BIOL 2020 Flashcards
1
Q
Briefly describe the movement of cargo through the endomembrane system
A
Protein transcribed in cytosol, binds to ER membrane and moves into lumen or stays on membrane, goes to golgi for processing via vesicles, then post-golgi vesicle to either PM or extracellular or lysosome.
2
Q
What does pulse-chase mean in terms of the experiment?
A
Pulse is the incubating of the cells with radioactive proteins, and the chase is the wait time that varies to see where proteins are in the cell at different times/steps of the secretory pathway.
3
Q
How is GFP used to track proteins?
A
GFP fusion with cell protein by incorporating GFP into corresponding DNA so that protein is green fluorescent once translated.
4
Q
What are microsomes? What is an experiment that allows them to be obtained?
A
Microsomes are functional golgi and ER membrane vesicles that are created during the homogenization of the cell. One the homogenization and centrifugation takes place, the post-nuclear supernatant can be further centrifuged to obtain a microsome pellet.
This technique is known as sub-cellular fractionation, as these microsomes can be sorted further via a density centrifugation.
5
Q
What is the principle behind sec mutants?
A
If you don’t know how something works, break it and see what happens.
6
Q
Whose work with sec mutants allowed us to see the different classes of secretory mutations?
A
Randy sheckman (2013 Nobel Laureate) worked with yeast and selected heat sensitive mutant yeast to investigate where different different mutation would affect the sec pathway.
7
Q
What are the 5 classes of sec mutants based on the accumulation of protein?
A
Class A: accumulate in cytoplasm before ER binding
Class B: accumulate in ER (no budding)
Class C: accumulate in post-ER vesicles (no golgi fusion)
Class D: accumulate in golgi (no budding)
Class E: accumulate in sec vesicles (no PM fusion or exit to extracellular).
8
Q
What are 3 main function of the smooth ER?
A
- Calcium storage (important in cell signalling).
- Steroid hormone synthesis
- Detoxification
9
Q
Where do free ribosomes and memb-bound ribosomes direct their proteins to go?
A
Free ribosomes: remain in cytosol, nucleus, peripheral cytosolic leaflet proteins, proteins for mitochondria.
Memb-Bound ribosomes: proteins for secretion, integral membrane proteins, proteins destined to remain in an endomembrane compartment.
10
Q
How do membrane bound ribosome proteins and free ribosome proteins differ in terms of their import mechanism?
A
Membrane-bound ribosome proteins are involved in co-translational import.
Free ribosome proteins are imported post translationally.
11
Q
What is the signal hypothesis and who created it?
A
Gunter Bloebel (1999 Nobel laureate) created the signal hypothesis which has 3 points:
- All ribosomes are the same.
- Amino acid signals on new proteins (the signal sequences) direct growing polypeptides where to go (tells free ribosome to go the ER or tells ER ribosome to deposit).
- That protein will then be fed into ER lumen at the same time as being translated (co-translational import).
12
Q
Describe the translocons role in co-translational import.
A
The translocon is located on the ER membrane and has 3 regions: the SRP receptor, the pore, signal peptidase.
The SRP (signal recgongnition particle bound to peptide also binds to SRP receptor on translocon.
After this, the polypeptide is red through the pore and into the ER lumen.
Once the polypeptide is in, the singal peptidase cleaves the singal sequence on the polypeptide (allows release of polypeptide).
13
Q
What is SRP?
A
Signal recognition particle recognizes the signal sequence on an mRNA, binds to it, and based on that signal/what SRP binds, the polypeptide can migrate accordingly via the facilitation of the SRP.
It is composed of 6 polypeptides and some RNA.
14
Q
How did sub-cellular fractionation experiments provide evidence for Bloebel’s theory?
A
In this experiment, 2 vitro and 1 vivo condition samples were examined using gel electrophoresis monitoring the length of proteins. In one test tube (vitro 1) it contained RNA, amino acids, and ribosomes. The second test tube(vitro 2) had all the same plus microsomes. When comparing size of proteins, test tube 2 with microsomes showed polypeptides of similar length to that of the vivo sample, whereas test tube 1 lacking microsomes (and therefore the translocon) showed larger proteins.
This can be explained as the lack of microsomes/ER and golgi membranes lacked the translocon and therefore the signal sequence could not be cleaved, resulting in larger proteins.
15
Q
What kind of transmembrane protein would be created from an internal stop-transfer sequence and terminal ER signal sequence?
A
COO- terminal on cytosolic side and amino terminal inside lumen.
16
Q
What kind of transmembrane protein would be created from a single internal start-transfer sequence?
A
Amino end of cytosolic side and COO- end in the lumen
17
Q
What is a multi-pass integral membrane protein?
A
Once with multiple stop/start transfer sequences so that there are multiple portions of protein on cytosolic side and inside the lumen.
18
Q
What enzyme is responsible for adding sugars to dolichol phosphate on cytoplasmic side on ER?
A
Glycosol transferases add carbohydrates to lipid (dolichol phosphate) on cytoplasmic side.
19
Q
What enzyme is responsible for flipping the sugar chain in dolichol phosphate over to the lumen side? What is this flipping process called?
A
Translocation is the process in which flippase coordinates the flip of the sugar chain attached to dolichol phosphate to lumen.
Sugars are continued to be added the sugar chain once
20
Q
What sugars on the sugar chain thats attached to dolichol phosphate complete the chain? What can happen after this sugar chain is complete?
A
3 glucoses complete the sugar chain, allowing oligosaccharide protein transferase to remove the sugar chain (core oligosaccharide) off of dolichol phosphate and add it to an asparagine of a protein.
21
Q
What does calnexin do? What happens if it doesn’t do its job properly the first time?
A
Calnexin is a molecular chaperone that ensures proper folding of the newly created protein. Once calnexin ensures proper folding, the final glucose can be removed via glucosidase 2 (recall that the final oligosaccharide on the protein has 3 glucoses, 2 are removed before calnexin, and this last one is removed after).
If calnexin “didn’t do its job properly” (i.e., protein is mis-folded, indicative of hydrophobic regions that are not tucked away) UGGT (a glycosol transferase) can recognize this improper folding and add another glucose to that calnexin can try again.
22
Q
What happens if protein will not fold properly after several cycles of UGGT calnexin interactions?
A
A transporter will recognize this protein that won’t fold properly and will feed it back into the cytoplasm via reverse translocation to be destroyed by a proteasome.
23
Q
Describe the structure of the proteasome.
A
A barrel shaped protein degrading machine.
Consists of a cap at each end that binds to ubiquitin-tagged proteins (meaning tagged for degradation).
Proteases that do the actual degrading of the protein.
ATPases that drive the degradation process by powering the proteases.
24
Q
What enzymes are involved in tagging a protein with ubiquitin?
A
E3 is a ubiquitin ligase that recognizes the misfolded protein intended for degradation and will transfer ubiqutins, being carried by the other 2 enzymes: E1 and E2, onto this protein.
Once protein is polyubiquitinated it can bind to the cap(s) of the proteasome.
25
What molecular chaperone is involved in the unfolded protein response occuring when too many unfolded proteins are present?
Bip/Hsp70.
26
Briefly describe the process of protein processing in the ER, beginning at entry.
The protein is first transported into the ER lumen via the translocon. Post translational modifications taken place in the ER, such as glycosolation. N-linked glycosolation, in specific, is involved in quality control. Quality control ensures the proper folding of proteins. If a protein will not fold properly, it can exit the ER by reverse translocation to be destroyed by the proteasome.
27
What does Bip do when there are too many unfolded proteins in the ER?
Bip abandons the sensors on the ER membrane to help with protein folding.
28
What are the two ways that the Bip sensors can be activated once Bip leaves?
They can either a) dimerize, to phosphorylate elF2a which will then bind to the small subunit of ribosomes to slow translation, and/or b) Undergo proteolytic cleavage so that the cytosolic portion of the censor can go to the nucleus and act as a transcription factor for genes associated with alleviating stress of misfolded protein response.
29
Who was the golgi complex named after?
Camillo Golgi, 1906 nobel laureate.
30
What is the function of the golgi?
Further post-translation modifications of proteins, specifically glycosylation (mainly further N-linked processing occurs).
There are many different glycosolation processes taking place which is why the golgi is comprised of separate compartments.
31
What is the older, no longer accepted model of the golgi?
The vesicular transport model: stating that cargo proteins are moved through the golgi cisternae via vesicles, indicating that the cisternae are stationary and the cargo passes through them by continuous vesicle budding.
32
Explain the cisternal maturation model.
The cisternal maturation model is the currently accepted-model of the golgi, and states that the cis cisternae is created by fusion of post-ER vesicles in the ERGIC. These vesicles, at the CGN, then mature becoming medial cisternae, and eventually reaching the TGN where vesicles can bud off.
In this model the cisternae are then not stationary, rather they mature, and the only vesicles involved are at first the fusion at CGN, budding at TGN, as well as the vesicles involved in transporting the resident enzymes.
The golgi resident enzymes move via retrograde vesicle back to their home cisternae as the cisternae matures.
33
What are the 3 main evidence points for the cisternal maturation model?
1. Labelling cargo proteins only ever reveal them in cisternae, never in vesicle while in the golgi network. This indicates maturation rather than vesicle transport.
2. Golgi-resident enzyme labelling shows them moving in vesicle and in cisternae in a retrograde fashion, suggesting they have a home compartment they are constantly being moved from.
3. Sec mutants involved in the inhibition of ER vesicle budding show that the golgi disappears, suggesting that the budding vesicles (which can't occur in these mutants) creates the golgi cisternae, explaining why the golgi disappears.
34
What are the 2 main functions of coat proteins?
1. Membrane curvature: assembly of coat proteins creates a mechanical force driving membrane to curve into vesicle shape.
2. They dictate what can enter the vesicle (via receptors) and what can interact with the membrane.
Different coat proteins are involved with different types of vesicles.
35
Explain what vesicles are involved with COP2, COP1, and clathrin coats.
COP2: ER to golgi movement, anterograde.
COP1: Golgi to ER, or TGN to CGN, retrograde.
Clathrin: TGN to out of cell, or PM to endosomes.
36
What monomeric G protein is involved with COP2 vesicles?
Monomeric G protein (a.k.a GTPase) involved wit COP2 vesicle formation is a Sar1.
37
What are the 2 adaptor proteins involved with COP2 vesicles?
Sec 23 and sec24, they form a heterodimer that binds the cytoplasmic tail of the transmembrane cargo receptors, Sar1 (it recruited them), and binds the outer-coat polypeptides.
38
What are the 2 outer coat polypeptides on COP2 coated vesicles?
Sec 13 and Sec31, which enable the vesicle to bud off.
39
How do the coat proteins fall off the vesicle?
The monomeric G protein hydrolyzes GTP.
40
Where would protein accumulate in a sec 23, 24, 13, or 13 mutant?
Sec 23, 24, 13, and 31 proteins are all involved in COP2 vesicle formation, which are the vesicle that move from the ER to the golgi. So if these proteins were mutated, they would not be able to bud from the ER and therefore they would accumulate there.
41
What is the GTPase (monomeric G protein) and outer coat proteins involved in COP1 vesicle formation?
Arf 1 is the GTPase, and there are 7 different outer coat proteins that form a coatamer with triskellion shape (b1 and alpha components).
42
Where do COP1 vesicles move in the cell?
They move retrogradely either from TGN to CGN, or from golgi to ER (i.e., when returning ER proteins back after COP2 delivered them).
43
How do proteins know to move retrogradely in COP1 vesicles?
The ER enzymes that need to move retrograde has a specific KDEL sequence (lys-asp-glu-lue) that can bind to specific receptors that allow them to move back to home compartment via COP1 vesicles.
44
Describe the structure of clathrin coated vesicles (outer coat).
Triskellion shape with 3 heavy chains and 3 light chains that can form a hexagon or pentagon shape. Hexagon shape is flat and can convert to pentagon shape to get rounded formation. Triskellion forms lattice around vesicle.
45
What are the 2 types of adaptor proteins that can recruit clathrin?
1. GGAs: vesicles going from TGN to endo/lysosomes via anterograde motion. The GTPase for this is Arf1.
2. AP2: vesicles coming from PM into endo/lysosomes via retrograde movement. The GTPase for this is no specified.
46
What is dynamin and what is its function?
Dynamin is a monomeric G protein that forms a ring around the clathrin coated vesicles. Once hydrolyzed, it pinches off the vesicle to be released.
47
What do GTP gamma S do?
Dynamin bound to GTP gamma S is activated, similarly to when its activated bound to normal GTP, but with gamma S it cannot hydrolyze and therefore the vesicle cannot be pinched off. This forms a neck like structure that the vesicle and membrane are attached to.
48
Describe the function of the lysosome and its part in the secretory pathway.
The lysosome is involved in intracellular digestion. It contains acid hydrolyases (meaning they are optimal in acidic conditions) and they degrade all of the main macromolecules.
They contain proton pumps to make themselves acidic.
Acid hydrolyases are made in ER, tagged with mannose-6-phopsphate in the golgi, and transported to lysosome due to their tag.
The low pH of the late endosome (right before it is converted to the lysosome) dissociates the receptor from the acid hydrolase (tagged with mannose 6 phosphate), so that the receptor can be recycled.
49
What kind of vesicles are destined to go to the lysosome?
Clathrin coated vesicles that contain acid hydrolyases tagged with mannose 6 phosphate.
Their adaptor protein GGA binds mannose 6 phosphate/receptor, Arf1 GTP, and clathrin outer coat.
50
What are Rabs and SNAREs?
Rabs are small GTP binding proteins that specify vesicle destination, approximately 60 types of them. They associate via a lipid anchor and recruit tethering proteins to loosely attach the vesicle.
SNAREs are membrane proteins that mediate vesicle fusion with alpha helix domains (coil to coiled): t SNARE on target membrane that will associate with v SNARE on vesicle membrane. This is the docking step of vesicle fusion.
51
Briefly describe the process of vesicle fusion that RABs and SNARES are involved in.
1. Target transport vesicle via lipid anchor.
2. Tethering occurs when rabs recruit tethering proteins to loosely attach the vesicle (tethering proteins connect to vesicle holding it in place).
3. Docking, now that vesicle is close by the help of tethering proteins, SNAREs coil together to pull vesicle close to PM.
4. Fusion is promoted by SNARE interaction .
5. Dissociation occurs when NSF uses ATP to untwist the SNAREs, release them along with Rab.
52
What vesicles are formed during receptor-mediated endocytosis and what coat proteins mediate this?
Clathrin coated vesicles are created in receptor-mediated endocytosis. There is no GTPase, rather ligand and receptor interaction causes diffusion to make a coated pit, where an invagination occurs and clathrin begins coating on cytoplasmic side.
Adaptor protein AP2 (4 polypeptide complex) facilitates this by connecting to the clathrin, the receptor (that extend through PM to connect to ligand.
53
What are the 2 main differences in receptor mediated endocytosis vesicle formation?
After this recap different dissociation of coat proteins in the other types of vesicles.
1. There is no monomeric G protein (rather ligand receptor interaction that diffuses to form coated pit).
2. In the other types of vesicles, the GTPase was what was responsible for the uncoating, since there is no GTPase here, and ATPase needs to come in and bind to coat proteins, hydrolyze, and allow dissociation.
Recall:
Arf1 invovled in clathrin coats from TGN is GTPase that can hydrolyze at end to dissociate coat proteins.
Arf 1 involved in COP1 coated vesicles undergoing retrograded movement is a GTPase that can hydrolyze to dissociate coat proteins (but no adaptor protein specified, rather their KDEL sequence on these proteins allow binding to various receptors).
Sar1 involved in COP2 coated vesicles undergoing anterograde movement from ER to golgi is a GTPase that can hydrolyze to dissociate coat proteins.
54
How are receptor-ligand complexes released after receptor mediated endocytosis?
After the ATPase uncoats the clathrin coat (releases clathrin and AP2), you are left with vesicle containing the cargo, but the cargo is still attached to the receptors from the initial coated pit formation. When the vesicle fuses with the early endosome, the pH increases slightly which reduces receptor ligand affinity, receptors are released back to PM.
55
After the clathrin coated vesicle fuses with late endosome what does it fuse with?
Can then fuse to form lysosome, sometimes fuses with clathrin vesicles coming from TGN, where pH increases and allows hydrolyases to be activated.
56
What are methods of acidification for the lysosomes and endosomes?
1. Proton pump in the lysosome.
| 2. Endosome fuses with lysosome that can take on new cargo.
57
What is phagocytosis?
The engulfment of relatively large particles, NOT facilitated by receptor-ligand interaction like in receptor mediated endocytosis. Here, pseudopods form to engulf materials which fuses with early endosome, and eventually lysosome for digestion.
58
What is autophagy and who studied it?
Yoshinori Ohsumi won nobel prize in 2016 for his work with autophagy, which is the destruction of organelles by isolation in a double membrane followed by fusion with a lysosome. He showed a mitochondria being sequestered in a autophagosome.
59
How do proteins get to the mitochondria?
They get there due to transit sequences that direct them there by binding to Hsp 70 and Hsp 90, which delivers them in an unfolded state.
60
What are the 2 fates once a protein reaches the mitochondria for entry?
Once it enters into the IM via the TOM (Transport outer membrane), it can either go through TIM 22 if it wants to reside in the IM, or through TIM 23 if it wants to go to the matrix.
Once in the matrix it binds to Hsp 60 chaperone, and mitochondrial transit peptidase will cleave its transit sequence.
61
What are each of the 3 main cytoskeletal elements made of (what are they polymers of?)
1. Microtubules are polymers of tubulin
2. Microfilaments are polymers of actin
3. Intermediate filaments are polymers of varying proteins
All polymers have non covalent linkages necessary for their dynamic nature.
62
Compare alpha and beta tubulin.
Similar amino acid sequence and 3D conformation allowing for easy linking, however, b tubulin has ability to hydrolyze GTP to GDP, so b tubulin may have GDP or GTP cap and alpha tubulin will only ever have GTP cap.
This creates a plus and minus end on MT. Plus end is b tubulin end, and minus end is alpha tubulin end.
63
Explain the role of Tau in the cell and what can happen if it is mutated.
Tau is a structural/classical MAP that helps maintain the structure of MTs. If mutated, MT structure can unstable, resulting in tangled MTs and Tau clumps. Clinically speaking, this can lead to frontotemporal dementia.
64
How were motor proteins first discovered.
Motor proteins were first discovered when studying the squid giant axon and vesicles were seen moving in both directions along the axon.
65
Compare dynein and kinesin.
Dynein moves toward minus end of cell and kinesin moves toward positive end of cell.
Both have head regions that hydrolyze ATP and tail regions that bind cargo.
Dynein requires dynactin to help bind MT and vesicle.
66
Explain the hand over hand mechanism seen with Kinesin.
1. ATP binding to leading head
2. This triggers the trailing head to swing in front (power stroke)
3. MT binding of new leading head takes place
4. ATP hydrolysis of new trailing head and ADP is released from leading head.
67
What are the 2 exceptions to kinesin that do not move toward plus end.
Kinesin 13 doesn't move.
| Kinesin 14 moves toward minus end.
68
Explain how kinesin motility assays work.
First secure the kinesin tails to coverslip preventing movement of kinesin.
Then add MT stained.
Then watch MT slide along kinesin head, this occurs because kinesin can move so MT does instead as a result of the head hand over hand mechanism.
69
Where are MTOCs in the cell and what is their function?
They are located perinucleur and near centre of cell.
The function to organize MT and as a result orient kinesin-mediated and dynein-mediated transport of vesicles, organelles, and vesicular tubular clusters.
70
What are centrosomes made of?
2 perpendicular centrioles and pericentriolar material (PCM).
Centrioles are composed of 9 triplet microtubules.
71
What is PCM and what is its significance in the cell.
PCM is a diffuse granular matrix that surrounds the centrioles. It seems to be recruited by centrioles to aid in MT nucleation.
It nucleates MT by providing a basis for growth: first has the base called gamma-tubulin ring complex, which is topped with a gamma tubulin ring that allows for alpha and beta tubulin to begin attaching.
72
Describe the structural hierarchy of MTs.
Tubulin comprises MT, and exists as a heterodimer of alpha and beta tubulin. These heterodimers form protofilaments, which then can make up a MT when 13 are linked together.
73
Explain dynamic instability model associated with MTs.
Growth can only occur at the beta/plus end where GTP can be hydrolyzed.
Plus end begins with GTP cap.
Caps promotes addition of dimers and MT will grow.
As it grows, GTP hydrolyzed to GDP, converting the GTP cap to a GDP cap.
Beta tubulin bound to GDP more prone to shrinkage (catastrophe).
74
What is the "9 + 2 array" ?
The microtubule structure seen in the axoneme of cilia and flagella. It has 9 doublet MT and 2 single central MT when looking at a cross section.
75
What would happen if the doublets weren't linked?
Axonemal dynein attached to one MT on a doublet walks along a MT of an adjacent doublet, this results in bending. Bending occurs because they are attached. If they weren't attached, dynein would just continue to walk along carry the MT with it and they would slide past each other.
76
What is plectin?
An IF associated protein that forms bridges between MFs, MTs, and IFs.
77
Briefly describe the hierarchical structure of intermediate filaments.
1. Coiled regions of two proteins linked together forms dimer, amino and c terminus are aligned (both on same side, therefore polar).
2. Two dimers join to form a tetrameric protofilament that align anti-parallel making this non polar
3. Protofilaments join together in bundles making up the IF (overall non polar).
78
How is the assembly of IF different from MT and MF assembly?
1. New tetramers are incorporated into middle of filament not the ends.
2. No ATP involved, instead phosphorylation dictates growth: phosphotases remove phosphate causing assembly, and kinases add phosphate causing disassembly.
79
What is keratin and what is it involved in in the cell?
Keratin is a type if IF and it comprises the nuclear lamina to provide support for the nuclear envelope.
They also comprise desmosomes and hemidesmosomes to provide tensional support to connect cells and cells to ECM.
80
Describe structure and assembly of MFs.
Monomers of actin (G-actin/globular actin) have 2 lobes with 2 domains and a cleft where ATP can bind. G-actin join together to form filamentous actin (F actin), and an MF is 2 filamentous actins wrapped around each other.
Monomers assemble by interacting with an ATP, allowing attachment, and hydrolysis of ATP once attached.
When assembled all monomers point in same direction, giving the MF polarity.
81
Describe S1 decoration experiment and how it proves polarity of MFs.
Removal of myosin head fragments (S1 fragments) in purified form and combining them with actin MFs result in arrowhead formation/appearance, demonstrating polarity.
This lead to the discovery of the two ends of MFs: barbed end (plus end) where growth occurs, and pointed end (minus end).
82
What is "treadmilling" when referring to actin MF assembly?
When there is a constant loss and gain of G monomers at minus end and plus end, respectively. Growth and shrinkage occurring at same rate, resulting in tread milling effect occurring at equilibrium.
83
Briefly describe myosin structure and the 2 types of myosin.
Myosin is a plus end directed motor with a head that hydrolyses ATP allowing movement.
There are conventional myosins (like type 2_ and unconventional myosins like type 1 and 5.
Type 2 has 2 heads, long tail, no bound cargo, and form bipolar filaments.
Type 1 has 1 head and type 5 has 2 heads, both are smaller and have no filament form and they bind vesicles.
84
Describe myosin 2 thick filament structure.
Multiple head and tail elements wrap around each other to form bundles/fibres called bipolar filaments.
Bipolar name comes from the opposing direction of myosin heads on the 2 sides of the "bare zone".
85
How does muscle contraction work in terms of the sarcomere.
In relaxed state, thin actin filaments are not interacting with heads on thick myosin filaments that are adjacent in the sarcomere.
To contract, heads on myosin filament walk on actin filament (recall this will be opposite directions on 2 sides of filament). This causes actin filaments to be pulled closer together, therefore contraction occurs.
86
What are the 2 types of actin organization found in the cell cortex?
1. Bundles: parallel fibres like filopodia (cellular extensions), actin interacting proteins that are bundling proteins facilitate this.
2. Varying 2d and 3D shapes.
Both these organizations are achieved via various actin interacting proteins.
87
What are nucleating actin interacting proteins and what is an example.
Example is Arp2/3 and it forms a nucleating centre, which mimics actin shape so G actin binds to it and continues to grow in whichever direction specified, specifically a 70 angle can be seen from original filament.
88
What are monomer sequestering actin interacting proteins and what is an example.
An example is Thymosin B4 which sequesters G actin monomers preventing them from polymerizing (controls levels of G actin available for filamentation).
89
What are end-blocking actin interacting proteins?
Capping proteins that cap ends of actin filaments.
Example CapZ that caps plus end of filament causing shrinkage).
Example Tropomodulin caps minus end causing growth.
90
What are monomer polymerizing actin interacting proteins?
Example is profilin which increases filament growth rates by promoting G actin addition at plus end (this competes with Thymosin B4 for G actin monomers).
So concentrations of profilin and Thymosin B4 will dictate whether actin filaments will grow or not.
91
What are depolymerizing actin interacting proteins?
Proteins (like Cofilin) that bind at minus end and cause depolymerization. Opposite effect of profilin.
92
What are cross-linking and bundling proteins?
Example: filamin hold filaments at 90 degree angles.
Example: Villim bundles proteins by holding them in parallel, what is seen in filopodia. `
93
What are filament severing actin associated proteins?
They do 2 jobs:
Example Gelsolin
1. Severe MF network causing meshwork to liquify
2. Cap plus ends, preventing polymerization.
94
What are membrane binding actin associated proteins/what do they do?
Examples: vinculin and dystrophin, they secure MFs to membrane allowing membrane to follow the actin movement.
95
What are 4 steps of cell locomotion and what facilitates these steps? Why is cell locomotion important for study?
All 4 steps facilitated by actin, first being extension, then adhesion, then translocation of cell body, then de adhesion.
Cell locomotion is typically good for cell and necessary, but can be bad like in cancer cell migration.
96
What are the 3 structural components seen when a cell is undergoing locomotion?
1. Tailing end
2. Lamellipodium (leading edge)
3. Filopodia, protrusions that feel out where cell is going.
97
Explain the extension step of cell locomotion.
The leading edge extends via polymerization of actin at its tip.
Arp2/3 complex is activated by WASP, which then allows polymerization at nucleating centres.
Profilin binds ATP attaching monomers.
Capping proteins terminate elongation to control direction of growth.
Cofilin depolymerizes old MF and those monomers can be added to barbed ends of new MF ends.
98
Differentiate basic properties of MF, MT, and IF (like flexibility, extensibility, etc.)
MTs: Hollow, stiff, inextensible, resist bend
MFs: helical, flexible, inextensible, contractile generating tension
IFs: Filament, tough and flexible, extensible that can withstand tension.
99
What are focal adhesions occurring during cell locomotion?
Focal adhesions are temporary attachment sites between the cell ECM and substrate below at the lamellipodium.
How cells adhere to plates in cell cultures.
Note these are different from hemidesmosomes, which are IFs attachments that are permanent.
100
What is meant by the nuclear envelope is dynamic?
it disintegrates during prophase of mitosis and reassembles at the end of mitosis.
In addition, it contains nuclear pore complexes that mediate movement through the envelope.
101
What is HGPS syndrome?
Premature ageing, at the cellular level it presents as an irregular shaped nucleus likely due to non-functionality of IFs.
102
What is the nuclear lamina and what is it made of?
Sits just below the nucleur envelope of the nucleus side and functions to support the envelope. It is comprised of intermediate filaments for support, including lamin a b and c.
Lamin a b and c disassemble during prophase via phosphorylation by kinases like CDC2 and reassemble after chromosome segregation via phosphotases.
103
Differentiate between euchromatin and heterochromatin.
Euchromatin is loosely packed and is available to be transcribed. Heterochromatin is condensed and cannot be transcribed, therefore is referred to as "silent". Heterochromatin is distributed near nuclear envelope.
104
What are nucleoli?
Regions within the nucleus that function to produce ribosomes. They are composed of protein and rRNA.
105
Briefly describe ribosome structure.
Large subunit (60S) with 4 types of rRNA, and small subunit (40S) with 1 type of rRNA that join to form 80S ribosome.
106
Explain Svedberg units and sedimentation coefficient.
Svedberg units describe the sedimentation coefficient coined by Theodore Svedberg, 1926 Nobel laureate.
The sedimentation coefficient refers to how quickly a substance can be sedimented during centrifugation, depending on both mass and surface area, explaining why the units are additive, rather their entire structure determines sedimentation and the structures aren't additive in surface area.
107
What are the regions within the nucleolus?
1. In centre there is the fibrillar centre (fc) that is mostly rDNA (dna that will be transcribed to form rRNA)
2. Dense fibrillar component (dfc) where the rDNA is converted to rRNA
3. Granular component (gc) where the ribosome assembly takes place, depends on the rRNA that is created.
108
Explain how the rRNA is created form the rDNA.
The long rDNA strand encodes genes necessary for ribosome assembly, called rRNA. These genes are transcribed via RNA polymerase and all the genes are first transcribed into one big transcript that can later be cut up to produce the smaller rRNA fragments seen in the ribosome subunits. The rDNA has 2 copies of the same gene in tandem, separated by small spacer DNA. This results in multiple rRNA copies that gradually get longer as RNA polymerase slides along rDNA.
109
Why are the ribosome subunits assembled separately?
They are assembled separately to have easier access out of the nucleus via the NPC. They then travel either to ER or cytoplasm where they can assemble when translation is necessary.
110
Briefly describe structure of the NPC.
Approximately 100nm in diameter and composed of proteins known as nucleoporins. There are about 30 nucleoporins per NPC and they exist in 8 fold symmetry (each protein occurs in multiples of 8).
The hole of NPC is plugged with fg nucleoporins (fg domain) meaning they have a lot of phenylalanine and glycine, making them hydrophobic, highly disordered, mesh like, and blocks diffusion.
111
How does import through NPC occur?
A nuclear localization signal (NLS) is required to transport cargo through the pore. This signal is approximately 8-10 amino acids long with strong positive charge (e.g., lys and arg) and often contains prolines.
112
Describe the processes of NPC import.
1a. NLS binds with importin, a transport receptor.
1b. Importin, with protein attached, also binds to cytoplasmic filaments on NPC.
2. Importin complex passes through pore and into the nucleus.
3. Ran GTP binds to importin in nuclear matrix and triggers the release of cargo (Ran GTP is a GTPase and so it becomes activated when its GDP is exchanged for a GTP and that allows it to release cargo from importin).
4. Ran + GTP exit nucleus via NPC
5. Ran GTP is then hydrolyzed causing the release of importin for reuse (inactivation)
113
What are 2 ways the activation and deactivation of GTPases can become faster?
1. Guanine exchange factors (GEFs) that put on a GTP to GTPase to activate.
2. GTPase Activation proteins (GAPs) help hydrolysis go quicker to deactivate.
114
What is the GEF for ran and the GAP for Ran?
GEF: RCC1, located in the nucleus promoting high concentration of importin-ran complex and low concentration of importin-ran-cargo complex.
GAP: RAN GAP1, located in cytoplasm allowing for low concentration of activated ran in cytoplasm and high concentration on inactive ran in cytoplasm.
115
Ways to break the Ran GTPase system?
1. Mutate RAN.
2. Add GTP gammas
3. Express RAN GAP in nucleus, will compete with RCC1 and there will be less activation of RAN GTPase an therefore less cargo will be able to be released.
116
What are the 5 histones and describe their structural arrangement.
H1, H2A, H2B, H3, H4. They are all enriched with small highly basic amino acids such as lys arg. They are highly conserved, except for H1 which is large.
2 copies of each of H2A H2B H3 H4 are assembled into an octomer that forms a nucleosome once wrapped in DNA. These bead-like octomers are connected via linker DNA, attached to each beach via H1.
117
How do steroid hormone receptors work?
They are inside the cytosol as the ligands are hydrophobic and can pass through the membrane directly. E.g., the sex hormones.
118
What is the purpose of a phosphorylation cascade?
To amplify the signal by generating more molecules to eventually cause a cellular response (can create 10^8 from just one original first messenger).
119
Differentiate between a kinase and a phosphotase.
Kinase: will phosphorylate something
Phosphotase: dephosphorylate something
120
What are the types of extracellular messengers?
1. amino acid derivatives (e.g., dopamine)
2. Steroids (cholesterol derived)
3. Gases (e.g., CO)
4. Polypeptides/proteins
5. Eicosinoids (Non-polar 20 carbon chains derived from fatty acids).
121
What are the types of receptors?
1. Ligand-gated ion channel
2. G-protein coupled receptors (GPCRs)
3. Receptor Tyrosine Kinase
4. Nuclear receptor (i.e., steroid hormones)
122
Describe the structure and basic function off GPCRs
Huge superfamily of proteins with 7 transmembrane spanning receptor (a helix domains): 3 loops on cytoplasmic side, 3 on exoplasmic.
Named due to heteromeric G protein interaction (can hydrolyse GTP to GDP to inactivate self).
Trimeric G proteins have a b and y subunits, a and y secure G protein to PM, a is connected to b and y complex.
123
How does the trimeric G protein in a signalling pathway become active?
A ligan bind to GPCR, causing GPCR to associate with the G protein. Ga will release GDP and acquire GTP, Gb and Gy dissociate from Ga and they are now active (2 subunits).
124
How does adenyl cyclase get activated and what does t do upon activation?
The active G protein (subunit a) will associate with adenyl cyclase (effector), activating it, allowing adenyl cyclase to make cAMP via 2 dephosphorylation events of ATP (2nd messenger).
125
GEF vs GAP
Gaps: GTPase active proteins: help hydrolysis go quicker (off switch)
Gefs: Guanine exchange factor: exchange GTP for GDP (turn things on)
126
What is the GEF for the trimeric G protein?
The GPCR
127
G Protein Receptor Kinases and result.
GRK phosphorylate GPCR to inactivate it causing desensitization of the GPCR.
This allows arrestin to come in an bind to GPCR, recruiting AP2, recruiting clathrin, inducing receptor mediated endocytosis.
128
What is the fate of a GPCR once in an endosome (after receptor mediated endocytosis by GRK).
1. More signalling via arrestin (GPCR can act on the endosome)
2. Lysosomal digestion
3. Recycle back to the PM
129
Second Messengers.
Small, non-protein molecules that rapidly diffuse in a cell to activate signalling molecules (amplification).
130
How is adenyl cyclase deactivated?
The hydrolytic function of the Ga subunit of trimeric G protein hydrolyzes itself (GTP to GDP), inactivating itself to rejoin with other subunits. Adenyl cyclase is then no longer activated.
131
What does the cholera toxin do?
Modifies the Ga to not be able to hydrolyse itself (stays bound to the effector aka adenyl cyclase).
This causes the continuous creation of cAMP, which causes epithelial cells to secrete salt into intestines, water follows, causing massive diarrhea.
132
Describe the stucture of protein kinase A (pKA) and how it changes upon cAMP.
Regulatory subunit combined with a catalytic subunit. Regulatory subunit has cAMP binding sites that when bound causes it to dissociate from the catalytic subunit.
The catalytic subunit has kinase ability allowing it to phosphorylate other proteins in the cell.
133
What is an allosteric regulator?
Modification of an enzyme activity by interaction with a compound binding to site other than the active site (in the case of the catalytic subunit in pKA, the regulatory subunit interacts with the non-active sit of the catalytic subunit but it still able to inhibit it).
134
Explain the role of adenyl cyclase/cAMP pathway in the flight or fight response.
Adrenaline (first messenger) bind to the GPCR, G protein is activated, activating adenyl cyclase, cAMP is produce (2nd messenger), pKA is activated which can liberate glucose from glycogen stores in muscles and/or activate CREB transcription factor which makes anabolic enzymes to synthesize glucose.
135
What are the 3 effector enzymes that make lipid derive second messengers, such as inositol triphosphate and diacetylglycerol.
Phospholipid kinases
Phospholipid phosphorylases
Phospholipases
136
Explain how phospholipase C (effetor enzyme) can create DAG and IP3.
Phosphotidylinositol (P1) on innter leaflet of PM has inositol ring capable of being phosphorylated by a phospholipid kinase.
Once double phosphorylated it is called PIP2 (phosphatidylinositol 4,5-biphosphate) which can be cleaved via phospholipase C (due to interaction of pH domain on into phospholipase C) DAG and IP3.
137
What does diacetylglycerol (DAG) and inositol tri P (IP3) do?
Remains in PM where it recruits PKC, IP3 triggers calcium ions release from smooth ER further activating PKC.
PKC will then phosphorylate many target proteins to elicit many responses like cell growth, ion channels, cell pH, secretion, cytoskeleton.
138
Explain role of Ca ion in the cell.
Calcium is another second messenger (via IP3) that is 10000 x more concentrated in SER than in cytoplasm. Once Ca in cytoplasm ,ATP driven pumps will pump it back to SER.
PKC (ubiq protein kinase), Gelsoin (actin severing protein), villin (actin organization), arrestin (termination of photoreceptor response) are all activated by calcium.
139
What is calmodulin
A calcium binding protein (4 binding sites) that has very low affinity for calcium. Calcium concentrations need to be very high for it to bind and induce response (this is good as there is always calcium present in cytoplasm in small amounts).
140
What ligands are typically involved in receptor tyrosine kinases (RTKs)?
Growth factor (GFs).
141
What are the two ways RTKs are activated/dimerized?
1. Ligand mediated dimerization via bivalent ligand
2. Receptor mediated dimerization via 2 monovalent ligands.
From there, the dimers transautophosphorylate (3 on each).
They are then active and serve as a docking station for other proteins
142
What classes of proteins with what domains interact with RTKs?
```
SH2 domain (Src homologous 2)
PTB domain (phosphotyrosine binding)
4 types of proteins have these:
1. Adaptor proteins (Grb2)
2. Docking proteins (IRS)
3. Transcription factors (STAT family)
4. Signalling enzymes (phospholipase C)
```
143
What is Ras?
Small monomeric G protein anchored to inner PM with GDP bound in inactive state.
144
What are the adaptor proteins in the Ras MAP pathway?
After GF ligand bind to RTK, GRB2 (SH2 domain) and SOS adaptor proteins bind to RTK.
This causes the activation of Ras by causing it to exchange GDP to GTP.
145
Explain what happens after Ras-GTP is activated?
First, active Ras-GTP recruits Raf and phosphorylated it (the effector protein and MAP kinase kinase kinase).
Raf phosphorylates MEK (the MAP kinase kinase).
MEK phosphorylates ERK (MAP kinase)
ERK phosphorylates Ets or Jun (transcription factors).
Jun and Ets induce transcription and produce proliferation genes.
146
What is the role of MKP1 in the Ras MAP pathway?
It is a gene transcribed as a result of the rad-MAP pathway which negatively feeds back and de-phosphorylates ERK.
147
What does MAP stand for?
Mitogen activating pathway (mitogen is a protein that activates mitosis).
148
What is a protoncogene vs an oncogene?
Protoncogene: genes with the potential (if mutated) to push cell into malignant state.
Oncogene: Genes that have been mutated such that the protein product promotes loss of growth control.
149
What was apoptosis first studied in?
C elegans (a transparent roundworm).
Only 1090 cells produced during development allowing easy following of all.
Found that 131 underwent apoptosis.
Hervits, Brenner, Sulsten won Nobel prize in 2002 for this.
150
What were the key proteins found in the C elegans apoptotic pathway?
Ced (cell death) 3, 9 (homologous ones were found in humans).
151
What are the features of apoptosis?
- Cytoplasm shrinkage
- Nuclear Fragmentation
- Loss of cell adherence abilities
- DNA digested by DNases (shown as - laddering pattern in gels)
- Flippase adds phosphotidylserine (phagocytic signal) to outer leaflet of PM
- Bleb like extensions
- Cell dismantled into apoptotic bodies
- Phagocytes ingest the bodies
152
Apoptosis vs necrosis
Apoptosis is very orderly, not causing damage to surrounding cells.
Necrosis is vey disorganized and leads to cell swelling, organelle swelling, leakage of contents into environment causing inflammation.
153
What main group of protein is involved in apoptosis and what are they?
Caspases (cysteine proteases): have cysteine in their active site, they can cleave other proteins on aspartic acid residues.
Harvitz discovered ced-3 protein was a cysteine protease.
154
How are caspases activated, basically?
They are produced as inactive procaspases and are activated once cleaved leading to a proteolytic cascade.
155
What do caspases target?
Protein kinases
Lamins
Cytoskeletal elements
Endonucleases
156
What is a death signal and what does it causes?
Death signal is a cytokine made by immune cells in response to high temperatures, viral infections, toxic chemicals. e.g., Fas L (fas ligand).
They initiate the extrinsic pathway to apoptosis.
157
Explain the pathways of Fas L death signal.
Fas L binds to Fas receptor in PM, adaptor proteins are recruited such as Fas associated death domain (FADD) and 2 procaspase.
The 2 procaspase 8 cleave each other to make the mature caspase 8 enzyme (initiator caspase).
Caspase 8 initiates apoptosis by cleaving and activating downstream caspases.
Caspase 3 is one of the main ones involved and it is the executioner caspase (initiates apoptosis)
158
Explain the mitochondria-mediated/intrinsic pathway to apoptosis usually normal function (when pathway is not initiated).
A survival factor (TF) will bind to RTK causing BAD protein to be phosphorylated and keep cell alive.
(As as long as TF is present BAD is phosphorylated).
159
What happens in BAD looses its P.
BAD (un-phosphorylated) will inhibit BCLx and BCL2, inhibiting them from inhibiting BAX.
BAX channel will open, cytochrome C will flow out of IM.
160
What does cytochrome C do once exiting the OM of mitochondria?
Combines with caspase 9 and Apaf1 (adaptor protein) to form the apoptosome.
161
What does the apoptosome do?
Activates caspase 3 (executioner caspase) which activates apoptosis.
162
How do docking proteins interact with RTKs?
Example:
IRS has PTB domain where a phosphate on the RTK can get bound.
IRS has domains itself which can get phosphorylated themselves allowing the recruitment of other proteins such as p13K and shp2 (serves as a docking station).
*p13k and shp2 can also just directly bind to RTK.
163
How do transcription factors interact with RTKs?
Members of the STAT family: 2 will get phosphorylated by RTK, allowing them to dimerize and move to nucleus
164
Explain how phospholipase C can interact with RTKs
Phospholipase C can interact with RTK instead of acting as an effector protein in GPCR pathway
165
What is anastais?
Cells that avoid/recovery from apoptosis.
Likely plays a role in cancer
*Even during apoptosis the cell holds onto a "lifeline" that can bring it back*
166
What are the cell conditions at the end of interphase (right before M phase)?
```
2 centrosomes (duplicated during S phase)
PM intact
NM intact
Chromatin non condensed
Nucleolus is visible
```
167
What was the key experiment that discovered the features of the cell cycle and what were the observations?
Adding radioactive thymine to asynchronus cell culture for 30 minutes.
Cells in S phase will incorporate it into DNA
Refresh media and wait (pulse chase type)
Use autoradiography to see labelled DNA
Observations:
1. Cells undergoing mitosis had no labelled DNA indicating DNA is no replicated during M phase
2. Only fractions of cells were labelled indicating S phase is only a small portion of the cell cycle
3. Gap of at least 30 minutes after initial labelling to when labelled DNA showed in compact chromosomes (indicating a gap between M and S phase).
168
What initiates mitosis?
MPF (maturation promoting factor)
169
What are the 2 proteins involved in maintaining the compacted mitotic chromosomes?
1. Consdensin: organizes DNA to be condensed and is activated via phosphorylation by MPF
2. Cohesion: Holds 2 sister chromatids together at centromere (during prophase it is concentrated exclusively in the centromere, before this it runs all along the arms).
170
Explain the centromere
Area of primary constriction containing many DNA repeats that are non-coding.
Repeats serve as spot for kinetochore to bind.
171
What is the kinetochore?
On the outer surface of centromere made of more than 100 proteins.
Serves at the attachment site between chromosome (centromere) and MT.
Location of some motor proteins involved in anaphase/important checkpoint.
If side of centromere is not connected to MT there is fibrous corona
172
What is the effect of the centrosomes migrating to opposite sides of the cell during prophase 1?
1. Effect on spindle formation
2. Defines poles of cell
3. Defines equator/metaphase plate of cell.
173
How and when is the nuclear lamina broken down?
Broken down during prophase via MPF kinase activity disassembles lamins.
174
How do chromosomes get to the pro-metaphase plate?
Chromosome binds to kinetochore microtubule and kinesin-related protein facilitates its "walking" to the plate by causing growth or shrinkage at the plus end.
This is called congression.
175
What state must the chromosomes be in to proceed to anaphase?
Chromosomes must be bi-oriented (attached to two MTs from opposite poles) and under tension.
This is known as the spindle-assembly checkpoint (allows for cohesion cleavage)
176
What happens during anaphase?
Cohesion is cleaved allowing sister chromatids to separate to opposite poles (the chromatids are already being pulled but the final cleavage is what allows chromatids to migrate).
177
What happens when the conditions to the spindle assembly checkpoint are met?
MAD2 will stop inhibiting cdc20.
cdc20 will combine with anaphase promoting complex (APC)
APC is an E3 ubiquitin ligase, so it adds ubiquitin to proteins to be destroyed, specifically securin.
Securin normally inhibits seperase protein, but now that it is gone seperase can cleave cohesion.
178
What are the 2 movements involved in chromosome separation?
Anaphase A: Movement of chromosomes to the poles via kinetochore MT
Anaphase B: Movement of spindle poles away from each other via astral and polar MTs.
179
Explain the process of Anaphase A
- Kinetochore MTs get shorter as tubulin is lost on both sides (via kinesin 13 depolymerizing)
- Other proteins connecting MT to kinetochore have heads walking to minus ends, pulling it closer
- Cytoplasmic dyenin may also play a role
180
Explain the process of Anaphase B
- Polar MTs get longer as tubulin is added to the plus end
- 2 Kinesin 5 motor proteins attached to each other with heads binding antiparallel on 2 polar MTs help them slide apart and elongate the spindle.
- Astral MTs get shorter as tubulin is removed from the plus end
- Cytoplasmic dyenin links astral MTs to the cell cortex (pulls spindle poles toward cortex by walking to the minus end).
181
What is the contractile ring theory?
Myosin 2 moves along ring of actin in the cortex (just below PM) cleaving cell into 2 daughter cells (cleavage furrow).
182
What is abscission?
The actual/final splitting of 2 daughter cells: after main cleavage furrow there is still a connection between the daughter cells called a midbody.
Midbody os a dense transient structure playing a role in localization of abscission..
183
Explain the early experiments and observations done to interpret the regulation of the cell cycle.
1. Fuse rat G1 cells and HeLa M cells: DNA is G1 cells began premature compacting indicating a diffusible signal in M-phase that triggers compaction of chromatin.
2. Fuse G2 cells with M: Same thing seen, further supporting idea
3. fuse S phase cells with M cells: same things seen (although DNA was more pulverized to to instability of DNA during replication), further supporting idea.
4. Fuse G1 with S phase: DNA is G1 began premature replication, suggesting diffusible signal in S phase triggering replication.
184
Basically explain cdk and cyclin levels and roles in the cell cycle.
Cdk (cyclin dependent kinase) levels remain constant throughout the cell cycle, cyclin levels vary throughout cell cycle. When cyclin concentration is high enough, cdk and cyclin will combine and allow phosphorylation ability of cdk.
Main cyclin-cdk complexes at G1, G2 , and M phase (at the checkpoints).
185
Who completed the early work in the control of the cell cycle (researched it)?
```
Leland Hartwell (cdks in yeast)
Paul Nurse (cdks in yeast)
Tim Hunt (cyclins in sea urchins)
Won Nobel prize in 2009
```
186
What controls the entry into M phase?
Aka the G2 checkpoint, mitotic cyclin and cdk levels determine this: complex is called MPF (maturation promoting factor).
MPF will phosphorylate proteins with threosine and serine residues causing cell to progress into M.
187
How is MPF activated (schizosaccharomyces pombe yeast)
Cdc2 mitotic cdk (inactive kinase) is combined with mitotic cyclin ( at threshold point) to make the mitotic cdk-cyclin complex with phosphorylated tryosine and threosine residues.
These residues are phosphorylated by CAK (cdk activating proteins adds P to thr161) and wee1 (adds P to try15). The kinase is still inactive until cdc25 removes P from try 15.
Once removed, the complex is active (MPF) and can initiate prophase
188
How is MPF deactivated?
Post mitotic fission:
APC/cdh1 complex will degrade cyclin and the cdc2 cdk is inactive again. This results in the exit of mitosis back into interphase.
189
What would happen if there was a mutation in wee1?
Recall wee1 phosphorylated try15 causing a pause before proceeding into M.
If this doesn't occur MPF is activated right away, cell isn't grown as much and daughter cells will be smaller.
190
What would happen if there was a mutation in cdc25?
Cdc25 stops the pause before M. If the pause isn't stopped, try15 remains phosphorylated, then the cell continues to grow. Daughter cells will be extremely large once try15 is finally de-phosphorylated.
191
Explain the role of ATM in the G1 checkpoint and how it affects p53.
ATM binds to ds breaks in DNA (this is what the cell looks for during the G1 checkpoint).
If ATM binds, it will phosphorylate chk2 (checkpoint kinase 2) which will phosphorylate p53 (now p53 is more long lived).
192
What happens when p53 is phosphorylated.
p53 normally degraded and short lived in cell but if phosphorylate it can:
1. cause transcription of p21, which will inactive g1 cdk (stalling S phase).
2. Potentially trigger apoptosis if damage is irreparable via activation of PUMA (p53 up-regulated products of apoptosis).
This will inactivate anti-apoptotic protein (BCL2).
BAX channel can open, cytochrome c exits, apoptosis is initiated.
193
What controls the cdc25 protein involved in the G2 checkpoint?
ATR phosphorylates chk1, which then phosphorylates cdc25, adaptor protein in the cytoplasm recognizes the phosphorylation on cdc25 and prevents in from entering the nucleus and removing P off of try 15, this in turn stalls the pause before M phase longer allowing for whatever breaks that have been detected in chromosomes to be fixed (ensures nondisjunction does not occur).
194
What is ataxia telongietasia syndrome?
Mutations in ATM or ATR genes resulting in cells prone to tumor formation, cancer is easier to occur.
These individuals cannot undergo chemical radiation because the DNA damage induced by radiation can't be repaired in these individuals (would just result in more tumor formations).
195
G0 Phase.
Cells that leave the cell cycle (permanent G1) and can be reversible (quiescent cells) or irreversible(terminally differentiated cells).
