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
Epigenetics of histones
each of the four main histones (H2A, H2B, H3, H4) have highly conserved n-tail domains that protrude from the nucleosome.
- subject to many post-translational modifications such as acylation, methylation, phosphorylation.
- regulated by a diverse set of histone acetylases and de-acetylases
- these modifications can persist through several cell divisions and in some cases are heritable.
Lupus
caused by production of antibodies to self antigens in the nucleus.
-able to study new sub-compartments in the nucleus
acute promyelocytic leukemia
caused by mutation in nuclear PML protein normally required to form a subcompartment of the nucleus, called PML bodies.
Spinal muscular atrophy
second most common neuromuscular disease of childhood, duchenne MS being most common.
inherited or mutation in the protein SMN (survival of motor neurons) beleived to be involved in RNA splicing.
describe the compartments of the nucleolus
Regions
1) Fibrillar center - region contains DNA not being actively transcribed
2) Dense fibrillar compartment - contains rRNA molecules actively being synthesized
3) Granular compartment - contains maturing ribosomal precursor molecules
what is the composition of the nucleosome?
2 copies each of histones H2A, H2B, H3, H4
Where are ribosomes made?
What is the size of the large rRNA precursor and how is this broken up?
Ribosomes are made in the nucleolus region of the nucleus.
Ribosome biogenesis involves forming a single 45s precurosr rRNA which is processed in the pre-ribosome by endo/exo-nucleases into 28s, 18s, and 5.8s
Ribosome biogenesis is complete following export from the nucleus
During mitosis, would you expect to see heterochromatin or euchromatin?
During mitosis, all of the chromosomes achieve their most condensed state…which is why we would expect to see heterchromatin (stains darker)
Which statement concerning human chromosomes is wrong?
A. Gametes contain 23 chromosomes.
B. Homologues carry different alleles of the same genes while sister chromatids have the same alleles.
C. Independent assortment to make gametes can have 2^46 possible outcomes.
D. Recombination between sister chromatids would not contribute to genetic diversity.
E. Recombination between homologues does contribute to genetic diversity.
C.
2^23
The frequency of genetic recombination between two loci on a given chromosome will determine: Selected Answer: Incorrect A. linkage of genetic traits. Answers: A. linkage of genetic traits.
B. linkage of RFLPs.
C. linkage of genetic traits to RFLPs.
D. Only A and B are correct
E. A, B, and C are correct.
E
