Nucleus Flashcards
inside nucleus
histone- protein that binds to DNA
nucleosome- protein dna complex containing 8 histone proteins
chromatin- repeating nucleosome structure
chromosome- region of chromatin that carries info relating to a specific set of genes (1/3 dna, 2/3 protein)- 23 pairs in nucleus
sister chromatids-identical copies of the same chromosome
mRNA processing
mrna is modified after transcription by adding a 5 prime CAP (modified guanine nucleotide) and 3 prime polyadenylation (adenosine monophosphate molecules)
splicing occurs
help with- stability of mRNA, binding to the ribosome and translation initiation, export from the nucleus
structure
biggest single organelle in eukaryotic cells
contains genetic material- packaged as chromatin
nucleolus- darker stained region
nuclear envelope is a double membrane (PL bilayer)
outer membrane is continuous with endoplasmic reticulum
nuclear laminar- part of cytoskeleton, maintaining structure
nuclear pore complex
large quaternary protein structure
aqueous channel that connects the nucleoplasm to cytosol
nuclear basket- cage like structure
nucleoporins- proteins that form the NPC
cystosolic fibrils- long fibres that reach out into cytosol that reach out to proteins
NPC has 8 fold rotational symmetry, meaning it has 8 nucleoporin proteins that form a ring structure
nucleoporins
NPC is formed of nucleoporin
30 different types of NP
500-1000 proteins (big proteins) in each NPC
nucleus isn’t static- lots of things moving in and out via NPC- dynamic
gap of npc is 9nm - 5KDa protein can pass straight through, 60 KDa can pass through but a bit harder, >60Kda cannot pass- need specialised mechanism
signal sequences
nuclear localisation signal- tell a protein to go into the nucleus
recognised by specific nuclear import receptor (importins)
NIR bind to FG repeats on nuclearporins in cytosolic domain of npc
proteins move through NPC binding sequentially to FG repeats
proteins are synthesised on cytosolic ribosomes containing an NLS
protein enters nucleus but is still bound to NIR
GTP
guanosine triphosphate
gtp binding protein- binds to GTP and acts as GTPase (hydrolyses GTP to GDP)
GAP- GTPase activating protein, binds to GTP binding protein and induces GTP hydrolysis
^part of recycling/ Ran cycle
GEF- guanine exchange factor, binds to GTP BP and causes it to release GDP + replaces it with GTP
Ran cycle- nuclear import
protein bound to NIR inside nucleus
RAN gtp binds to NIR- conformational change which releases protein
ran GTP now bound to NIR
exits nucleus through NPC
RAN binding protein binds to RAN gtp, releasing NIR and recycling it
ran BP and ran GTP bind
ran GAP hydrolyses GTP and forms GDP- releases from Ran BP
GDP can enter nucleus through NPC
GEF translates GDP back to GTP
mRNA export
mature mRNA is complexed with proteins- messenger ribonucleoprotein
mRNP export uses nuclear export receptors
receptors bind to adaptor proteins
adaptor proteins bind to the 5’ (prime) end of mRNA, interacting with the cap binding complex (CBC)
adaptor proteins can only bind if the mRNA has been properly processed
this complex binds sequentially to FG repeats and is transported out of the nucleus
nucleolus
chromatin exists in 2 forms within our cell
heterochromatin is transcriptionally inactive and densely packed (dark regions)- hinders transcriptional machinery
euchromatin is transcriptionally active and loosely packed
packed full of RNAs and proteins- darkly stained
nucleolus is region of nucleus where ribosomes are synthesises (ribosomal biogenesis)
very high demand
ribosomes
found in cytosol/ER
synthesised in nucleus
site of protein synthesis
made up of 2/3 rRNA and 1/3 protein
made up of large 60S (contains 5S, 5.8S, 28S rRNA + 49 proteins)
+ small subunit 40S (contains 18S rRNa and 33 proteins)
=80S ribosome
svedburg unit
unit = S
how quickly an organelle sinks to the bottom of a tube when it is centrifuged
non linear function influenced by mass, density and shape
rRNA genes
high demand on ribosomal biogenesis
4 rRNAs are encoded by more than 200 rRNA genes, split across 6 chromosomes
making it easy to synthesis RNA quickly
rRNA genes are clustered together on a DNA sequence known as tandem repeats
18S, 5.8S and 28S rRNAs are transcribed in nucloelus by rna poly 1
5S rRNA are transcribed by rna poly 3 in nucleoplasm
tandem repeats
speeds up rRNA transcription
contain
-spacer regions of different coding sequences (non coding dna sequence)
-coding sequence (18S, 5.8S, 28S)
-can generate rRNA very quickly as sequences are all next to each other
-everything is transcribed in chunks
45S pre rRNA formed by spacer regions
forming rRNA
-addition of ribosomal proteins (1/3) which enters via nucleoporin (ran cycle)
-forms large macromolecular structure (90S pre ribosome- 45S pre rRNA + protein)
-90S pre ribosomes are modified to form 3 rRNAs
