2 - the nucleus Flashcards
1
Q
what does the nucleus look like?
A
- largest organelle
- one nucleus per cell
- size varies across cells and organisms
- -determined by cell size
- increases during development and in cancer cells
- -helps diagnosis and prognose cancer
- the primary difference between prokaryotes and eukaryotes
2
Q
what are the differences between eukaryotic and prokaryotic nuclei?
A
- the primary difference between prokaryotes and eukaryotes
- eukaryotes have a membrane bound nucleus
- large, more complex interiors
- have single and double membrane bound organelles
- compartmentalization: allows for large size and segregation and organization of specific cellular functions
- organelles have unique and common factors for functioning and their biogenesis, maintenance, and turnover
- prokaryotes have a nucleoid region where the chromosome is located
- less DNA, DNA packaging
- little or no RNA processing
3
Q
what are the two main functions of the nucleus?
A
- compartmentalization of cellular genome and its activities
- coordination of cellular activities
4
Q
what is the significance of the separation from cytoplasm?
A
- allows for unique regulation of gene expression in eukaryotes
- in prokaryotes, mRNAs are translated while transcription occurs
- in eukaryotes, mRNAs have post transcriptional processing before moving out of the nucleus and translated in cytoplasm/ER
- gene expression is also controlled by the nuclear envelope which prevents transcription factors from entering
5
Q
what is the nucleoplasm?
A
- nucleoplasm
- fluid filled interior of nucleus
- highly organized
- has more than 30 specialized regions/subdomains that have specific functions
6
Q
what is the nucleolus?
A
- most obvious subdomains that are irregular shaped, dense, and granular in appearance
- found via electron microscopy
- size and number depend on the metabolic activity of the cell
- function in producing ribosomes
- site of ribosomal DNA gene transcription, rRNA processing, and initial stages of ribosomal subunit assembly
- final assembly of ribosomes occurs in the cytoplasm
7
Q
what are chromosomal subdomains?
A
- chromosomes during interphase are organized into discrete subdomains within the nucleus
- location of a gene is often related to its activity
- most actively transcribed genes are at the periphery of a chromosomal subdomain
- interchromosomal channels: regions between subdomains that act as barriers to prevent unwanted DNA to DNA or to protein interactions
- active genes from different subdomains or different regions on the same chromosomes extend into interchromosomal channels to form transcription factories where transcription factors are concentrated
- interchromosomal interactions occur when gene regulatory regions from one chromosome can activate a gene on another chromosome
8
Q
what are nuclear speckles?
A
- subdomains where mRNA splicing factors are concentrated
- like where pre mRNA processing occurs
- appear as speckles via fluorescence microscopy
- often located in interchromosomal channels next to transcription factories
- numerous and highly dynamic
- move quickly and grow or shrink depending on what the cell needs
9
Q
what is the nuclear matrix?
A
- insoluble protein network distributed throughout the nucleoplasm
- analogous to the cytoskeleton of the cytoplasm
- made of three major filament systems
- microtubules, actin microfilaments, and intermediate filaments
- serves a structural role
- maintains the overall shape of the nucleus
- serves as a scaffold
- organizes nuclear subdomains and anchoring protein factors
10
Q
what is the nuclear envelope?
A
- separates the inside of the nucleus from the cytoplasm
- serves as a barrier
- requires the regulated passage of molecules across it
- establishes the unique composition of the nucleus compared to cytoplasm and spatially regulates gene expression
- provides structural framework for the nucleus
composed of three main parts - nuclear membranes
- nuclear lamina
- nuclear pore complexes
11
Q
what are nuclear pore complexes?
A
- channels in the nuclear envelope
- large and highly complex
- responsible for regulating traffic of substances across membranes
- small polar molecules, RNAs, proteins
- typically 3000 to 4000 per nucleus
- number related to nuclear activity
- NPC is an 8 fold symmetrical structure organized around a large, central aqueous channel
- has several parts
- central scaffold: composed of integral bound/trans membrane nucleoporins
- anchors NPC to the nuclear envelope
- forms an aqueous central channel
- inner surface of the channel is lined by filament like nups
- called FG nucleoporins
12
Q
what are fg nucleoporins?
A
- possess unusual amino acid composition
- mostly hydrophilic polypeptides with short repeats of hydrophobic domains enriched in phenylalanines and glycines (FG domains)
- have a highly disordered 2 degree structure
- extended/flexible organization that fills the central channel
- FG domains extend into central channel
- form a mesh that limits the diffusion of macromolecules larger than about 40 kDa
- small molecules can move freely through the NPC in either direction
- molecules bigger than 40 kDa can’t move through freely and must be selectively imported or exported by an active process
- RNA and most proteins
- there is a size exclusion limit for the NPC
- discovered by using microinjected nuclear protein coated gold particles into cells
13
Q
what are nucleoporins?
A
- composed of about 40 different proteins called nucleoporins/nups
- highly conserved among all eukaryotes
- include integral and peripheral inner and outer membrane proteins
- several nups are related to COPII proteins involved in vesicle formation at the ER
- common evolutionary origin between nups and COPII proteins as they function to highly curve membranes
14
Q
what are y complexes?
A
- include cytoplasm and nuclear rings
- composed of structural nups
- located on the cytoplasmic and nucleoplasmic side of the NPC, respectively
- linked to the central scaffold and the cytoplasmic filaments or the nuclear basket
15
Q
what are cytoplasmic filaments?
A
- cytoplasmic filaments are long filament shaped structural nups that extend into the cytoplasm
- involved in nuclear receptor cargo protein recognition and import from the cytoplasm
16
Q
what is the nuclear basket?
A
- the nuclear basket is a basket like structure made of structural nups located on the nucleoplasmic side of the NPC
- linked to the y complex nuclear ring
- involved in nuclear receptor cargo protein import and export to the cytoplasm
17
Q
what are nuclear membranes?
A
- inner and outer membranes/phospholipid bilayers arranged in parallel
- separated by nuclear envelope lumen
- serve as barriers to the passage of ions, solutes, and macromolecules across the membrane
- outer membrane is continuous with the rough ER
- ribosomes are attached to cytoplasmic surface of the outer membrane
- nuclear envelope lumen is continuous with ER lumen
18
Q
what is the nuclear lamina?
A
- located on the inner surface of the nuclear inner membrane
- network mesh of long, filament like proteins
- ABC nuclear lamins
- evolutionarily related to proteins that form intermediate filaments in the cytoskeleton network
- provides mechanical support to nuclear envelope
- binds to nuclear inner membrane integral proteins
- serves as scaffold for attachment of chromatin and nuclear matrix to the nuclear envelope
19
Q
what happens if the nuclear lamin genes are mutated?
A
- mutations in lamin genes are responsible for several human diseases
- e.g. hutchinson gilford progeria syndrome
- premature aging in children
- due to a point mutation in lamin A gene leading to a truncated lamin protein
- results in breakdown of nuclear lamina
- changes in nuclear envelope morphology and function
