Week 8 Flashcards
What make up the cytoskeleton?
Actin filaments,
Intermediate filaments,
Microtubules
What is the important function of the cytoskeleton?
Muscular movement to the transport of molecules thorough out the cell.
Normal embryonic development.
Abnormalities in the cytoskeleton result in what?
Diseases affecting every tissue in the body.
Many drugs work by targeting the cytoskeleton. And can be cancer treatment by interupting cell division
What is the structure of microfilaments?
Two intertwined strands of actin, each polymer of actin subunits
7nm in diameter
Function of microfilaments?
Maintenance/change of cell shape,
Muscle contraction,
Cytoplasmic streaming,
Cell motility,
Cell division,
Exo/endocytosis,
Cell junctions
What are microfilaments made of?
G and F actin
(G assembles into long, helical F-actin polymers)
ATP holds together the two lobes of the actin monomer
What are cellular extensions driven by?
Actin polymerization
Do microfilaments have microvilli?
Yes.
Allows movement of cells,
Increased absorption,
Also allows sound absorption
What are actin motor proteins?
Myosins (I, II, V (different functions according to shape)
hey use energy derived from ATP hydrolysis to “walk” along actin filaments.
Enables roles in contraction or transportation
What is Phalloidin?
Toxins that bind to F-actin preventing disassembly.
Can be used as a florescent tag to stain actin.
Treatment can be eating red meat.
true or false, actin is a globular protein?
true
What is the structure of microtubules?
Hollow tubes, wall consists of 13 columns of tubulin molecules
25nm diameter, with 15nm lumen
What are the functions of microtubules?
Maintenance of cell shape,
Cell motility,
Chromosome movements in cell division,
Organelle movements
What are the protein subunits of microtubules? (tubulin polymers)
Tubulin, a dimer consisting of a-tubulin and b-tubulin
What is the importance of microtubule-binding drugs?
Used to treat a variety of diseases, including cancer.
EG the cancer drug taxol used for ovarian cancer, breast cancer and lung cancer which prevents microtubule disassembly and therefore cell division in cancer cells.
Size of cilia vs flagella?
Cilia - 2-20um
Flagella - 10-200um
However, both have the same stucture
What are the two main types of microtubules?
cytoplasmic - more dynamic and loosely organized found in the cytoplasm.
axonemal - highly organized and stable found in cilia flagella and basal bodies
how are microtubules formed?
By polymerization of tubulin dimers
What is the role of cytoplasmic microtubules?
maintenance of cell shape, cell movement, transport of vesicles within the cell and the separation of chromosomes during cell division.
What is the structure of intermediate filaments?
Fibrous proteins supercoiled into thicker cables.
8-12nm in diameter
Intermediate filament protein subunits?
One of several proteins (eg keratins), depending on cell type
Functions of intermediate filaments?
Maintenance of cell shape,
Anchorage of nucleus and certain other organelles,
Formation of nuclear lamina,
What can intermediate filament abnormalities lead to?
Amyotrophic lateral sclerosis (ALS)
What is the main function of cell junctions?
Form a barrier, provide structure
What are impermeable cell junctions?
Prevent passage of molecules between cells
eg tight junctions
What are Adhesive cell junctions?
Mechanically hold cells together
eg Adherens junctions, Adhesive Desmosomes
What are communicating cell junctions?
Passage of small molecules between cells
Eg gap junctions, chemical synapses
What is the fence function of tight junctions (impermeable junctions)?
Separate the apical plasma membrane from the basal plasma membrane, thus allowing them to have different compositions. This also gives rise to cellular polarity.
What is the barrier/gate function of tight junctions (impermeable junctions)?
Prevent molecules from leaking between adjacent cells.
What is the importance of tight junctions in the stomach?
keep the apical membrane components separate from the basal membrane components.
In the gut, this stops acid being pumped into the blood stream.
What does the apical membrane contain?
Proton and chloride ion pumps that generate HCl.
Tight junctions, keep the components in this membrane separate from basal membrane components
What are the two types of Tight Junctions?
Transmembrane protein (physical barrier, adhesion, permeability)
Cytosolic proteins (Scaffolding, signalling, polarity)
What are the four types of anchoring junctions?
Adherens, desmosomes, focal adhesions, hemi-desmosomes
What are Adherens Junctions (Anchoring junctions)?
Mainly Cadherins,
Connect cells to actin filaments,
Intracellular signalling regulator
(enable stretching)
What are desmosomes (Anchoring junctions)?
Mainly cadherins (eg desmoglein, desmocolin),
Connect cells to intermediate filaments
Structural integrity, withstand mechanical stress
What are focal adhesions (Anchoring junctions)?
Mainly integrins
Connect (extra-cellular matrix) ECM to actin filaments
What are hemi-desmosomes (Anchoring junctions)?
Mainly Integrins,
Connect (extra-cellular matrix) ECM to intermediate filaments
What do hemi-desmosomes do?
Attach the cell base to the basal lamina forming basement membranes, linking to the cytoskeleton
Features of gap junctions (communicating junctions)?
- Most common
- In tissues of animals
- Are tiny pores composed of 6 connexins on one cell (proteins) (each has 12 form the junction)
- Communication function, pass electrical signals, signalling molecules, share ions
- Permeability depends in intercellular pH and intracellular Ca levels
How many connexins does a gap junction have?
12 connexins (6 from each cell) form the gap junction
They form cylindrical channels
Where are gap junctions abundant?
Cardiac and Muscle Tissue
Why do cells divide?
Growth,
New organisms (unicellular organisms),
Cell replacement
What is binary fission?
Simple process where bacteria duplicate their DNA and segregate their chromosome into two daughter cells
Why is eukaryotic cell division more complex than prokaryotes?
Nuclear membrane
Multiple chromosomes
Organelles
Process of binary fission?
What is G1 (Gap) phase of cell division?
Cell grows,
Prepares to replicate DNA
What is S (synthesis) phase of cell division
Cell grows,
Synthesis of duplicate DNA ready for mitosis
What is G2 phase of cell division?
Cell grows,
Prepares for mitosis: DNA is checked for errors
Define mitosis
Physical process of segregating chromosomes into daughter cells
Summarise M phase of cell division (mitosis)
Chromosomal segregation and cell separation (mitosis and cytokinesis)
What is interphase of cell division?
Cell grows, replicates its DNA and gets ready for mitosis
Contains phases: G1, S, G2
G2 of interphase?
Intact nuclear envelope,
Chromosomes replicated - remain indistinct as loosely packed chromatin fibres
Centromere replicated (MTOCs)
Microtubules extend radically forming asters
What form asters during G2 interphase?
Microtubules
Prophase?
Chromatin fibres condense, forming discrete chromosomes
Nucleoli disappear
Centrosomes move away from each other
Mitotic spindle begins to form
Prometaphase?
Breakdown of nuclear envelope,
Some microtubules attach to chromosomes at their kinetochores
Other microtubules interact with those from opposite poles
Metaphase?
Centrosomes at opposite poles
Chromosomes align on the metaphase plate
Sister kinetochores attached to microtubules coming from opposite poles
Anaphase?
Begins with separation of centromeres,
Sister chromatids move towards opposite poles of cell
Each chromatid becomes a new chromosome
Poles move further apart
Telophase?
Elongation of cell by microtubules
Daughter nucleoli begin to form at poles of cell
Nuclear envelopes form
Chromatin begins to decondense
During telophase, what is also occurring?
Cytokinesis where a cleavage furrow forms pinching and pulling apart the cytoplasm/cell membrane to form 2 daughter cells
What are the stages of mitosis?
Interphase (G1, S, G2)
Prophase
Prometaphase
Metaphase
Anaphase
Telophase (cytokinesis)
What does the mitotic spindle consist of?
Tubules
Centrosomes (organises microtubules)
Chromatid pairs (arranged on the metaphase plate)
What are tubules within the mitotic spindle split into?
Astral
Kinetochore (pull chromosomes to opposite poles of the cell)
Non-kinetochore/polar (involved in elongation of the cell
What is the function of the mitotic spindle?
To organise chromatids along the metaphase plate and then pull sister chromatids apart
How do astral microtubules work? What does it use to function?
Uses dynein
Pull astral microtubules towards poles during prophase,
Tubules de-polymerise and shorted
Hold astral tubules in place during metaphase and later
What do kinetochore tubules do? What do they use to function?
Uses dynein
Attach chromosomes to tubules
Pull on tubules during anaphase - chromosomes move towards centrosomes
Tubules de-polymerise and get shorter
What do non-kinetochore/polar tubules do? What do they use to function?
Uses kinesin
Motors attached to tubule from either side where polar tubules overlap
Motors push tubules away in opposite directions during meta/anaphase
Tubules polymerise and get longer
Cell elongates
How is chromosomal separation/segregation achieved?
By a combination of pushing (kinesin) and pulling (dynein) forces
How does dynein provide a pulling force during mitosis?
Kinetochore motors pull chromosomes towards centrosome/pole
Astral motors pull centrosomes towards inner face of plasma mem
Both shorted depolymerise tubules
How does kinesin provide a pushing force during mitosis?
non-kinetochore/polar motors polymerises tubules to drive poles of spindle apart, this elongates cell to aid telophase/cytokinesis
How are chromosome pairs separated by mitotic spindle fibres in anaphase?
Proteins holding sister chromatids together are inactivated so separate.
Kinetochore tubules use dynein to ‘walk’ a chromosome to nearest pole
Tubules shorted by depolymerisation at the kinetochore ends
Non-kinetochore tubules elongate whole cell
Describe cytokinesis in animal cells?
Microfilaments form a ring at the furrow
Ring contracts - owing to interaction between actin and myosin filaments
Furrow deepens until cell is pinched in two
Cytokinesis in plant cells?
Cell plate forms at equatorial plane of the cell
Cell wall forms - from plate contents
What is a spot desmosome?
Spot desmosomes spot-weld cells together, attached on the inside of the cell to keratin filaments, which serve to spread the stresses from the spot desmosome throughout the cell
What is step one of binary fission?
Chromosome replication begins. Soon after, one copy of the origin moves rapidly toward the other end of the cell
Step 2 binary fission?
Replication continues. One copy of the origin is now at each end of the cell
Step 3 binary fission?
Replication finishes. The plasma membrane grows inward, and new cell wall is deposited.
Two daughter cells result
What is the medical significance of gap junctions?
Gap junction channels are reduced in number or changed in distribution in a variety of cardiac diseases .
Cellular functions of connexins related to medical diseases?
Mediate electrical communication to effect contraction and motility
Autism Spectrum disorder
Cellular functions of connexins related to medical diseases?
Allow passage of molecules and ions between cells
Gastrointestinal infections
Cellular functions of connexins related to medical diseases?
Modulate cell polarity and directional migration
Inflammatory bowel disease
What is the Cadherin/Catenin complex within Adheren junctions?
E-cadherins are transmembrane proteins (in epithelial phenotype) that occur within the paracellular space between epithelial and endothelial cells. It binds to cytosolic partner (b-catenin) and to the e-cadherin on adjacent cell which forms the barrier. This is a calcium dependent process
Cadherin/Catenin complex is involved in signal transduction, for example regulation of wind-signalling which occurs during development.
What happens if cadherin bonds between epithelial cells are broken?
If cadherin bonds between epithelial cells are broken, (eg no Ca) this leads to depolarisation and dedifferentiation of epithelial cells - they’d start to lose their apical and lateral domains, and they become more rounded.
What is treadmilling?
At positive end of the F-actin, G-actin monomers form and bind onto filament.
At negative end G-actin monomers disassemble.
G-actin is an ATPase so only ATP can be added to + end of filament and addition of G-actin disassociates at - end, this occurs simultaneously so the full length stays the same at any one point.
So the addition at the positive end and loss at negative end retains the original length of filament.
What is diapedesis?
The process of cells changing shape to be able to fit through small spaces such as those between cells.
For example, white blood cells do this within the blood.
Another process driven my actin polymerisation is cell extensions. How?
Formation of structures like filopodia, lamellipodia, pseudopodia. This distorts cell shape allowing for cells to sense their environment or polarise following a chemoattractant for example. And this process plays an essential role during fertilisation for example.
(Cytoplasmic streaming is this but in plant cells and how amoebae move within their environment)
How do actin and myosin II have essential roles in cytokinesis?
A ring of actin filaments known as cleavage furrow separates the cytoplasm and cell membrane from each other, allowing the formation of two genetically identical daughter cells
How do microtubules self-assemble?
by binding of a-tubulin and b-tubulin make a dimer and this dimer is arranged into a spiral formation, leading to a polymer formation: resulting in a hollow tube.
What does tubulin require, unlike microfilaments, to form?
Guanine Triphosphate (GTP).
Therefore, each tubulin monomer contains a GTP binding site
What happens when GTP is bound to a-tubulin compared to b-tubulin?
When GTP is bound to a-tubulin, it cannot be hydrolysed; its trapped at the dimer interphase. But when bound to B-tubulin, it can be hydrolysed to GDP. So GTP bound tubulin favours growth and forms strong bonds that don’t break, so when dimers are added the positive end is stable. Hydrolysis to GDP favours disassembly as bonds are weaker.
What happens if all GTP is hydrolysed in tubulin?
If all GTP hydrolysed the tubular fall apart in process called catastrophe and is rescued when GTP bound dimers stabilise the filament again.
Constant recycling of GTP hydrolysis to GDP is known as what? What does this lead to?
Microtubule dynamic instability.
Leading to a conformational change, generating energy required for cell movement to occur.
(And this rate of growth takes place about every 20 seconds, Rate of catastrophe occurs every 70 seconds, during the loss over 1000 dimers can be lost per second. But this drives the movement of microtubules.)
What is Taxol, how does it interfere with Tubulins (microtubules)?
Taxol binds and stabilises microtubules (tubulin) which increases polymerisation. It attaches to the b-tubulin and prevents disassembly, interferes with its function and therefore has profound affects.
How is Taxol used in the clinic as an advantage, despite its negative affects on b-tubulin?
tubulin has a role in the cell cycle, so if we can prevent disassembly and mitotic spindle cannot be formed we can stop cell division, so can be used as a cancer therapy.
What is the structure of a microtubule?
They consist of a core axonemal microtubule ensheathed in an extension of the plasma membrane.
9 doubles of microtubules are arranged in a ring and connected to 2 central microtubules by radial spokes - 9 + 2 arrangement.
How are microtubule doublets connected?
Sidearms composed of Dynein, which contracts at the expense of ATP forcing the doublets to move relative to each other.
Crosslinks prevent the doublets sliding past each other, so that the flagella or cilia bend.
How does nexin crosslinking in microtubules cause “walking” moves within flagella and cilia?
Nexin forces tubules to bend, the force causes the tubules to eventually flick which generates the wave-like motions in cillia/vili
Prokaryotic flagella vs eukaryotic flagella?
Prokaryotic: Made of protein subunits, protrude through cell wall and cell membrane, stiff and twirl like a propeller
Eukaryotic: A bundle (9+2) of microtubules (made of protein), covered by cell membrane, whipping action
What is the function of Kinesin and dynein microtubule motor proteins?
Both transport membrane bound vesicles, proteins and organelles along microtubules at the expanse of ATP.
What is the difference between kinesins and dynein microtubule motor proteins?
Kinesins move cargo towards + end of microtubules, while dynein transport cargo towards - end of microtubules.
Where are intermediate filaments usually found?
Cardiac tissue
Polymerisation of what protein dimers form this component of the cytoskeleton?
Alpha-beta Tubulin
