The Cell Cycle Flashcards
(190 cards)
1
Q
What is the term for the cycle of duplication and division of a cell?
A
Cell cycle
2
Q
What are the two major phases of the cell cycle?
A
- S phase
- M phase
3
Q
What does S phase stand for in the cell cycle?
A
DNA synthesis
4
Q
How long does S phase typically take in a mammalian cell?
A
10–12 hours
5
Q
What occurs during M phase of the cell cycle?
A
- Mitosis
- Cytokinesis
6
Q
What is the approximate duration of M phase in a mammalian cell?
A
Less than an hour
7
Q
What are sister chromatids?
A
Identical copies of a chromosome linked together
8
Q
During which stage of mitosis do sister chromatids align at the spindle equator?
A
Metaphase
9
Q
What happens to sister chromatids at the start of anaphase?
A
They are separated and pulled to opposite poles
10
Q
What is the length of interphase in a typical human cell cycle?
A
Approximately 23 hours
11
Q
What is the G1 phase in the cell cycle?
A
Gap phase between M phase and S phase
12
Q
What is the G2 phase in the cell cycle?
A
Gap phase between S phase and M phase
13
Q
What is the role of the G1 phase?
A
To monitor the environment and prepare for DNA replication
14
Q
What is the specialized resting state that cells can enter during G1 phase called?
A
G0 phase
15
Q
What happens at the Start point in the G1 phase?
A
Cells commit to DNA replication
16
Q
What is endoreduplication?
A
Multiple rounds of S phase without intervening M phases
17
Q
What method can be used to measure the DNA content of cells?
A
Flow cytometry
18
Q
What is the main function of the cell cycle?
A
To duplicate DNA and segregate it into daughter cells
19
Q
What is the significance of the cell-cycle control system?
A
To regulate the progression of the cell cycle
20
Q
What phases are separated by gap phases in the cell cycle?
A
G1 and G2
These phases regulate cell-cycle progression.
21
Q
What are the basic principles of the cell-cycle control system?
A
Operates like a timer triggering events in a set sequence
The control system is independent of the events it controls.
22
Q
What happens if DNA synthesis is not completed successfully?
A
The control system delays progression to M phase
This prevents the segregation of incompletely replicated chromosomes.
23
Q
What type of switches does the cell-cycle control system consist of?
A
Binary switches (on/off) that launch events irreversibly
This ensures complete execution of cell-cycle events.
24
Q
How does the cell-cycle control system maintain reliability?
A
It is robust and adapts to various conditions
This allows it to function effectively even if some components fail.
25
What are the three major regulatory transitions in the cell cycle?
* Start (restriction point) in late G1
* G2/M transition
* Metaphase-to-anaphase transition
## Footnote
These transitions are critical for cell-cycle progression.
26
What are cyclin-dependent kinases (Cdks)?
A family of protein kinases that regulate the cell cycle
## Footnote
Their activity fluctuates as the cell progresses through the cycle.
27
What are the four major classes of cyclins?
* G1/S-cyclins
* S-cyclins
* M-cyclins
* G1-cyclins
## Footnote
Each class is involved at different stages of the cell cycle.
28
What is the role of G1/S-cyclins?
Activate Cdks in late G1 to trigger progression through Start
## Footnote
Their levels fall during S phase.
29
What do S-cyclins do?
Stimulate chromosome duplication and early mitotic events
## Footnote
Their levels remain elevated until mitosis.
30
What is the function of the APC/C in the cell cycle?
Initiates the metaphase-to-anaphase transition
## Footnote
It is a regulatory protein complex involved in cell cycle control.
31
In vertebrates, how many Cdks are there?
Four Cdks
## Footnote
Each interacts with different classes of cyclins.
32
What is the function of cyclins in relation to Cdks?
Cyclins activate Cdks by binding to them
## Footnote
Without cyclins, Cdks have no activity.
33
What do protein phosphatases do in the cell-cycle control system?
They reverse the effects of Cdks and other kinases by removing phosphate groups.
34
What is the analogy used to explain protein phosphorylation levels?
The level of protein phosphorylation is compared to the level of water in a sink, dependent on the rate of water flow in (kinase activity) and out (phosphatase activity).
35
What is the significance of Protein phosphatase 2A (PP2A)?
PP2A is a critical regulator of Cdk substrates during the cell cycle and comes in multiple forms based on its regulatory subunit.
36
What is the role of cyclins in substrate phosphorylation order?
Cyclins direct Cdk partners to specific target proteins, influencing the timing and order of substrate phosphorylation.
37
What mechanisms contribute to the switchlike behavior of cell-cycle transitions?
Positive feedback mechanisms generate robust, all-or-none regulatory effects critical for cell-cycle progression.
38
What initiates the activation of M-Cdk at the G2/M transition?
The activation of the protein phosphatase Cdc25 removes inhibitory phosphates from M-Cdk, leading to rapid activation.
39
What triggers the metaphase-to-anaphase transition?
The transition is triggered by the anaphase-promoting complex (APC/C) leading to protein destruction.
40
How does the activity of M-Cdk change during mitosis?
M-Cdk activity continues to rise as the cell progresses through mitosis.
41
What happens to Cdc25 and Wee1 during the activation of M-Cdk?
Cdc25 is activated while Wee1 is inhibited, resulting in increased M-Cdk activity.
42
How does the cell ensure the timing of protein phosphorylation during the cell cycle?
Timing is influenced by the activation of cyclin–Cdk complexes, substrate affinities, and the opposing actions of phosphatases.
43
What is the key regulator of the metaphase-to-anaphase transition?
The anaphase-promoting complex, or cyclosome (APC/C)
## Footnote
APC/C is a member of the ubiquitin ligase family of enzymes.
44
What does the APC/C catalyze the ubiquitylation and destruction of?
Two major proteins:
* Securin
* S- and M-cyclins
## Footnote
The destruction of securin activates a protease that separates sister-chromatid pairs.
45
How does the APC/C ensure that cyclins and other proteins are kept at low levels until the next cell cycle?
By transitioning from Cdc20 to Cdh1 during late mitosis and G1
## Footnote
Cdh1 maintains APC/C activity after anaphase.
46
What is the role of Cdc20 in the cell cycle?
Cdc20 activates APC/C during metaphase to trigger the destruction of securin and cyclins
## Footnote
This results in chromosome segregation in anaphase.
47
What happens to M-Cdk activity in late mitosis?
M-Cdk activity declines, leading to increased accumulation of CKIs like Sic1
## Footnote
This helps keep M-Cdk activity low after mitosis.
48
What triggers the release of braking mechanisms that suppress Cdk activity in G1?
Rising G1/S-Cdk activity
## Footnote
This initiates a new cell cycle.
49
What is the role of protein kinases and phosphatases in the cell cycle?
They modify Cdks to regulate their activity
## Footnote
Examples include CAK, Wee1 kinase, and Cdc25 phosphatase.
50
How does M-Cdk influence M-cyclin gene expression?
M-Cdk promotes the expression of M-cyclin genes, creating a positive feedback loop
## Footnote
This loop is turned off as cells exit from mitosis.
51
What is the overall function of the cell-cycle control system?
To trigger cell cycle events and ensure they are properly timed and coordinated
## Footnote
Central to this system are the cyclin-dependent protein kinases (Cdks).
52
What is the result of M-Cdk activity during the G2/M transition?
It triggers the events of early mitosis and chromosome alignment on the mitotic spindle
## Footnote
M-Cdk also activates APC/C–Cdc20.
53
What is the central event of chromosome duplication?
DNA replication
## Footnote
DNA replication must occur with extreme accuracy to minimize mutations and every nucleotide must be copied once to prevent gene amplification.
54
What are the two distinct steps in the initiation of DNA replication?
1. Licensing of replication origins in late mitosis or early G1
2. Activation of Mcm helicases in S phase
55
What is the origin recognition complex (ORC)?
A large multiprotein complex that binds to replication origins
56
How is chromatin structure duplicated during S phase?
By synthesizing chromatin proteins and assembling them onto newly synthesized DNA.
57
What are the two forms of chromatin structure?
* Heterochromatin
* Euchromatin
58
What is the role of cohesin in sister-chromatid cohesion?
Cohesin forms a ring structure that holds sister chromatids together.
59
What is the main mechanism by which sister-chromatid cohesion is maintained during S phase?
Cohesin is loaded around unduplicated chromosomes before S phase.
60
What enzyme is involved in disentangling sister DNA molecules?
Topoisomerase II
61
What major events occur during M phase?
* Mitosis
* Cytokinesis
62
What are the five stages of mitosis?
* Prophase
* Prometaphase
* Metaphase
* Anaphase
* Telophase
63
What triggers the events of early mitosis?
An increase in M-Cdk activity at the G2/M transition.
64
What is the significance of sister-chromatid cohesion?
It facilitates the attachment of sister chromatids to opposite poles of the mitotic spindle.
65
What is the outcome of defects in sister-chromatid cohesion?
Major errors in chromosome segregation.
66
What is the second major event of M phase following mitosis?
Cytokinesis
## Footnote
Cytokinesis divides the cell into two halves, each with an identical nucleus.
67
What triggers the events of early mitosis?
An increase in M-Cdk activity at the G2/M transition
## Footnote
M-Cdk is a cyclin-dependent kinase that regulates the cell cycle.
68
What are the two major parts of mitosis from a regulatory point of view?
* Early mitosis (prophase, prometaphase, and metaphase)
* Late mitosis (anaphase and telophase)
## Footnote
Each part is governed by distinct components of the cell-cycle control system.
69
What is the role of M-Cdk in mitosis?
M-Cdk induces the assembly of the mitotic spindle, chromosome condensation, and breakdown of the nuclear envelope
## Footnote
It also rearranges the actin cytoskeleton and the Golgi apparatus.
70
What triggers sister-chromatid separation during mitosis?
Destruction of securin by the APC/C
## Footnote
This liberates a protease that cleaves cohesin.
71
What is cohesin?
A protein complex with four subunits that holds sister chromatids together
## Footnote
It consists of two SMC proteins (Smc1 and Smc3) and associated subunits (Scc1 and Scc3).
72
What is the function of condensin in mitosis?
Helps configure duplicated chromosomes for separation
## Footnote
Condensin is involved in chromosome condensation and sister-chromatid resolution.
73
What is the main function of the mitotic spindle?
Chromosome segregation
## Footnote
It is a bipolar array of microtubules that pulls sister chromatids apart in anaphase.
74
What are the three types of spindle microtubules?
* Kinetochore microtubules
* Non-kinetochore microtubules
* Astral microtubules
## Footnote
Kinetochore microtubules attach to sister chromatids, non-kinetochore provide stability, and astral microtubules help position the spindle.
75
What is the role of the centrosome in mitosis?
Nucleates a radial array of microtubules
## Footnote
The centrosome consists of pericentriolar material surrounding a pair of centrioles.
76
What happens during prophase of mitosis?
Replicated chromosomes condense and the mitotic spindle assembles
## Footnote
The nuclear envelope remains intact at this stage.
77
During which phase do chromosomes align at the equator of the spindle?
Metaphase
## Footnote
Kinetochore microtubules attach sister chromatids to opposite poles.
78
What occurs during telophase?
Daughter chromosomes arrive at spindle poles and decondense
## Footnote
A new nuclear envelope reassembles around each set.
79
What is the function of cytokinesis?
Divides the cytoplasm into two daughter cells
## Footnote
This is achieved by a contractile ring of actin and myosin filaments.
80
Do all cells have centrosomes?
No, some cells, such as those of higher plants and oocytes of many vertebrates, do not have centrosomes.
81
What are the three general categories of microtubule-associated proteins involved in spindle assembly?
* Nucleating factors
* Regulatory proteins
* Motor proteins
82
What is the primary function of microtubules in the spindle?
They are the building blocks of the spindle and are required for its assembly.
83
What complex is most important for nucleating microtubule assembly?
The γ-tubulin ring complex (γ-TuRC).
84
What is centrosome maturation?
The process where the number of γ-TuRCs in each centrosome increases greatly at the beginning of mitosis.
85
What do mitotic chromosomes generate to promote microtubule formation?
Local signals that activate γ-TuRC.
86
What is dynamic instability in microtubules?
A state where individual microtubules are either growing or shrinking and switch between the two states.
87
How does entry into mitosis affect microtubule lifetime?
The average lifetime of a microtubule decreases dramatically.
88
What regulates microtubule dynamics during mitosis?
Microtubule-associated proteins (MAPs) and depolymerization factors.
89
What are the two families of microtubule-dependent motor proteins?
* Kinesin-related proteins
* Dyneins
90
What is the function of kinesin-5 in spindle assembly?
It slides antiparallel microtubules in opposite directions to push their minus ends toward the poles.
91
What role does cytoplasmic dynein play in spindle organization?
It connects new microtubules to those nucleated at a centrosome and helps focus the spindle poles.
92
What is the relationship between centrosome duplication and chromosome duplication?
Both use a semiconservative mechanism and must replicate once and only once per cell cycle.
93
What initiates spindle assembly in cells containing centrosomes?
The movement of centrosomes apart along the nuclear envelope in prophase.
94
What is required for the attachment of the spindle to sister-chromatid pairs?
The removal of the nuclear envelope.
95
What initiates the breakdown of the nuclear envelope?
Phosphorylation of several subunits of the nuclear pore complexes by M-Cdk.
96
What is the role of M-Cdk during mitosis?
M-Cdk phosphorylates components of the nuclear lamina, leading to nuclear lamina disassembly and detachment of lamin proteins and chromosomes from the nuclear envelope.
## Footnote
This process incorporates these components into the membranes of the endoplasmic reticulum.
97
What initiates centriole duplication during the cell cycle?
G1/S-Cdk and Plk4 initiate centriole duplication, assembling a new centriole (procentriole) at a single site on the side of each parent centriole.
## Footnote
The elongation of the procentrioles is usually completed in G2.
98
What changes occur in centrosomes upon entry into mitosis?
The tether between centrosomes is removed, the immature parent centriole acquires appendages (centriole maturation), and the centrosomes separate, forming a new spindle between them.
## Footnote
These changes are triggered by protein kinases M-Cdk, Plk1, and other regulators.
99
What is the significance of centriole disengagement after mitosis?
Centriole disengagement is required for centriole duplication in the subsequent cell cycle, ensuring centrioles only duplicate once per cycle.
100
How do mitotic chromosomes influence spindle assembly?
Mitotic chromosomes create a local environment that favors microtubule nucleation and stabilization, actively participating in spindle formation.
## Footnote
They can stimulate the formation of new microtubules around them.
101
What is the role of the GTPase Ran in mitotic spindle formation?
Ran-GTP activates microtubule-regulatory proteins, stimulating local nucleation and stabilization of microtubules around chromosomes.
## Footnote
This process is essential for spindle assembly in the absence of centrosomes.
102
What are the functions of kinesin-5 and dynein in spindle assembly?
Kinesin-5 organizes microtubules into antiparallel bundles, while dynein focuses minus ends into discrete poles at each end of the bipolar array.
103
What is bi-orientation in the context of mitosis?
Bi-orientation is the attachment of sister chromatids to opposite poles of the mitotic spindle, ensuring correct separation during anaphase.
104
What prevents both kinetochores from attaching to the same spindle pole?
Sister kinetochores are constructed in a back-to-back orientation, which helps to prevent incorrect attachments.
105
What is the role of the Ndc80 complex in microtubule attachment?
The Ndc80 complex attaches to the plus end of the microtubule, allowing polymerization and depolymerization while remaining connected to the kinetochore.
## Footnote
The Ndc80 complex is crucial for the stability of kinetochore-microtubule attachments during mitosis.
106
What role does tension play in kinetochore function?
Tension indicates correct bi-orientation and increases microtubule attachment affinity at the kinetochore.
## Footnote
High tension shuts off inhibitory signals that would otherwise weaken microtubule attachments.
107
What occurs to chromosomes at the metaphase plate during mitosis?
Chromosomes are tugged back and forth, eventually aligning equidistantly between the two spindle poles.
## Footnote
This positioning is critical for the accurate separation of sister chromatids.
108
What are the three major forces acting on chromosomes during mitosis?
* Kinetochore-generated poleward force
* Microtubule flux
* Polar ejection force
## Footnote
These forces work together to ensure proper chromosome movement and alignment during mitosis.
109
What drives the kinetochore toward the spindle pole during anaphase?
Depolymerization at the plus end of the microtubule generates a poleward force.
## Footnote
This movement does not require ATP or motor proteins; it is driven by energy stored in the microtubule.
110
What is microtubule flux?
Microtubule flux is the process where microtubules are pulled toward spindle poles and dismantled at their minus ends.
## Footnote
This contributes to the poleward forces acting on chromosomes during metaphase and anaphase.
111
What is the role of kinesin motors in the polar ejection force?
Kinesin-4 and kinesin-10 motors transport chromosomes away from the spindle poles.
## Footnote
This force helps align sister-chromatid pairs at the metaphase plate.
112
What initiates sister-chromatid separation during mitosis?
The anaphase-promoting complex/cyclosome (APC/C) initiates separation by ubiquitylating and targeting regulatory proteins for destruction.
## Footnote
This process involves the degradation of securin, which releases separase to cleave cohesin.
113
What happens to cohesins during the transition from metaphase to anaphase?
Cohesins are abruptly removed, allowing sister chromatids to separate and move to opposite poles.
## Footnote
The removal is facilitated by separase cleaving the Scc1 subunit of cohesin.
114
What is the significance of dephosphorylation in the later stages of mitosis?
Dephosphorylation is required for spindle disassembly and the re-formation of daughter nuclei in telophase.
## Footnote
This process is partly due to the inactivation of Cdks, which occurs when APC/C targets cyclins for destruction.
115
What triggers the events of early mitosis?
M-Cdk triggers the events of early mitosis.
116
What is the role of APC/C in anaphase?
APC/C stimulates the destruction of the proteins that hold the sister chromatids together.
117
What happens during the transition from metaphase to anaphase?
Sister chromatids suddenly and synchronously separate and move toward opposite poles of the mitotic spindle.
118
What does the spindle assembly checkpoint ensure?
The spindle assembly checkpoint ensures that cells do not enter anaphase until all chromosomes are correctly bi-oriented on the mitotic spindle.
119
What are the two processes of chromosome movement during anaphase?
Anaphase A and Anaphase B.
120
What characterizes anaphase A?
The initial poleward movement of the chromosomes accompanied by shortening of the kinetochore microtubules.
121
What characterizes anaphase B?
The separation of the spindle poles themselves.
122
What type of motor proteins are involved in spindle-pole separation during anaphase B?
Dynein and Kinesin-5.
123
What happens at telophase?
The two sets of chromosomes are packaged into a pair of daughter nuclei.
124
What is the first major event of telophase?
The disassembly of the mitotic spindle.
125
What is required for the completion of a normal anaphase?
Dephosphorylation of Cdk substrates.
126
What happens to nuclear pore complexes during telophase?
Nuclear pore complexes are incorporated into the nuclear envelope.
127
What does the spindle assembly checkpoint depend on?
A sensor mechanism that monitors microtubule attachment at the kinetochore.
128
What triggers the activation of APC/C by Cdc20?
The proper attachment of all sister-chromatid pairs to the spindle.
129
What is the role of phosphatases during mitosis?
Phosphatases drive protein dephosphorylation and the completion of mitosis.
130
What occurs to the nuclear envelope during telophase?
It re-forms around the chromosome cluster.
131
What is cytokinesis?
The division of the cytoplasm in two.
132
In which phase of the cell cycle does cytokinesis occur?
It follows every mitosis.
133
What is the contractile ring composed of?
* Actin filaments
* Myosin II filaments
* Structural and regulatory proteins
134
When does cytokinesis begin in animal cells?
In anaphase.
135
What structure appears on the cell surface to initiate cytokinesis?
A cleavage furrow.
136
What happens to the contractile ring during cytokinesis?
It gradually contracts and is disassembled after completion.
137
What is the function of the contractile ring?
To generate the force that divides the cytoplasm in two.
138
What triggers the assembly of the contractile ring?
The local formation of new actin filaments.
139
What is the role of RhoA in cytokinesis?
It controls the assembly and function of the contractile ring.
140
What activates RhoA at the future division site?
A guanine nucleotide exchange factor (GEF) called Ect2.
141
What is the midbody in cytokinesis?
The narrow membrane bridge between the daughter cells.
142
What process completes cytokinesis?
Abscission.
143
What is the primary role of the mitotic spindle in cytokinesis?
To ensure division occurs at the right time and place.
144
What type of cells undergo cytokinesis without a contractile ring?
Early Drosophila embryos, some mammalian hepatocytes, and heart muscle cells.
145
What do formins do in the context of cytokinesis?
They nucleate the assembly of parallel arrays of linear, unbranched actin filaments.
146
What role do astral microtubules play in cytokinesis?
They help focus centralspindlin and Ect2 at the equator.
147
What is the primary method of membrane addition during cytokinesis in plant cells?
Transport of small membrane vesicles from the Golgi apparatus.
148
What happens to the endoplasmic reticulum (ER) during mitosis?
The ER is released and largely intact, then cut in two during cytokinesis
## Footnote
The ER is continuous with the nuclear membrane and is reorganized during cell division.
149
What is the fate of mitochondria and chloroplasts during cell division?
They are inherited by daughter cells as they are usually present in sufficient numbers
## Footnote
This allows for safe inheritance during cell division.
150
What is the significance of spindle positioning during asymmetric cell division?
It allows the mother cell to segregate cell-fate determinants to one side
## Footnote
This ensures that the daughter cells develop along different pathways.
151
What triggers cytokinesis after anaphase?
Dephosphorylation of Cdk targets due to Cdk inactivation
## Footnote
This process ensures that cytokinesis occurs at the correct time.
152
What is meiosis?
A specialized nuclear division process that produces haploid cells from diploid cells
## Footnote
Meiosis is essential for sexual reproduction and includes two rounds of chromosome segregation.
153
What distinguishes meiosis I from meiosis II?
Meiosis I segregates homologs, while meiosis II separates sister chromatids
## Footnote
This results in the production of four haploid nuclei from one diploid nucleus.
154
What is the outcome of a single diploid nucleus entering meiosis?
It produces four haploid nuclei, each carrying either the maternal or paternal copy of each chromosome
## Footnote
This genetic variation is crucial for sexual reproduction.
155
What occurs during the meiotic S phase?
Chromosome duplication takes place
## Footnote
This results in pairs of sister chromatids that are linked by cohesin complexes.
156
What is a key difference in how chromosomes line up during meiosis compared to mitosis?
Duplicated homologs pair and recombine in meiosis, while chromosomes line up individually in mitosis
## Footnote
This pairing is essential for genetic diversity.
157
What is meiosis?
A form of nuclear division involving one round of chromosome duplication followed by two rounds of chromosome segregation.
158
What is produced at the end of meiosis?
Four genetically different haploid nuclei.
159
What is the primary difference between meiosis and mitosis?
Meiosis involves homologous chromosomes pairing and crossing over, while mitosis does not.
160
What is crossing-over?
A genetic recombination event where DNA exchanges occur between homologous chromatids.
161
What is a chiasma?
The thin connection seen microscopically between homologs after crossover events.
162
What must occur for homologs to segregate properly during meiosis I?
Homologs must recognize and associate physically.
163
How do sister kinetochores attach during meiosis I?
Both sister kinetochores attach to the same spindle pole.
164
What is the role of cohesin during meiosis I?
Cohesin holds sister chromatids together and is selectively removed during anaphase I.
165
How does the process of crossing-over contribute to genetic diversity?
It creates new combinations of alleles in gametes.
166
What are 'hot spots' in the context of crossing-over?
Regions on chromosomes where double-strand breaks occur frequently.
167
What proteins help pull homologs together during pairing?
Recombination complex.
168
What triggers homolog separation at the onset of anaphase I?
Loss of arm cohesion due to cohesin removal.
169
What is nondisjunction?
The failure of homologs to separate properly during meiosis, leading to gametes with abnormal chromosome numbers
170
What are the three fundamental processes that determine organ and body size?
* Cell growth
* Cell division
* Cell survival
171
What are the three major classes of extracellular signal molecules that regulate cell behavior?
* Mitogens
* Growth factors
* Survival factors
172
What do mitogens do?
Stimulate cell division by triggering G1/S-Cdk activity
173
What is the role of growth factors?
Stimulate cell growth by promoting protein synthesis and inhibiting degradation
174
What do survival factors promote?
Cell survival by suppressing apoptosis
175
What state do cells enter in the absence of mitogenic signals?
G0 (a specialized nondividing state)
176
Which cells are typically in a terminally differentiated G0 state?
Most neurons and skeletal muscle cells
177
What is the primary control mechanism for cell division in animal cells?
Mitogens acting in the G1 phase of the cell cycle
178
What is the role of E2F proteins in the cell cycle?
Activate transcription of genes required for S-phase entry
179
What does the activation of G1-Cdk complexes lead to?
Increased expression of genes encoding G1/S-cyclins and S-cyclins
180
What is the effect of DNA damage on the cell cycle?
Blocks progression through the cell cycle
181
What happens when DNA is damaged?
The cell-cycle control system can detect DNA damage and arrest the cycle at Start and G2/M transitions
## Footnote
This prevents entry into the cell cycle and mitosis.
182
What is the effect of Chk1 and Chk2 on Cdc25 protein phosphatases?
Chk1 and Chk2 phosphorylate Cdc25, inhibiting its function
## Footnote
This inhibition helps block entry into mitosis.
183
What happens when a replication fork fails during DNA replication?
The same mechanisms that respond to DNA damage detect stalled replication forks and block entry into mitosis
## Footnote
This prevents segregation of partially replicated chromosomes.
184
What is replicative cell senescence?
A phenomenon where human cells divide a limited number of times before undergoing permanent cell-cycle arrest
## Footnote
Fibroblasts typically go through 25–50 population doublings.
185
What is the role of growth factors in cell growth?
Growth factors stimulate cell growth by binding to cell surface receptors and activating intracellular signaling pathways
## Footnote
These pathways increase protein synthesis and nutrient uptake.
186
What is mTORC1 and its function?
mTORC1 is a protein kinase that regulates cell growth by stimulating metabolic processes
## Footnote
It activates targets involved in protein and lipid synthesis.
187
How do proliferating cells coordinate their growth and division?
Cells must ensure that cell size doubles with each division
## Footnote
The mechanisms for this coordination can vary among different organisms and cell types.
188
What happens to muscle cells and nerve cells regarding cell cycle?
They can grow dramatically after permanently withdrawing from the cell cycle
## Footnote
This allows for growth without division.
189
What is the consequence of a malfunctioning DNA damage response?
Accumulation of genetic damage leads to an increased frequency of cancer-promoting mutations
## Footnote
Mutations in the p53 gene occur in at least half of all human cancers.
190
What is the fate of cells with severe DNA damage in multicellular organisms?
They undergo apoptosis instead of continuing division
## Footnote
This is to prevent the propagation of genetic damage.
