1-2 Nuclear Structure and Function Flashcards
Describe structure of nuclear envelope
a double membrane barrier between nucleus and cytoplasm. Punctuated by nuclear pores where all traffic into and out of nucleus occurs
- outer - contiguous with ER, has ribosomes, proteins translated on these insert into lumen within membrane
- inner - attached to and supported by nuclear lamina
Describe structure and function of the nuclear lamina
Meshwork of intermediate filaments formed from three proteins Lamin A, B, and C.
Attaches to inner membrane of Nuclear envelope and chromatin to stabalize envelope and organize chromosomes
Describe the structure of nuclear pores
Nuclear Pore Complexes (NPC) are proteinaceous channels through nuclear envelope. Where all molecules enter/exit nucleus.
Arranged in 8-fold symmetry forming channel which small proteins (44kDa or less) can passively diffuse. Proteins 60kDa or larger must contain a nuclear localization signal (NLS)
What happens to the lamin proteins during mitosis?
Lamin phosphorylation at mitosis induces disassembly of the nuclear lamina which causes nuclear breakdown
process reversed after mitosis, causing re-assembly of the nucleus
Describe the structure of the nuclear pores
Nuclear Pore Complexes (NPCs) are proteinaceous channels which traverse the nuclear envelope. Pathway for all molecules moving into and out of nucleus.
Describe the basic mechanism of transport through the pores
- Carrier proteins recognize transport signals (NLocalS/NExportS) on substrate.
- Carrier protein (importin/exportin) binds substrate and escorts through pore
- Cargo/Carrier protein dissociate, carrier recycled back through pore.
The functions of the nuclear pores allows for..
different protein and RNA compositions between the nucleus and cytoplasm
What is the “cargo” exportin binds to in the nucleus?
mRNA, tRNA, or rRNA
Transport in either direction is dependent on?
Small GTPase called RAN and two associated factors
- RanGTPase activating protein (GAP) on the cytoplasmic side (both have letter P)
- Ran GTPase exchange factor (GEF) on the nuclear side (both have letter E)
What powers the transport in and out of the nucleus?
The assymetric distribution of GTP bound ran(activated) vs GDP-bound ran sets up a gradient which drives reactions.
Describe protein import specifically
- Import requires Nuclear Localization Signal on a substrate which is bound by importin carrier protein.
- Importin directs docking of complex to fibrils extending from cytoplasmic face of NPC
- translocation of complex
- GTP bound ran directs dissocation of the carrier and cargo by binding to importin and lowering importin’s affinity for the cargo.
- RanGTP bound importin is then recycled out of the nucleus into the cyctoplasm, where RanGTP is hydrolyzed to RanGDP by GAP, causing it’s dissocation from the importin.
Describe protein export specifically
- Export requires Nuclear Export Signal on the substrate which is bound by exportin carrier protein and RanGTP.
- Exportin directs movement through the NPC to the cytoplasm
- Once in the cytoplasm, GAP hydrolyzes the RanGTP to RanGDP, which dissociates the complex due to change in affinity. Releasing the cargo, Ran-GDP, and exportin which is recycled back into the nucleus.
Describe the nucleoulus
non-membrane bound sub-compartment of the nucleus assembled on the rDNA repeats of 5 different autosomes with 2 homologes each for a total of 10 chromosomes.
sidenote: rDNA is ribosomal DNA which is a sequence that codes for rRNA
Describe the pathway of ribosomal biogenesis
- begins in nucleolus, transcription of genes for 45s rRNA large precursor
- single 45s precursor RNA formed and processed into pre-ribosome by endo/exo nucleases into 28s, 18s, and 5.8s.
- 18s incorporated into small subunit, 28s, 5.8s, and 5s (made elsewhere by RNA poly 3) incorporate into large ribo subunit
- ribosome exported from nucleus
Describe the structure and relative numbers of Chromosomes
-extremely long, linear polymers of double stranded DNA coated with structural/regulatory proteins called chromatin
- 22 different autosomes, 2 sex chromosomes
- haploid cell - 22 autosomes, 1 sex =23
- diploid cell - 44 autosomes, 2 sex = 46
what is required to stabilize and duplicate chromosomes?
Three prominent sequences are required to stabalize and duplicate chromosomes
- Replication origin- initiation of DNA replication
- Centromers - segregation of duplicated chromosomes at mitosis and meiosis
- Telomeres - stablility and replication of chromosome ends. Protects chromosomes against shortening during DNA replication
There are two levels of packaging in ____phase cells.
interphase
euchromatin - less condensed, DNA being actively transcribed
Heterochromatin - more condensed, not currently being expressed
When are chromosomes most condensed?
During Mitosis
DNA packaging is mediated by _____. Types?
Histones. 5 Kinds (H1, H2A, H2B, H3, H4)
Loss of function mutation that involves RanGEF is likely to have what effect?
loss of mRNA export from nucleus
Describe how chromatin is formed
1) Positive residues of histones attatch to negative residues of DNA.
2) 8 histones (2 of each type) combine to form a histone octamer (nucleosome)
3) Multiple nucleosomes coil together and stack
4) resultant packed fiber of nucleosomes is 30nm thick and called chromatin
Define “gene”.
Genes code for proteins such as RNAs (rRNA, tRNA, mRNA, snRNA, snoRNA, microRNA, IncRNA).??????????/
Describe nuclear architecture
The nuclear matrix is a dense, underlying fibrous network composed mainly of proteins that provides structure/organization to the nucleus.
-Chromosomes attatch to the matrix through sequences called matrix associated regions (MARS)
-the matrix helps bring together regions of chromosomes being actively transcribed to specific foci (transcription/splicing factories)
When does nuclear architecture change? how?
During Mitosis. Most eurkaryotes (including humans) undergo open mitosis which nucleus disassembles.
- Transcription stops
- Nucleolus dissolves
- Chromosomes condense
- Modification of the lamins (phosphorylation) causes the nuclear envelope to break down into vesicles
After mitosis - Lamins dephosphorylated.
nuclear matrix/envelope reassembles around de-condensed chromosomes
chromosomes fuse to recreate complete nucleus
rDNA transcription resumes creating 10 pre-nucleolar foci that fuse to reform nucleus
