Phys Concepts 1.17 Flashcards
1
Q
What is the cell cycle?
A
The orderly sequence of events by which a cellduplicates its contents and divides in two
2
Q
The cell cycle is driven by specific molecular signals present in the __
A
cytoplasm
3
Q
The cell cycle consists of 2 phases, what are they?
A
- mitotic (M) phase (mitosis and cytokinesis)
- interphase (cell growth and copying of chromosomes in preparation for cell division)
4
Q
Interphase is about _% of the cell cycle
A
90
5
Q
Interphase can be divided in subphases, what are they?
A
- G1 phase: first gap
- S phase: synthesis
- G2 phase: second gap
- G0 phase: resting phase, postmitotic quiescent
6
Q
What is interphase?
A
cell growth and copying of chromosomes in preparation for cell division
7
Q
What is the grey? What is the peach?
A
grey: interphase
peach: mitosis
8
Q
What is G1?
A
first gap phase
9
Q
What is the cell preparing for in G1?
A
entering DNA synthesis phase
10
Q
T/F: Cell is metabolically active in G1.
A
True
11
Q
What does G1 require? What synthesis is occuring?
A
- Requires nutrients & growth factors
- RNA, protein, lipid and carbohydrate synthesis occurs
12
Q
T/F: No organelles are duplicated in G1
A
False, many are
13
Q
How long is G1?
A
variable (6-24 hours)
- short in embryonic and cancer cells
- rapid or non-existent in rapidly dividing cells
14
Q
What phase is DNA and chromosomal protein synthesis occuring?
A
S
15
Q
What is the duration of S phase?
A
approx. 7-8 hours in a typical mammalian cell w/ a 16 hour cycle
16
Q
What is the cell committed to in S phase?
A
cell division
17
Q
T/F: growth factors are needed in S phase
A
False
18
Q
What phase does DNA replication occur?
A
S phase
19
Q
What is G2 phase?
A
second growth phase
20
Q
What occurs at G2 phase?
A
enzyme, protein, and ATP synthesis
21
Q
What is the duration of G2?
A
lasts approximately 3 hours in a typical mammalian cell with a 16-hour cycle
22
Q
What is M phase?
A
mitotic phase
23
Q
What does the cell undergo during M phase?
A
mitosis and cytokinesis
24
Q
What is the duration of the M phase?
A
1-2 hours
25
M phase is about _% of the cell cycle
10
26
What state is the cell in in G0 phase?
state of withdrawal from cell cycle
27
Is the cell dividing or preparing to divide in G0?
neither
28
What is the cell doing during G0?
Instead, the cell is “doing its job” - performing it’s function within the tissue
29
What are examples of cells in G0?
hepatocytes, neurons
30
What type of cell is G0 common in?
differentiated cells
31
In order to progress through the cell cycle, a variety of __ must be turned on.
signals
32
What happens if the environment is not favourable or there are errors in DNA?
the cell cycle is "paused" at several main check points (aka "transitions")
33
What are checkpoints of the cell based on?
series of biochemical switched to initiate a specific cell-cycle events
34
What is the cell cycle control system?
Checkpoints, biochemical switches that decide whether the cell cycle continues
35
what are features of the biochemical switched?
1. Generally binary (on/off) to launch an event in a complete & irreversible fashion
2. Robust & reliable
3. Adaptable & modified to suit specific cell types
36
How are biochemical switched robust & reliable?
Contains back up mechanisms to ensure efficacy under variable conditions & if some components fail
37
How are biochemical switched adaptable and modified to suit specific cell types?
Responds to specific intracellular or extracellular signals
Cyclin dependent kinases (Cdks)
38
What are checkpoints (transitions)?
Points in the eukaryotic cell division cycle where progress through the cycle can be halted until conditions are suitable for the cell to proceed to the next stage
39
What are checkpoints regulated by?
- Factors within the cell, mostly controlled by the “health” or “state of preparation” of the cell
- Factors from outside the cell – i.e. messages from other cells within the same tissue or distant cells
40
Where are the 3 major regulatory transitions?
1. Start Transition (aka G1/S)
2. G2/M transition
3. Metaphase-to-anaphase transition (aka M-to-A)
41
For most cells, the G1/S seems to be the __-__ and __ step of the cell cycle
rate-limiting; committing
42
Which phase is the rate-limiting and committing step of the cell cycle?
G1/S
43
__ __ MUST be present for a cell to pass through the following checkpoints
specific signals
44
If cell detects problem inside or outside the cell, what happens?
it will block progression beyond the checkpoint
45
If extracellular conditions are not appropriate for cell proliferation, what happens?
central control system blocks progression through the start transition
46
If there is a problem with completion of DNA replication, what happens?
cell will be held at G2/M checkpoint
47
If all the chromosomes are not attached to the spindle, what happens?
cell will be held at M phase (metaphase to anaphase)
48
What are the keys to the cell cycle control system?
cyclin-dependent kinases (Cdks)
49
What are Cdks responsible for?
cyclical changes in phosphorylation of intracellular proteins that initiate/regulate the major events of the cell cycle
50
What are Cdks controlled by?
cyclins
51
What do cyclical changes in cyclin protein levels result in?
cyclic assembly and activation of cyclin–Cdk complexes at specific stages of the cell cycle
52
What is needed to progess through a checkpoint? What is the exception of this?
- Correct & functional cyclin-cdk complexes
- Exception: M-to-A checkpoint is a little different
53
What are the 4 different class of cyclins?
1. G1 cyclins: cyclin D
2. G1/S cyclins: cyclin E
3. S-cyclins: Cyclin A
4. M cyclins: Cyclin B
54
What do G1 cyclins (cyclin D) form complex with?
Cdk4 or Cdk 6
55
What is G1 cyclin (cyclin D) used for?
Involved in G1 phase of cell cycle, needed for initiation of transcription of G1/S cyclins to help promote passage through start transition
56
What do G1/S cyclins (Cyclin E) form complex with?
Cdk2
57
What does G1/S cyclins (cyclin E) do?
- Bind Cdk’s at the end of G1 & help trigger progression through start transition
58
When do levels of cyclin E decrease?
in S phase
59
What does S-cyclin (cyclin A) form a complex with?
Cdk1 and Cdk2
60
What does cyclin A do?
Bind Cdks after progression through start transition & helps stimulate chromosome duplication during S phase
contributes to control of some early mitotic events
61
What does Cyclin B form a complex with?
Cdk1
62
What does cyclin B do?
Bind CdKs to stimulate entry into mitosis at the G2/M transition
63
When do levels of cyclin B decrease?
mid-mitosis
64
How do cyclin-Cdk complexes work?
cyclins function by activating the Cdk
65
How does cyclin protein activate Cdk?
directs it to a specific target protein
66
T/F: A cyclin-CdK complex can induce different effects at different times in the cell cycle
True
67
Why can cyclin-Cdk complex induce different effects at different times in the cell cycle?
Since accessibility of CdK substrates change during the cell cycle
68
Proteins that function in mitosis mat only become available for phosphorylation in what stage?
G2
69
How does progression through the metaphase-to-anaphase checkpoint occur?
via regulated proteolysis
70
What is regulated proteolysis?
regulated break down of proteins
71
What is APC/C stand for?
anaphase promoting complex
72
What is APC/C a family of?
ubiquitin ligase
73
What is APC/C used for?
Used to stimulate proteolytic destruction of specific regulatory proteins
- APC/C polyubiquitinates specific target proteins for destruction in proteasomes
74
What are target proteins of APC/C?
securin, M-cyclins, S-cyclins
75
__ __ bind to specific receptors to stimulate cellular growth and proliferation
growth factors
76
What do growth factors turn on in G1 phase?
early response genes and delayed response genes
77
What do early response genes code for?
transcription factors - induce transcription of delayed response genes
78
What do delayed response genes code for?
usually Cdks, cyclins, or other proteins needed for cell division
79
In G1, in response to binding of growth factor, what happens?
Cyclin D and then E are transcribed and translated and form complexes with their Cdk's
80
What is the complex of cyclin D and Cdk4 & Cdk6 called?
G1-Cdk complex
81
What is the complex of cyclin E and Cdk2 called?
G1/S-Cdk complex
82
What allows progression through the G1/S checkpoint?
active G1-Cdk and G1/S-Cdk
83
At the G1/S checkpoint, Active G1-cdk (and G1/S-cdk) complex will target a protein called? And do what to it?
RB and phosphorylate it
84
How does RB function?
transcription co-repressor
85
Hyperphosphorylation of RB will __ RB
inactivate
86
What does inactive RB release? What does this cause?
transcription factor E2F, allowing transcription to proceed
87
Transcription of cyclin E => G1/S-cdk complex form, does what?
promotes passage through the start transition
88
In early S phase, cyclin D (G1-cdk complex) and E (G1/S-cdk complex) are targeted for?
destruction
89
What promotes progression through S phase?
desruction of cyclin D (G1-cdk complex) and E (G1/S-cdk complex)
90
Active S-cdk complex allows?
progression through the S phase of the cell cycle
91
S-Cdk complex levels are still high in?
G2
92
M-cyclin levels begin to rise in?
G2
93
M-cyclin levels begin to rise in G2 forming?
M-Cdk complex
94
What is the M-Cdk complex needed for?
to pass through G2/M checkpoint
95
At the end of G2, what is destroyed?
S-cyclins
96
How are S-cyclins destroyed?
Targeted for proteolysis by APC/C
97
Why do we need to be able to control activity of M-cyclins?
so that mitosis doesn't start too soon
98
Once the M-Cdk complex is assembled, what happens to it immediately?
it is inhibited vis phosphorylation
99
When the cell is ready for mitosis to begin, the M-Cdk complex is __-__.
de-phosphorylated
100
When can the cell progress through G2/M checkpoint & mitosis begins?
once M-Cdk is de-phorphorylated
101
M-cdk complex is needed for?
activation of various proteins needed in mitosis
102
Where is the final checkpoint?
metaphase-to-anaphase (M-to-A) checkpoint
103
APC/C complex targets a protein called securin by ubiquitylation for?
destruction by a proteosome
104
What is securin?
an inhibitory protein that protects protein linkages that hold sister-chromatid pairs together in early mitosis
104
What does destruction of securin activate?
protease that separates the sister chromatids allowing progression to anaphase
105
At the end of mitosis, M-cyclins are targeted for?
destruction by APC/C
105
What does destroying M-cyclins do?
inactivates most Cdks in cell
106
When many Cdks in the cell are inactivated at the end of mitosis, what happens?
many proteins phosphorylated by Cdks from S phase to early mitosis are dephosphorylated by various phosphatases in the anaphase cell
107
What is required for the completion of M phase, including the telophase and then cytokinesis ?
Dephosphorylation of Cdk targets
108
In unfavourable conditions, what happens to the cell cycle?
can be paused at any of the main checkpoints
109
When is progression through G1 delayed?
- DNA is damaged by radiation, chemicals, or errors
- Absence of nutrients or growth factors
- Abnormal cell size
110
When is entry into M prevented?
- DNA replication is not complete
- Presence of DNA damage
- Abnormal cell size
111
When is progression through M-to-A prevented?
Chromosomes are not properly attached to mitotic spindle
112
What are two important molecular mechanisms in unfavourable conditions?
- Cdk inhibitory proteins (CKIs)
- Proteins coded by tumour suppressor genes
113
What does binding of Cdk inhibitory proteins do?
- Inactivates cyclin-Cdk complex
- stimulate rearrangement in structure of Cdk active site
114
Who are CKI's primarily used by? What for?
used by cells to govern the activities of G1/S- and S-Cdks early in cell cycle
115
What are 3 important CKIs?
p16, p21, p27
116
What does p16 inhibit?
CyclinD-cdk4 & CyclinD-cdk5 (G1-cdk complex)
117
What does p21 inhibit?
CyclinE-cdk2 (G1/S-cdk complex)
CyclinA-cdk2 & CyclinA-cdk1 (S-cdk complex)
Cyclin B-cdk1 (M-cdk complex)
118
What does p27 inhibit?
CyclinA-cdk2 & CyclinA-cdk1 (S-cdk complex)
CyclinE-cdk2 (G1/S-cdk complex)
Cyclin B-cdk1 (M-cdk complex
119
What are 2 key tumour suppressor genes?
p53, RB
120
What do p53's do?
- Recognizes and binds damaged DNA
121
Why do unstressed cells have lower levels of p53?
since it will be bound by a protein called Mdm2 and be degraded
122
What does RB do?
- recognize damaged DNA
123
What form is RB found in?
active form
124
In the presence of DNA damage, __ will be __, releasing __
p53; phosphorylated; Mdm2
125
T/F: In the presence of DNA damage, p53 is degraded
False
126
Active p53 binds __ and promotes the transcription of __
DNA; p21
127
An inactive G1/S-cdk complex will pause the cell cycle at the ____ transition
start
128
In the presence of a growth suppressor signal or DNA damage __ is transcribed.
p16
129
What does p16 inhibit? What was this needed for?
G1-Cdk complex which was needed to inactivated RB
130
Since p16 was transcriped, RB remains activated and bound to __.
E2F
131
What does RB bound to E2F cause?
- no transcription of G1/S-cyclins or S-cyclins
- cell cycle is paused at start transition
132
The cell cycle progression can also be inhibited due to contact with?
- other cells - density-dependent inhibition
- basement membrane or other matrix component - anchorage dependence
133
What is contact inhibition regulated by?
cadherins and beta-catenin
134
What do survival signals promote?
the cell cycle and prevent apoptosis
135
What pathway is used by survival signals to promote cell cycle and prevent apoptosis?
PI3K-Akt-mTOR C pathway
136
How does Akt in the PI3K-Akt-mTOR C pathway promote cell cycle progression?
- Akt activates/increases:
Cyclin A --> activation of CDK-1
Cyclin D --> activation of CDK-4/6
Akt decreases/inactivates: p21 and p27
137
What is the template for DNA duplication?
The DNA double helix acts as a template for its own duplication
138
What will each daughter cell inherit?
- DNA double helix containing 1 original strand and 1 new strand
139
What are the 4 steps of DNA replication?
- strand separation
- primer creation
- DNA replication
- primer removal
140
What bonds help contribute to stability of DNA?
- H-bonds b/w complementary base-pairs
- Phosphodiester bond within sugar-phosphate backbone
- Hydrophobic effect due to base-stacking
141
in step 1 (strand separation), what is needed in order for DNA synthesis to begin?
DNA double helix must open up
142
What are two proteins are used to open up the double helix?
- DNA helicase
- single stranded binding proteins
143
What does DNA helicase do?
unwinds double helix - breaks H bonds
144
What do single stranded binding proteins do?
Bind tightly and cooperatively to stabilize the single strand conformation
145
In step 1 (strand separation), what does opening the double helix create?
replication fork
146
What is used at the replication fork to synthesize both new daughter strands of DNA?
DNA polymerase
147
What are limitations of DNA polymerase?
- can only add nucleotides
- only works in 5' to 3' direction
148
Why is a primer built in stage 2?
DNA polymerase can only add nucleotides to an existing strand of DNA (can only elongate a strand of nucleic acid)
149
What does the primer in stage two serve as?
base-paired chain on which to add new nucleotides
150
What is the primary made of in step 2?
RNA
151
What makes the primer in stage 2?
DNA primase
152
Step 2: Since DNA can only be synthesized in the 5’ to 3’ direction, the replication fork has an __ __
asymmetric structure
153
What is the difference between the leading strand and lagging strand in step 2?
- Leading strand: synthesized continuously
- Lagging strand: synthesized discontinuously. The direction of nucleotide polymerization is opposite to the overall direction of DNA chain growth
154
How many primers are required to start replication on the leading strand?
only one
155
How many primers are needed to start replication on the lagging strand?
one for each Okazaki fragment
156
What step is when DNA polymerase adds nucleotides in the 5' to 3' direction?
step 3 - DNA replication
157
In step 4, how are RNA primers removed and replaced with what?
by DNA repair system replaced w/ DNA
158
What does DNA ligase do in step 4?
joins 3' end of new DNA fragment w/ 5' end of previous fragment
159
How does supercoiling occur?
As the replication fork moves along the double-strand DNA, anything in front of the replication fork will become overwound forming supercoils.
160
What does DNA topoisomerase do? What does this allow?
- relieves the super-helical tension by breaking the phosphodiester bond
- allows the two sections of the DNA helix to rotate freely & relieve tension
- phosphodiester bond will re-form as DNA topoisomerase leaves
161
Why are histones needed in DNA replication?
so newly replicated DNA can be packaged into nucelosomes
162
When does histone synthesis occur?
during the S phase of the cell cycle
163
What do histone chaperones assist?
formation of assembly of histone octomer & nucleosomes
164
Only about _ mistake occurs for every 10^10 nucleotides during DNA replication
1
165
What are the DNA proofreading mechanisms?
- DNA polymerase activity
- exonucleolytic proofreading
- strand-directed mismatch repair system
166
When does DNA polymerase proofreading activity take place?
just prior to a new nucleotide being covalently added to the growing daughter chain
167
How does the DNA polymerase proofreading activity work?
- Correct nucleotide has higher affinity for the DNA polymerase than an incorrect nucleotide
- So, energetically, incorrectly paired nucleotides are less favourable and therefore more likely to diffuse away before the DNA polymerase can add them by mistake
168
When does exonucleolytic proofreading occur?
immediately after an incorrect nucleotide has been covalently added to the growing daughter chain
169
How does exonucleolytic proofreading work?
- An incorrectly added nucleotide will not provide an effective 3’-OH end for DNA polymerase to add on the next nucleotide
- Separate catalytic site on DNA polymerase will initiate DNA polymerase to move in the 3’ --> 5’ direction, cliping off any unpaired or mispaired residues
- Catalytic site: 3’-to-5’ proofreading exonuclease
170
What deals with the problem of the lagging strand ending up with a shorter DNA fragment on the daughter strand once the RNA primer has been removed?
telomeres
171
What are telomeres?
- Eukaryotes have specialized nucleotide sequences at the end of their chromosomes
- in humans: many repeats of GGGTTA
172
What are telomere DNA sequences recognized by?
telomerase
173
What does telomerase do?
replenish telomere sequences each time a cell divides
174
What cells have full telomerase activity?
stem cells
175
What cells have low/minimal telomerase activity? Why?
- most cells
- telomeres gradually shorten until a descendant cell inherits chromosome that lack telomere function
- Initiates a response causing them to withdraw permanently from the cell cycle and cease dividing - Called replicative cell senescence
176
What is replicative cell senescence?
when a cell will cease dividing due to low telomerase activity and telomere function is lacking
177
What does telomerase recognize?
the tip of an existing telomere DNA repeat on the parent strand and elongate it in the 5’ to 3’ direction
178
What does telomerase use as a template?
a intrinsic RNA template
179
After telomerase is done its job, what happens?
DNA polymerase can complete the replication
180
What are the 5 phases of mitosis?
Prophase
Prometaphase
Metaphase
Anaphase
Telophase & cytokinesis
181
What is the end-product of mitosis?
two identical daughter cells
182
What occurs during prophase?
- Chromosomes condense
- Since chromosomes have been replicated they consist of 2, closely-associated sister chromatid
- Mitotic spindles assemble between the two centrosomes
- Centrosomes have been replicated and are being moved apart
183
What is a centrosome? What does it consist of?
- protein organelle
- Consists of a pair of centrioles surrounded by a cloud or amorphous material (called pericentriolar matrix)
184
T/F: Centrosomes undergo replication during the cell cycle in preparation for mitosis
True
185
What happens during prometaphase?
- Nuclear envelope breaks down
- Chromosomes attach to spindle microtubules via a protein called a kinetochore
186
What happens during metaphase?
- Chromosomes align at the equator of the cell (halfway between the spindle poles)
- Kinetochore microtubules attach sister chromatics to opposites poles of the spindle
187
What happens during anaphase?
- Chromatids synchronously separate forming two daughter chromosomes
- Kinetochore microtubules get shorter while spindle pole moves apart
- Both these processes contribute to separation of chromosomes
188
What happens during telophase?
- Daughter chromosomes arrive at poles of spindle
- Chromosomes decondense and a new nuclear envelope reassembles around each set
189
What happens during cytokinesis?
Cytoplasm divides in two forming two daughter cells
190
How would you describe the number of chromosomes in each of the following stages of the cell cycle: (n, 2n, 4n, other)
A) G1 of the cell cycle
B) Prophase
C) After cytokinesis
A) 2n
B) 2n
C) 2n
