Final Flashcards
Nucleus
largest organelle – typically one nucleus per cell
- nuclear size varies from cell-to-cell and between organisms usually determined by cell size (i.e., cytoplasmic volume)
increases during development and in cancer cells – used for cancer diagnosis/prognosis
primary difference between prokaryotes and
eukaryotes nucleus
Eukaryotes possess a membrane-bound nucleus
Prokaryotes possess a ‘region’ (nucleoid) where chromosome
is located; less DNA, less DNA packaging, and limited/no RNA processing
Differences between prokaryotes and eukaryotes (cell)
Eukaryotes: larger, structurally & functionally more
complex interiors
possess single and double membrane-bound organelles
Cellular “compartmentalization” allows for larger
size and segregation and organization of specific
cellular functions
Each organelle contains both unique and common
factors for functioning (e.g., metabolism) and their
biogenesis (formation), maintenance, and turnover
2 Functions of the nucleus
- Compartmentalization of the cellular genome and
its activities
e.g., site of DNA replication, transcription & RNA
processing
e.g., site where translation components (ribosomes,
mRNAs, tRNAs) are synthesized - Coordination of cellular activities
e.g., control of metabolism, protein synthesis,
reproduction (cell division), etc
Nucleoplasm
the cytoplasm of the nucleus
fluid-filled interior of nucleus – highly organized
* consists of >30 specialized regions (‘subdomains’) that participate
in specific functions (Note: nuclear subdomains are not membrane bound)
* E.g., Nucleolus
* most conspicuous nuclear subdomain - irregular shaped, dense and granular in appearance (via EM)
* size and number (1-5 nucleoli) depend on metabolic activity of cell
(↑cellular activity ↑protein synthesis ↑size/number of nucleoli)
* function in producing ribosomessite of ribosomal DNA (rDNA) gene transcription, rRNA processing, and initial stages of ribosomal subunit (rRNA + protein) assembly final assembly of ribosomes (used for protein synthesis) occurs in
cytoplasm
Where are chromosomes located in nucleus?
Chromosomes during interphase are organized into discrete subdomains within nucleus
* location of a gene is often related to its activity
most actively transcribed genes (i.e., decondensed euchromatin) are found at periphery of chromosomal ‘subdomains
interchromosomal channels
regions between chromosome
subdomains that serve as barriers to prevent unwanted DNA-DNA and/or DNA-protein interactions
Chromosomal subdomains
active genes (euchromatin) from different subdomains
(or from different regions of same chromosome) extend into interchromosomal channels to form transcription factories
where transcription factors are concentrated
interchromosomal interactions
interchromosomal interactions – “kissing chromosomes” gene regulatory regions from one chromosome activate
gene(s) on another chromosome
Nuclear speckles
subdomains (appear as ‘speckles’ via fluorescence microscopy) where mRNA
splicing factors concentrated (i.e., where pre-mRNA processing occurs)
* often located in interchromosomal channels next to transcription
factories
* numerous and highly dynamic – often move quickly and grow/shrink
and change in number depending on needs of cell
Nuclear matrix
insoluble fibrillar-like protein network (‘mesh’) distributed throughout nucleoplasm
* analogous to cytokeleton network in cytoplasm
composed of 3 major filament systems: microtubules, actin microfilaments & intermediate filaments
serves a structural role – maintains overall shape
of nucleus
- serves as a ‘scaffold’ – responsible for organizing nuclear subdomains and anchoring protein factors (e.g., proteins involved in DNA replication, transcription, RNA processing, etc)
- very little is known about composition and
assembly/disassembly of nuclear matrix
Nuclear envelope
separates the contents (e.g., genome) of nucleus from surrounding cytoplasm
serves as a barrier – requires regulated passage of molecules (e.g., RNA and proteins) between nucleus and cytoplasm
- establishes unique composition of nucleus (compared to cytoplasm) and spatially regulates gene expression
- provides structural framework for nucleus
- composed of 3 main parts:
nuclear membranes
nuclear lamina
nuclear pore complexes
Nuclear membranes
inner and outer nuclear membranes - two concentric membranes (phospholipid bilayers)
arranged in parallel
- inner and outer membranes separated by nuclear envelope lumen (10-50 nm diameter)
- membranes serve as barriers to passage of ions, solutes, and macromolecules
between nucleus and cytoplasm
outer nuclear membrane
is continuous with rough endoplasmic reticulum (RER) ribosomes attached to cytoplasmic surface of outer membrane (functionally similar to RER)
nuclear envelope lumen is continuous with ER lumen
inner nuclear membrane
unique protein composition (functionally distinct from outer membrane)
- outer and inner membranes joined (highly curved) at nuclear pore complexes
nuclear lamina
ocated on inner surface (i.e., nucleoplasmic side)
of nuclear inner membrane
* network (‘mesh’) of long, filament-like proteins
ABC nuclear lamins – evolutionarily related to proteins that form intermediate filaments
in 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 nuclear envelope
ABC nuclear lamins
evolutionarily related to proteins that form intermediate filaments
in cytoskeleton network
Hutchinson-Gilford Progeria Syndrome
mutations in LAMIN genes responsible for several human diseases
rare, characterized by premature aging in children
(e.g., hair loss, wrinkles, artery damage) – death by early adolescence due to a point mutation (sporadic – occurs during in embryo development) in LAMIN A gene leading to truncated lamin protein results in destabilization /breakdown of nuclear lamina causes aberrant changes in nuclear (envelope) morphology
and function recently….promising advances using CRISPR/Cas9 genome
editing-based (gene) therapy in mice
NPC
Nuclear Pore Complex
channels (‘doorways’) in nuclear envelope
- responsible for regulated trafficking (import & export) of all substances between nucleus and cytoplasm
small, polar molecules (e.g., nucleotides for DNA/RNA synthesis)
RNAs - mRNA, tRNA, rRNA
proteins - e.g., transcription factors, RNA-binding proteins, ribosomal (subunit) proteins, and cyclins [see later]
- typically, 3000-4000 per nucleus – number of NPCs related to
nuclear activity
Nups
nucleoporins
~40 different proteins in NPC
highly conserved among all eukaryotes
include both integral and peripheral inner and outer
nuclear membrane proteins
overall structure of NPC:
8-fold symmetrical structure organized around large, central aqueous channel
Parts of the NPC:
central scaffold
FG nucleoporins
Y complex
cytoplasmic filaments
nuclear basket
central scaffold
part of NPC
composed of integral/trans
membrane-bound nucleoporins
- anchors NPC to nuclear envelope membranes (at junction of outer and inner membranes)
- forms aqueous central channel (~20-40 nm wide pore)
FG nucleoporins
inner surface of channel lined by ‘filament-like’
Nups - FG nucleoporins
possess unusual amino acid composition
hydrophilic polypeptides with short repeats of
hydrophobic domains enriched in phenylalanines and glycines (‘FG domains’)
