5. Nuclear Structure Flashcards
major features of the nucleus?
surrounded by double layered nuclear envelop
outer membrane continguous with ER
inner membrane covered by nuclear lamina and chromatin
nuclear pores in inner and outer envelopes; thats how they fuse
nucleolus is site of ribosome synthesis and assembly (can see multiple nucleoli)
*heterochromatin @ periphery usually
active vs inactive DNA
euchromatin: active
heterochromatin: inactive and dense
describe the lamina network
fx?
2 types of proteins: A & B
-A: network and nucleoplasm
B- primarily in netowrk
they are rod like proteins with globular head and tail formed by coiled coil dimers
fx: shape and structural stability to the nucleus; some also link w/ actin to membrane with ECM
mutations in type A lamins?
results in laminopathies
1) emery dreifuss muscular dystrophy
2) hutchinson-gilford progeria–> change in shape of nucleus and decreased heterochromatin in periphery
how is the lamin network regulated?
regulated by phosphorylation
-both A & B phosphorylated at onset of mitosis—> so phosphorylation actual disrupts the nuclear envelop so that they can do mitosis
-they simply self assembly again by removal of phosphates
what does the nuclear pore complex consist of?
cytoplasmic side has filaments that regular nuclear import and export. primary component her is Ran BP2 protein
nuclear side is nuclear basket: 8 protein filaments that converge in a ring. primary component here is the Tpr proteins which interacts with dynamic network of proteins
- heterochromatin associate with the pore
- specific role in mRNA and rRNA export
how is GTP hydrolysis facilitated? how is GDP switch to GTP facilitated?
GTP—GDP hydrolysis uses a GTPase activating protein
GDP switch to GTP uses a guanine exchange factor
how do things get through the nuclear pore complex?
if smaller than 40-60 kDa, can freely diffuse.
larger molecules:
1) for export:
cargo binds to exportin and Ran-GTP in nucleus. Lots of Ran GTP in nucleus forming gradient. they go through the filaments in the cytoplasmic side. rEmember this filament has Ran BP2 protein which hydrolyses the GTP and so it kicks off the cargo protein on the cytoplasm side
2) for import: Ran GDP is in high concentrate in the cytoplasm. wants to move into the nucleus:
-imporin binds cargo protein
they enter nuclear envelop pore complex which has nuclear exchange factor associated with the heterochromatin that will cause Ran to release GDP, allowing it to bind GTP.
what is chromatin exactly?
how do you pack chromosomes?
structure of chromosomal DNA?
charge?
DNA and its histone proteins make up chromatin
to package- simple make the active DNA more accessible to transcription factors :
- have 4 histones: H1, H2A, H2b, H3, and H4
- form an octomer with 2 copies each of H2A, H2B, H3, and H4
- 147 bp of DN wraps around histone octamer to form nucleosome
-35 bp linker sequence btwn nucleosomes
basic aa on the surface of histone octamer facilitate interaction with DNA and the histone wrapping helps it so that negative charges don’t repel each other
what triggers condensation of chromatin during cell division? why is this important?
phosphorylation of histone triggers condensation and highly compacts the chromatin.
describe the open and close configuration for nucleosomes
closed is 30nm and inactive heterochromatin–COILED
open is beads on a string and unboun active. predominantly euchromatin
intermediate is ‘poised’ ready to go silent but can be activated pretty quickly
what does the chromatin stain and why?
where is the constitutive and facultative heterchromatin?
DNA has negative charge so is basophilic and the heterochromatin picks the dye stronger than the euchromatin/since its so condensed.
constitutive heterochromatin is near the periphery of the nucleus and links to lamina; gene poor and repetitive sequences
facultative- reversibly transitions btwn closed and open configuration
size of fibers is? and facilitated by what linker histone?
H1 is linker and fiber is 30nm wide
what is the importance of chromatin fibers?
loops form when part of it unwinds/decondenses to enable transcriptional activation—> replication machinery can get to it.
-loops also allow distant DNA to com into close proximity
what are telomeres
chromosome ends. tandem repeats go GGGTTA
