Chapter 18 Flashcards
Cell cycle phases
M, G1, G2, S
Most cell cycle duration differences are due to the
legnth if gap phases
Cells can also exit the cell cycle for an extended time by entering
G0 phase
What must be met before the cell cycle can transition to the next phase
Checkpoints
must bind cyclins to become
activated and phosphorylate their
target proteins
Cyclin-dependent kinases (CDKs)
Cdk regulation is carried out by
phosphorylation
Drives the cyclic assembly and activation of cyclin-Cdk complexes
Changes in the concentration of cyclin
The cell-cycle control system depends on
cyclin accumulation and degradation
Continual transcription leads to a gradual increase in the
concentration of each cyclin over the course of the cell cycle
Abrupt degradation of cyclins returns the Cdks to an
inactive state
Degradation of cyclins occurs via the
proteasome
Cyclins are tagged with a chain of ubiquitin by an enzyme called
the
anaphase-promoting complex (APC/C)
To activate a Cdk, two conditions must be met:
(1) The Cdk must bind a cyclin to form a cyclin-Cdk complex
2. each cyclin-Cdk complex must be
dephosphorylated
Cdk activity can also be blocked by the
binding of a Cdk inhibitor
M-Cdk is inhibited by phosphorylation via the
kinase Wee1
M-Cdk is activated by dephosphorylation by the
phosphatase Cdc25
what is the target if phosphorylation
Cdk (not cyclin)
Cdk activity thus requires both ___ on inhibitory sites
cyclin binding and dephosphorylation
To progress into mitosis, M-Cdk must be
dephosphorylated by
Cdc25
Once M-Cdk is activated it phosphorylates and activates nore Cdc25 which ___, it is a __ feedback loop, and increase in M-Cdk activity at the start of __
dephosphorylates, positive, M phase
important to
prevent transmission of
mutations or inaccurate
chromosome separation
checkpoint
Protein kinases respond to
DNA damage by
phosphorylating and
activating
p53
activates transcription of a Cdk inhibitor
p53
p53 is an important…
tumor suppressor gene, it many cancers
If the damage cannot be
repaired, p53 causes the
cell to
die
Match the scenario with the outcome:
___ The cell detects that its DNA is not fully replicated.
____ The cell detects that its chromosomes are not properly attached to the mitotic spindle.
Outcomes:
(a) The cell would delay entry into M phase
(b) The cell would delay exit from M phase.
a
b
Match the scenario with the outcome:
____ The cell lacks a protein required for a checkpoint mechanism that operates in G2
____ The cell lacks a protein required for a checkpoint mechanism that operates in G1
Outcomes:
(a) The cell would enter S phase under
conditions when normal cells would
not.
(b) The cell would enter M phase under
conditions when normal cells would not.
b
a
the cell reorganizes and distributes its
components equally into two daughter cells in what phase
M phase
M phase includes
mitosis and cytokinesis
pulls the duplicated chromosomes apart
during mitosis
mitotic spindle
divides the cytoplasm into two halves
during cytokinesis
contractile ring
Mitosis occurs in __ stages
5
Before M phase when events must take place
The cell’s DNA must be fully replicated, The centrosome must be
duplicated
Replicated chromosomes are held together at the
centromere
Before entering mitosis, sister chromatids
condense
Sister chromatids are held
together by protein complexes
called
cohesins
are protein complexes that help carry out
chromosome condensation
condensins
Phosphorylation of condensins by M-Cdk triggers the
assembly of
condensin complexes onto the DNA
The duplicated centrosomes become
spindle poles
New microtubules grow out from the
spindle poles and form the
mitotic spindle
originate from one spindle pole and interact with
microtubules from the other spindle pole
Interpolar microtubules
originate in random directions from either spindle pole and do
not interact with other microtubules
Astral microtubules
bind the kinetochores of duplicated chromosomes
Kinetochore microtubules
are protein complexes that form on the centromere of each
chromosome, attach to the plus end of microtubules
kinetochores
Before M phase begins, two critical events must be completed
1) The DNA of the chromosomes is replicated
2) The centrosome is duplicated
Interphase consists of what other phases
G1, S, G2
What phase: chromosomes condense, kinetochores assemble onto chromosomes
Prophase
What phase: The mitotic spindle assembles between the two centrosomes
Prophase
Phosphorylation of condensins by MCdk promotes…
prophase
are protein complexes that help carry out chromosome condensation
Condensins
Phosphorylation of condensins by MCdk triggers the assembly of
condensin complexes on DNA
Mitosis stage 2:
Prometaphase
What phase: The nuclear envelope breaks down and microtubules of mitotic spindle attach kinetochores on chromosomes
Prometaphase
Phosphorylation of nuclear lamins promotes
nuclear envelope breakdown during prometaphase
Mitosis stage 3:
metaphase
What phase: The chromosomes are aligned midway between the spindle poles, The kinetochore microtubules on each sister chromatid attach to
opposite poles of the spindle
Metaphase
Mitosis stage 4
anaphase
What phase: The sister chromatids separate and are pulled toward the
spindle pole to which they are attached
anaphase
What phase: The kinetochore microtubules shorten, and the spindle poles
also move apart
anaphase
During anaphase, degradation of cohesin allows…
chromosome segregration
Sister chromatids are held
together by protein complexes
called
cohesins
The cohesin linkage during anaphase is destroyed by a protein called
separase
Before anaphase, separase is held in an active state by
securin
At the beginning of anaphase, securin is targeted for degradation by
APC/C
Anaphase A or B
spindle poles are pushed and pulled apart
kinetochore microtubules shorten, moving chromosomes
toward spindle poles
B
A
Mitosis stage 5:
Telophase
What phase: The two sets of chromosomes arrive at the spindle poles, a new nuclear envelope reassembles around each set of
chromosomes
Telophase
Dephosphorylation of nuclear lamins allows
reassembly of the nuclear envelope during telophase
What divides the cell into 2
cytokinesis
The cytoplasm is divided in two by a ____
contractile ring of actin
and myosin
contractile ring of actin and myosin
contractile ring
Sliding of actin filaments against the myosin filaments generates a
force that creates a
a cleavage furrow and divides the cell in two
Match the stage of M phase to the events that occur during
that stage:
___ Alignment of the chromosomes at the spindle equator
___ Breakdown of the nuclear envelope
___ Pinching of the cell in two
___ Re-formation of the nuclear envelope
___ Condensation of the chromosomes
___ Separation of sister chromatids
A. prophase
B. prometaphase
C. metaphase
D. anaphase
E. telophase
F. cytokinesis
C
B
F
E
A
D
Kinetochores assemble on what region of chromosome
centromere
If cells are no longer needed, they can remove themselves via
apoptosis
Cell death plays a critical role during
animal/body development
In adults, the rate of cell death = the rate of __
cell proliferation
Apoptosis allows cells to die __
individually
engulf the apoptotic cell before its contents can leak out
Phagocytic cells
cells that die as a result of
acute injury undergo
necrosis
The cell swells and bursts is
necrosis
This triggers a potentially damaging
inflammatory response
necrosis
proteases that are responsible for apoptosis
Caspases
Caspases are made as inactive precursors called
procaspases
cleave and activate executioner caspases
Initiator caspases
Procaspases are activated in response to signals that
induce apoptosis
Active executioner caspases __ many proteins in the cell
degrade
control release of the protein cytochrome c from
mitochondria into the cytosol
Bcl2 family proteins
Caspase activity is tightly regulated by the
Bcl2 family of proteins
Two Bcl2 family members, ___ and ___ , promote cytochrome c release. Other family members inhibit release.
Bax and Bak
Once released into the cytosol, cytochrome c activates initiator
procaspases by promoting the assembly of a large protein complex called
the
apoptosome
In a multicellular organism, the fate of individual
cells is controlled by
signals from other cells
Three major categories of signal proteins
Survival factors promote cell survival
* Mitogens stimulate cell division
* Growth factors stimulate cell growth
Survival factors usually act through
cell surface receptors
When survival factors cell surface receptors are activated, they turn on intracellular signaling pathways that…
inhibit apoptosis
Most survival factors activate signaling pathways that regulate members
of the
Bcl2 family
stimulate cell division by promoting
entry into S phase
Mitogens
Most mitogens are secreted proteins that bind
cell-surface
receptors
Mitogens intracellular signaling pathways act mainly by
promoting the
transition from G1 phase into S phase
some factors can act as both __ and __
by promoting both cell growth and division
mitogens and growth factors
binds transcription regulators and prevents them
from turning on genes required for cell division
Retinoblastoma protein (Rb)
Mitogens trigger the activation of
G1-Cdk and G1/S-Cdk
Role of G1-Cdk and G1/S-Cdk
phosphorylate Rb protein
phosphorylate Rb protein alters the ___ of Rb and it releases ___ so that it can activate ___ required for ___ into __ phase
conformation, bound transcription regulators so they can activate genes required for entry into S phase
promote the
accumulation of proteins and other
macromolecules
Growth factors intracellular signaling pathway
promote protein synthesis and inhibit protein degradation
Growth factors
Binding of a growth factor to a
receptor tyrosine kinase (RTK) initiates an
intracellular signaling pathway that leads to
activation of
Akt and Tor
Example of what stimulates
protein synthesis and inhibits protein
degradation
Akt and Tor
