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first year uni cards > phgy 170 final- module 5 > Flashcards

phgy 170 final- module 5 Flashcards

1
Q

what is the cytoskeleton?

A
  • The cytoskeleton is a network of structural proteins that are found in all cell types
  • This filamentous array occupies a large portion of the cytosol and extends throughout the cytoplasm from organelle to the plasma membrane
  • It is the cytoskeleton that permits common cellular functions such as signaling and vesicular transport to occur, as well as giving some cells unique properties, including cell motility
  • The cytoskeleton also defines the shape of a cell and the distribution of cellular contents
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2
Q

what are the 3 classes of of proteins that make up the cytoskeleton in eukaryotic cells that contribute to cellular strength and permit cellular function?

A

—–> INTERMEDIATE FILAMENTS
- The primary purpose of intermediate filaments is to add mechanical strength to cells

—–>MICROTUBULES
- The primary purpose of microtubules is to support trafficking within cells

—–>ACTIN
- The primary purpose of actin is to support cellular motility, or other larger-scale movements like contraction

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3
Q

what are the 3 main functions of the cytoskeleton?

A

—–> BINDING
- Cytoskeletal proteins bind to a target (such as another protein) which includes similar proteins, to form polymers

  • Polymers: molecules made up of a large number of repeating units

—–> CONFORMATION
- when the cytoskeletal proteins bind, they undergo conformational changes

—–> FUNCTION
- the function of these proteins is defined by the number and type of cytoskeletal proteins that are bound

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4
Q

what are intermediate filaments?

A
  • Within the cytoskeleton, the intermediate filaments are analogous to the bones of the body, such that these filaments supply strength to cells allowing them to resist changes of shape
  • They are the strongest filaments of the cytoskeleton and provide great mechanical strength
  • Intermediate filaments are polymers and their expression is tissue and cell specific
  • The assembly and disassembly of intermediate filaments is controlled by post-translational modification of the individual proteins
  • The intermediate filaments can absorb the greatest amount of stress by deforming/elongating
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5
Q

what are the major classes of intermediate filaments?

A
  • Every cell in the body performs a unique function
  • Consequently, each cell faces different types of mechanical stress
  • Due to this, cells have evolved the ability to express different intermediate filaments, reflective of the specific needs of the cell
  • Intermediate filaments are organized into different classes based on their protein types, which then dictates their distribution, and functions
  • Class I and II keratins are the most common intermediate filaments in humans
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6
Q

what proteins are found in the epithelial cells?

A

acidic and basic keratins what provide tissue strength and integrity

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7
Q

what proteins are found in muscle, glial, mesenchymal and perphevin cells?

A
  • Desmin, GFAP, vimentin and perophevin that provide sarcomere organization and integrity
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8
Q

what proteins are found in neurons?

A

neurofilaments that allow for axon organization

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9
Q

what proteins are found in the nucleus?

A

lamins, that provide nuclear structure and organization

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10
Q

why might there be a need for tissue specific expression of intermediate filaments?

A

different cell types express separate classes of intermediate filaments based on their functions. For example, neurons have the important role of conveying information electrically throughout the body. Neurofilaments, one of the type of intermediate filaments, are required to support this role by organizing and providing structural support to the unique structure of the neuron

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11
Q

what is the primary structure of intermediate filaments?

A
  • The strength of intermediate filaments comes from how the individual proteins are packaged and assembled into polymers
  • Similarly to other proteins, intermediate filaments do not gain their properties until they are assembled
  • Recall from module 2 that the primary structure of a newly synthesized protein is just a polymer of amino acids linked together by peptide bonds
  • Peptide bonds: a stable covalent chemical bond that allows for the sharing of electrons between pairs of atoms, formed between the carboxyl group of one molecule and the amino acid group of another
  • At this stage, all proteins have the same strength and an intermediate filament protein is no stronger than any other protein in the body
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12
Q

what is the secondary structure of intermediate filaments?

A
  • If you examine the secondary structure of intermediate filaments, you will begin to notice where their strength comes from
  • Recall from module 2 that proteins’ secondary structures can be alpha-helices, beta-pleated sheets and random coils
  • Intermediate filaments are very rich in alpha-helices, giving them some of their properties
  • Alpha-helices are responsible for the long, coiled structure of filaments, and the hydrogen bonds stabilize the structure as they resist the stretching of the filament and prevent its collapse
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13
Q

What is the tertiary and quaternary structure of intermediate filaments?

A
  • Although the secondary structure of intermediate filaments hints at where they get their properties from, it isn’t until you examine the tertiary and quaternary structures that you will fully understand their function

—–> MONOMER
- The coiled monomer is the tertiary structural level

—–>DIMER
- 2 coiled monomers come together to form a dimer
These monomers wrap around each other to form what is called a coiled coil

  • The coiled coil structure allows for maximum contact (hydrogen bonding) between the 2 peptides, and thus conveys tremendous strength to the dimer
  • If you recall from module 2, when 2 peptides combine like this dimer, it is described as quaternary structure
  • This will further build as the intermediate filaments form tetramers and eventually complex polymers as mature filaments

—–>TETRAMER
- The next level of organization is the formation of tetramers)

  • 2 dimers assemble in an antiparallel (NH2 and COOH termini on opposite ends) staggered manner
  • Since the dimers have again aligned lengthwise, the hydrogen bonding and future strength of the intermediate filament have further increase
  • This newly formed tetramer is considered to be the fundamental building block of intermediate filaments
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14
Q

How do intermediate filaments assemble?

A
  • The intermediate filament building blocks (tetramers) come together spontaneously in 3 stages to form filaments

—–> The FIRST step involves the formation of what is called a unit-length filament

  • This is formed by 8 tetramers coming together (20 nm)

—–> The SECOND step is when unit-length filaments come together to form an immature filament

  • These interact loosely end-to-end

—–> The THIRD step is when the immature filament compacts to form a mature filament

  • This is the final step to create a fully assembled intermediate filament
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15
Q

what is the post-translational modification of intermediate filaments?

A
  • Post-translational modifications control the shape and function of intermediate filaments, just like they do for other proteins
  • These filaments can be modified by all types of post-translational modification, such as phosphorylation and glycosylation

—> PHOSPHORYLATION: the addition of phosphate groups
—> GLYCOSYLATION: the addition of sugar groups

  • Such modifications typically occur in the head and tail domains of the intermediate filament subunit proteins
  • Although it is known that intermediate filaments undergo post-translational modification, an understanding of the consequences of such modifications remains unclear
  • Overall, phosphorylation leads to the dissolution of an intermediate filament into unit-length filaments, and when these phosphates are removed by enzymes called phosphatases, the intermediate filaments will spontaneously reform
  • The disassembly and assembly is important during cellular processes such as cell division
  • Prior to cell division, the cytoskeleton collapses as the cell is partitioned into 2 cells
  • After division, the cytoskeleton reforms
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16
Q

what are the 3 main types of specialized intermediate filaments?

A

—–> LAMINS
- A type of intermediate filaments found solely in the nucleus, that forms the nuclear matrix, a dense network to protect chromatin

—–> DESMINS
- A type of intermediate filament that does not form long, thin filamentous structures, but more so connects different cellular structure together
- It is important for muscle structural integrity

—–>KERATIN
- An important intermediate filament that binds to desmosomes to form a complex
- Keratin makes up your hair, skin, and nails

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17
Q

what are microtubules and what are their primary purpose?

A
  • The primary purpose of microtubules is cellular trafficking
  • Trafficking is the movement of proteins, vesicles and some cellular organelles within the cytoplasm
  • A very important concept to note is that these movements are not random within the cytoplasm
    The microtubule network defines how things are trafficked throughout the cytoplasm and they create specific routes by which their cargo can travel
  • Travel can be bi-directional along a single microtubule and the cargo can attach or detach anywhere along its length
  • This intracellular distribution network determines where things move within a cell and can be assembled or disassembled to create or remove routes, respectively
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18
Q

what are microtubule organizing centers?

A
  • In contrast to intermediate filaments, the assembly of microtubules is more organized and does not occur spontaneously
  • Microtubule assembly requires many proteins and occurs in regions called the microtubule-organizing centres (MTOCs)
  • MTOCs are the cellular structures from which microtubules arise
  • Depending on where the microtubules need to be assembled, they can be at different locations within a cell
  • An example of an MTOC is the centrosome
  • The centrosome is located near the nucleus of the cell, and during cell division it is copied so that the two resulting centrosomes can form the poles of the mitotic spindles
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19
Q

what is the protein structure of microtubules and what are they composed of?

A
  • Microtubules are made of specific proteins called tubulins
  • Like intermediate filaments, they are composed of DIMERIZED PROTEINS
  • These tubulins represent a very large family of cellular proteins with many different functions
  • We will focus on 2 specific tubulins: alpha-tubulin and beta-tubulin
  • Alpha-tubulin and beta-tubulin are both globular (glob-like in shape) proteins with similar shapes, that can bind very tightly together in a head-to-tail fashion to form a dimer
  • Both tubulin proteins bind to a GTP molecule, and beta-tubulin can cleave its GTP to GDP
    When bound to GDP, beta-tubulin has a shape change
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20
Q

what is the process of microtubule polymerization?

A
  • The formation of microtubules from alpha and beta tubulin-dimers is very dynamic
  • If the polymer made from individual tubulin dimers can reach a critical length, it will continue to grow

—–> Dimers form polymers
- Dimers will spontaneously assemble into unstable polymers that can quickly fall apart

—–>Polymer growth
- Once a polymer of at least 6 dimer subunits form, it is more stable and it may grow laterally and longitudinally (PROTOFILAMENT)

—–>Protofilament tubes
- Eventually, protofilaments will form a sheet and will assemble into a tube of 13 protofilaments

  • This is the nucleation site for microtubule elongation
  • Even in its tubular form, the microtubule is in a dynamic state of assembly and disassembly

—–>assembly/ disassembly
- At the ends of a microtubule, dimers continue to come and go

  • If the rate of assembly is greater than disassembly, the microtubule grows
  • Conversely, the microtubule shortens if disassembly occurs faster than assembly
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21
Q

what is the process of microtubule assembly?

A
  • Recall, that an alpha-tubulin always has GTP bound to it, while beta-tubulin may have either GTP or GDP
  • When GTP is bound to beta-tubulin, dimer polymerization is favoured and the dimers will attach to each other, aka→ assembly
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22
Q

what is the process of microtubule disassembly?

A
  • When beta-tubulin’s GTP is hydrolyzed to GDP, the dimer undergoes a conformational change that promotes depolymerization, aka → disassembly
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23
Q

Are microtubules polar? If so, what are the main characteristics?

A
  • Since microtubules are formed by the end-to-end polymerization of dimers, the ends are different and are said to have polarity
  • One end of the microtubule is the plus end, while the opposite is the minus end

-Although growth occurs at both ends of a microtubule, they do not grow at equal rates

  • There is a preference for dimer binding at the PLUS END, so microtubules extend from that end much faster than from the minus end
24
Q

what is dynamic instability in microtubules?

A
  • Depending on their type, cells may change their shape or have a high need to move things around within themselves
  • Because of these varied needs, microtubules need to be very responsive, and have the ability to grow or shrink or even change direction very quickly in response to changes in the cellular environment
  • The ability to rapidly grow or shrink is called DYNAMIC INSTABILITY

—–> GTP cap
- A growing microtubule has a cap of GTP subunits at its tip

—–>Hydrolysis
- GTP hydrolysis occasionally exposes GDP-bound subunits at the tip

—–>Depolymerization
- Following hydrolysis, rapid catastrophic depolymerization occurs

—–>Recap
- The microtubule resumes growing when GTP-bound dimers are available, until another change in the cellular environment is detected

25
Q

how does catastrophe occur during dynamic instability in the microtubule?

A
  • Recall how microtubule growth originates in MTOCs, and how tubulin dimers are preferentially added to the plus end

As long as there are GTP-bound tubulin dimers present, then the microtubule will continue to grow

  • When GTP is converted to GDP on the tubulin dimers at one end, they rapidly fall off which can initiate CATASTROPHE, which is the rapid depolymerization of tubulin dimers at the plus end, resulting in a shortening of the microtubule

—–>Measures against microtubule catastrophe:
- The catastrophes that occur due to microtubule dynamic instability are not unstoppable or permanent

  • A microtubule catastrophe can be averted or reverse through mechanisms such as capping and rescue

—–> Catastrophe aversion: CAPPING
- Once a microtubule is of desired length, the plus end can be bound by microtubule capping proteins, which add tremendous stability to microtubules and will keep them polymerizes even if their dimers are in the GDP-bound form

—–>Catastrophe reversal: RESCUE
- Even if a catastrophe has occurred, that does not mean that the microtubule will shrink and completely disassemble

  • Catastrophe can be halted or even reverse by rescue
  • Rescue can occur spontaneously if there are enough GTP-bound dimers present, but it can also occur in the presence of some other proteins
  • Not much is known about the proteins that can assist in rescue, but we should know about their existence
26
Q

microtubules are made up of ________ and found in the cytoskeleton and cytoplasm

A

tubulin

27
Q

tubulin is a _______ protein

A

globular

28
Q

TRUE or FALSE?

the growing and shrinking of tubulin depends on the rate at which GTP is hydrolyzed

A

TRUE!

29
Q

when the tubulin slows in binding to the GTP cap _____, occurs

A

catastrophe

30
Q

centrosomes anchor many microtubules to their surface using the _______ ________ _____ ________

A

gamma tubulin ring complex

31
Q

what is the treadmill effect?

A

where one end (positive) goes in to rescue the negative end, and is continually in catastrophe

32
Q

what are the benefits of dynamic instability?

A

-dynamic instability is essential for 2 reasons

—-> it allows cells to explore their cytosol and rapidly create new pathways for trafficking depending upon a cell’s constantly changing needs

—-> it allows cells to exert force. Any molecule attached to a microtubule near the plus end will be transported through the cell as the microtubule grows or shrinks

33
Q

what are microtubule associated proteins?

A

-There are many different proteins that associate with microtubules

  • So far, we have already been introduced to microtubule capping proteins, which help stabilize microtubules, and rescue-associated proteins that help to stop microtubule catastrophe
  • However, there are many other microtubule associated proteins (MAPs) that have other functions, such as assisting in stability, cross linking, bundling, and even cutting microtubules
  • One particular class of MAPs that you will now focus on are the microtubule-based motor proteins

-Motor proteins are the proteins that actually control trafficking, while the microtubules control where molecules can go

  • Motor proteins bind to the cargo that needs to be trafficked, then bind to the microtubule and “walk” along it
34
Q

what are motor proteins, and what are the two most common types?

A
  • The two most common microtubule-based motor proteins are KINESIN and DYNEIN
  • In general, kinesins move along microtubules towards the plus end while dyneins move to the minus end
  • The figure shows the basic structures for both kinesin and dynein
  • Their heads contain microtubule-binding domains, whereas their tails bind the cargo that needs to be trafficked
  • Both kinesin and dynein contain two heads that literally walk along the microtubule
  • This walking consumes cellular energy in the form of ATP

note that the stalks and heads, bind the microtubule, and the tails that bind the cargo

35
Q

what are the steps of the “walking” of motor proteins?

A

—-> Step 1
- Head 1 is bound to the microtubule, and head 2 is bound to ADP

—–> Step 2
- The walking movement is initiated by ATP binding to head 1, which causes a conformational change that includes head 2 swinging around

—–> Step 3
- Once head 2 is over a binding site, it binds to the microtubule and releases the ADP

—–> Step 4
- The ATP at head 1 then undergoes hydrolysis, so it is now ADP bound to head 1, which causes it to release from the microtubule

—–> Step 5
- The entire process is then repeated but with ATP now binding to head 2, causing head 1 to swing around

36
Q

what are some similarities and differences between actin filaments and microtubules?

A
  • The actin cytoskeleton is similar to the microtubule cytoskeleton in some ways, yet dramatically different in others

—–> Composition
- Both actin filaments and microtubules are composed of globular proteins

—–> Movement
- Motor proteins are used to initiate movement along both cytoskeletal proteins

—–> Network formation
- Actin and microtubules differ in the networks they form

  • Microtubules move cargo throughout the cell along a dynamic network
  • In contrast, actin forms a stronger network that contributes to both the structure of the cell and to large scale movements such as muscle contraction
  • Most distinctly, this means that the actin cytoskeleton can move the cell itself
37
Q

what are the basic structures and functions of actin monomers?

A
  • The basic building block of the actin cytoskeleton is the MONOMERIC ACTIN PROTEIN, or actin monomer
  • Cells can express several different types of actin monomers, which allow the cells to match the monomers to their specific functional needs
  • There is a barbed (+) end and a pointed (-) end
  • Similar to intermediate filaments, actin monomers come together to form long, thin, actin filaments

—–> Structure
- These filaments are very similar to a double coil structure, meaning that the actin monomers are binding to each other both longitudinally (end to end) and laterally (side to side)

  • The combination of longitudinal and lateral bonding means that actin filaments have high tensile strength and can withstand pulling forces that would pull microtubules apart

Tensile strength: resistance to breaking under tension

—–> Polarization
- Similar to microtubules, actin filaments are polarized, meaning that their ends are NOT THE SAME CHARGE

  • This plus end is often called the barbed end, while the minus end is called the pointed end
  • These names were derived from electron microscopic examinations that described their shapes
  • The barbed appearance of the plus end is due to the association of other proteins such as myosin, which we will learn about later this section
38
Q

what is the process of actin polymerization?

A
  • Like microtubules, actin monomers bind to nucleotide phosphates
  • However, in contrast to tubulin that binds GTP/GDP, actin monomers bind to ATP/ADP
  • The binding of ATP to an actin monomer promotes assembly or polymerization, whereas the binding of ADP discourages polymerization and may lead to disassembly of the filament
  • There is a preference for ATP, so if there is a constant source of ATP, then the ADPs are replaced by them
39
Q

what are the stages of actin polymerization?

A

—–> Stage 1: NUCLEATION
- 2 actin monomers can dimerize, but nucleation occurs when a third actin monomer binds to the dimer to form a nucleus trimer

  • This timer forms the core on which the rest of the actin filament can form
  • Although it is structurally simpler than an MTOC, it serves the same purpose

—–> Stage 2: ELONGATION
- Additional actin monomers are added to the nucleus and as the actin filament elongates, it is important to note that it can occur from both directions, starting at the nucleus, so both ends of the filament can lengthen

  • However, like microtubules, polymerization is favoured at the plus end
  • Actin polymerization is also dynamic with actin monomers both being added and being removed
  • If the balance favours adding monomers, then the filament elongates

—–> Stage 3: STEADY STATE
- eventually, the rate of assembly equals the rate of disassembly and net actin filament elongation ceases

40
Q

what is the process of actin treadmilling?

A

-Since actin filaments both polymerize from a trimer nucleus, both the plus and minus ends are exposed

  • Because the minus end is exposed, the dynamic adding and removing of monomers can occur and this allows for the actin filament to undergo what is called treadmilling
  • Treadmilling is the favoured addition of monomers to one end with the same rate of monomer removal at the other end
  • This effectively keeps the actin filament the same length but can result in the filament moving within the cell

—–> Regulation of actin treadmilling
- Treadmilling is regulated by the ATP-actin concentration compared to the ADP-bound actin

  • The critical concentration of ATP-actin to polymerize is lower at the plus end than it is at the minus end
  • This means that if the ATP-actin concentration is just right, monomers are added to the plus end and removed from the minus end to cause treadmilling
  • If the ATP-actin concentration increases above the critical concentration for the minus end, then actin monomers can again be added to that end
  • Treadmilling gives the cells the ability to rapidly adjust the actin cytoskeleton, much faster than the intermediate filaments which require phosphorylation for disassembly
41
Q

what are actin binding proteins?

A

-Actin filaments are tremendously dynamic, but their assembly and disassembly are not random or out of control

  • Just like other proteins, actin can also be modified by phosphorylation, alkylations, and disulphide bonds
  • Other proteins can also bind to actin to modulate the structure and function of the actin cytoskeleton
  • Collectively, these proteins are called actin-binding proteins and are important in the regulation of the actin cytoskeleton for disassembly
42
Q

what are the 7 types of actin binding proteins?

A

—–> MONOMER-BINDING PROTEINS
- Proteins that bind directly to the actin monomers and influence polymerization

—–> NUCLEATING PROTEINS
- Proteins that bind to actin polymers to increase their stability and can allow for growth of a new branch

—–> CAPPING PROTEINS
- Proteins that bind to the plus or minus end and can stabilize the polymer to prevent disassembly and further assembly

—–> SEVERING AND DEPOLYMERIZING PROTEINS
- proteins that can bind to the actin polymer and sever or induce disassembly respectively

—–> CROSS-LINKING PROTEINS
- Proteins that allow the side-to-side linkage of actin polymers to form bundles of actin filaments

—–> MEMBRANE ANCHORS
-These link actin filaments to nonactin structural proteins similar to those integral to the plasma membrane such as integrins

—–> ACTIN-BINDING MONOMERS
- Proteins that bind to the actin filament allow movement

43
Q

what are actin binding motor proteins?

A
  • Myosins are the most commonly studied actin-binding motor proteins
  • There are 18 different families of myosins, each of which perform specific roles in specific cell types
  • All myosins are multi-subunit proteins
  • The different subunits are called either light chains or heavy chains based on their relative size
  • The protein is also organized into 3 different domains: the motor, regulatory and tail domains

—–> MOTOR
- The motor domain, formed by the heavy chain, binds to the actin filament of ATP

—–> REGULATORY
- The regulatory domain, formed by a heavy chain and two light chains, moves back and forth as the myosin moves along an actin filament

—–> TAIL
The tail domain binds to other cellular proteins or other myosins

44
Q

what are the steps of movement of actin-binding motor proteins?

A
  • the movement of myosin along an actin filament is an energy dependent process which requires ATP energy

—–> HYDROLYSIS
- With ATP bound to the motor domain, the myosin is unbound to the actin filament

  • Hydrolysis of the ATP to ADP and inorganic phosphate causes a conformational shift in the regulatory domain, swinging it like a lever

—–> ACTIN BINDS
- The motor domain then binds to the actin filament

  • The inorganic phosphate is released from the myosin, causing another conformational change and pulling the myosin along the actin filament
  • The ADP is then released and the binding of new ATP causes the myosin to unbind from the actin filament

—–> MOVEMENT

  • in most instances, myosin moves toward the barbed, or plus, end of the actin filament
45
Q

TRUE OR FALSE?

The vast majority of cell movement occurs within a cell

A

TRUE!

46
Q

what is the process of cellular migration?

A
  • The majority of cell movements involved the physical deformation of cells or the movement of cargo within cells; however, there are many cell types that do physically move around → migration

—–> Coordination within the cytoskeleton
- Like all cellular processes you have learned about in the course, cellular migration is not a random event

  • It takes an incredible amount of coordination for a cell to be able to move from one place to another while keeping its cellular contents intact and functional
  • Importantly, this coordination must arise from within the cell, specifically the cytoskeleton, because all of the forces that underlie migration must originate from inside the cell itself

—–> Rapid and dynamic assembly and disassembly
- In discussing the 3 major classes of cytoskeletal proteins, you have focussed on how these proteins assemble into filaments and disassemble back into protein monomers or dimers, very rapidly and dynamically

  • These features are essential in order to generate the forces and coordination necessary for cell migration
47
Q

how do actin filaments assist in cellular migration?

A
  • To move itself, a cell uses actin filaments and their associated motor proteins to generate internal pushing and pulling forces
  • Cellular migration is initiated when actin filaments polymerize near the plasma membrane and physically push it outwards
  • Recall that actin filament polymerization uses a combination of actin–binding proteins to not only extend and stabilize the actin filament, but to also disassemble other actin filaments if more actin monomers are needed
  • The pushing forces of the extending actin filament on the plasma membrane do not rip the membrane due to the hydrophobic interactions between the membrane phospholipids holding it together
48
Q

what are the types of actin filaments responsible for cellular migration?

A
  • Three different types of actin filaments can push against the cell membrane
  • It is important to note that cytoskeletal remodelling may need to occur throughout the cytosol during movement, and not just at the leading and following edges of the cell

—–> FILOPODIA
- Filopodia are thin, parallel bundles of filaments
All of the filaments have the same polarity with the plus end facing the membrane and the filopodia extend in the direction of the intended movement

—–> LAMELLIPODIA
- Lamellipodia are larger, sheet-like bundles of actin filaments

  • Again, these structures are polar, with the plus end towards the plasma membrane
  • Lamellipodia form broader structures that distend a wider amount of the plasma membrane in the same direction as the filopodia
  • As the filopodia and lamellipodia extend, plasma-membrane bound integrins bind to the extracellular matrix
  • Internally, the actin filaments bind to the integrins as anchors

—–> STRESS FIBRES
- Stress fibres are the third actin filament type that forms around the integrins

  • They resemble the filopodia, but their polarity is different
  • Instead of growing towards the membrane, the plus ends are oriented towards the cytosol
  • These stress fibres are rich in motor proteins and are anchored to the integrins that allow the actin filaments to move forward
  • At the trailing edge of the cell, integrins are internalized and recycled, and actin stress filaments are disassembled so the actin monomers may be used elsewhere
49
Q

what are the 2 main overarching phases of the cell cycle?

A

INTERPHASE and MITOSIS

50
Q

what are the 3 main phases of interphase?

A

G1, S and G2 phase

51
Q

TRUE or FALSE?

A

majority of the cell’s life occurs in the G1 phase

52
Q

what are cell cycle checkpoints and what are their purpose?

A
  • The cell expands a great deal of energy to divide, thus making division an expensive activity
  • During division, cellular processes stop and massive rearrangements happen, making it also very risky
  • Cells have therefore developed switches, or checkpoints, to control these points of transition between phases to avoid unnecessary energy waste
  • There are many checkpoints, but this course will only focus on four

-The functional unit of checkpoints are cyclins and cyclin-dependent kinases (CDKs)

  • Cyclins are a class of proteins associated with progression through the cell cycle
  • CDKs bind to their respective cyclins to become activated
  • Once activated, these kinases phosphorylate other proteins to trigger the next stage of the cell cycle
53
Q

how are the apoptotic pathways altered in cancer cells?

A
  • cancer cells must have the ability to evade apoptosis and to manipulate the cell cycle to facilitate continuous replication
54
Q

what is the p53 protein and what is its function?

A

-A key example of the importance of cell cycle checkpoints and apoptosis in controlling cancer is the p53 protein

  • P53 is a tumour suppressor protein that ensures cells with damaged DNA don’t divide
  • P53 performs its guardian functions at the G1/S checkpoint
  • If DNA damage is undetected before replication, then mutations will carry forward to future generations of that cell
  • Therefore, p53 must act in the G1 phase, prior to the start of DNA replication
  • It can also initiate apoptosis in cells with damaged DNA
  • If this protein is dysfunctional, cells acquire the ability to evade apoptosis and replicate uncontrollable
  • Over 50% of tumours have a mutation in the gene that codes for p53
55
Q

what is Li-Fraumeni Syndrome (LFS)?

A

-LFS is a hereditary genetic condition that can be passed down generationally

  • It results in an increased risk of cancer in people with the gene
  • Individuals with LFS often have a mutation in the gene that codes for p53, which controls apoptosis and progression through the cell cycle
  • Without a functioning p53 protein, people with LFS are at a much greater risk for developing cancer

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