nuclear envelope to break down prior to mitosis, the lamins must
be phosphorylated. A kinase will do this.
nuclear membrane is not permeable to random stuff
transport between cytoplasm and nucleus is extremely regulated - things can
enter and leave through the nuclear pore
thousands of nuclear pores on every nucleus
Each nuclear pore contains a nuclear pore complex inside the nuclear pore. it
acts as a plug. it is the darkly stained area inside the pore.
the proteins that make up the nuclear pore complex are the nucleoporins
only need to know 4 different parts to it
spoke ring assembly: turquoise in diagram. connects through the inner
membrane to the outer membrane
nuclear basket: dark blue. looks like a basket. hangs down into the nucleoplasm. asymmetrical. it is only inside the nucleus. doesn’t go outside at all.
cytoplasmic filaments. red filaments that extend out into the cytoplasm. wave
around like tentacles in the cytoplasm.
central transporter: pink tunnel on the inside.
8 - fold symmetry (from top view or from bottom view)
Nuclear Localization Signals (NLSs)
sequence of specific amino acids that tells the cell its supposed to go inside the
these sequences can vary, can be at any part of the protein. usually has a lot of
positive amino acids. rich in K, R, P.
even they have a NLS, they must be escorted by a chaperone
importins- into the nucleus
exportins- out of the nucleus
importins: 3 subunits: NLS protein attaches to alpha importin then to beta
step1 : they all clump together and make a group. NLS on the bottom, then a,
then b. (receptor cargo)
step 2: the receptor cargo complex docks on a cytoplasmic filament
step 3: conformational change occurs, and it causes the NPC to “swallow” it
step 4: once the Receptor-Cargo enters the nucleus, Ran-GTP (a gtp-ase)
dissociates the receptor from the cargo (removes alpha and NLS from beta)
step 5: Ran-GTP transports importin B back to the cytoplasm and the GTP is
hydrolysed by GDP.
alpha is stuck inside. so it binds to exportin, which exports it back out to the
is a GTP-ase. can turn a GTP to a GDP.
on when bound to GTP
off when bound to GDP
can turn GTP off by hydrolyzing GTP - lots of enzymes that catalyze this process.
for example, GAP.