Chp. 17 Cell Cycle
APC/C: what it is, what it does, how it does it
- APC/C is a member of the ubiquitin ligase family of enzymes - it regulates the proteolysis that triggers the metaphase-to-anaphase transition - The APC/C is activated in mitosis by association with Cdc20, which recognizes specific amino acid sequences on M-cyclin and other target proteins - With the help of two additional proteins called E1 and E2, the APC/C assembles polyubiquitin chains on the target protein. - The polyubiquitylated M-cyclin is then recognized and degraded in a proteasome
Cell-cycle arrest or apoptosis induced by excessive stimulation of mitogenic pathways.
- Abnormal Proliferation Signals Cause Cell-Cycle Arrest or Apoptosis, Except in Cancer Cells - Abnormally high levels of Myc cause the activation of Arf, which binds and inhibits Mdm2 and thereby increases p53 levels (inactivates G1/S-Cdk) - Depending on the cell type and extracellular conditions, p53 then causes either cell cycle arrest or apoptosis.
The contractile ring: what it does, how it does it, and its assembly
- Actin and Myosin II in the Contractile Ring Generate the Force for Cytokinesis - assembly of the contractile ring results in part from the local formation of new actin filaments, which depends on formin proteins that nucleate the assembly of parallel arrays of linear, unbranched actin filaments - after anaphase, the overlapping arrays of actin and myosin II filaments contract to generate the force that divides the cytoplasm in two
Low Tension in Microtubule Attachment to the Kinetochore
- Bi-orientation Is Achieved by Trial and Error - When a sister chromatid pair is unattached to the spindle or attached to just one spindle pole, there is little tension between the outer and inner kinetochores. - The protein kinase Aurora-B is tethered to the inner kinetochore and phosphorylates the Ndc80 complex in the outer kinetochore - This reduces the affinity of microtubule binding. - Microtubules therefore associate and dissociate rapidly, and attachment is unstable.
Step-by-step activation of M-Cdk
- Cdk 1 associates w/ M-cyclin as the level of M-cylin gradually rises - The resulting M-Cdk complex is phosphorylated on an activating site by the CAK and on a pair of inhibitory sites by the Wee1 kinase. - The resulting inactive M-Cdk complex is then activated at the end of G2/onset of mitosis by the phosphatase Cdc25. - Cdc25 is further stimulated by active M-Cdk, resulting in positive feedback. - This feedback is enhanced by the ability of M-Cdk to inhibit Wee1. - M-Cdk Drives Entry Into Mitosis
Step-by- step mechanism for the stimulation of cell growth by extracellular growth factors and nutrients
- Cell Proliferation is Accompanied by Cell Growth - Proliferating Cells Usually Coordinate Their Growth and Division 1) Growth factor binds to cell-surface receptors, leading to the activation of PI 3-kinase 2) PI3K promotes protein synthesis through a sig. pathway that leads to the activation of the protein kinase TOR - extracellular nutrients such as amino acids also help activate TOR 3) TOR activates transcription regulatory proteins (e.g. Myc) and S6 kinase (S6K) and inhibits 4E-BP, which leads to indirect activation of eIF4E - this all results increased production of ribosomes and stimulation of protein synthesis - it also inhibits protein degradation 4) the stimulated transcription regulatory proteins activate the transcription of various genes that promote cell metabolism and growth. - 4E-BP is an inhibitor of the translation initiation factor eIF4E
What else does cell-cycle control depend on?
- Cell-Cycle Control Also Depends on Transcriptional Regulation
What happens if extracellular conditions are unfavorable during the cell cycle?
- Cells delay progress through G1 and may even enter a specialized nondividing/resting state known as G0 - Some cells remain in G0 forever (e.g. neurons, skeletal muscle cells), transiently (e.g. hepatocytes), or regularly withdraw from and re-enter cell-cycle (e.g. fibroblasts)
Centriole replication (include when it occurs, composition of centrosome, and process of replication)
- Centrosome Duplication Occurs Early in the Cell Cycle and it's initiated by G1/S-Cdk - The centrosome consists of a centriole pair and associated pericentriolar matrix - At a certain point in G1, the two centrioles of the pair separate - During S phase, a daughter centriole begins to grow near the base of each mother centriole and at a right angle to it. - The elongation of the daughter centriole is usually completed in G2. - The two centriole pairs remain close together in a single centrosomal complex until the beginning of M phase, when the complex splits in two and the two daughter centrosomes begin to separate. - Each centrosome now nucleates its own radial array of microtubules (called an aster), mainly from the mother centriole.
The two processes of anaphase and what forces drive them
- Chromosomes Segregate in Anaphase A and B - Anaphase A: Separated sister chromatids move toward the poles - Depends on two major poleward forces: 1) microtubule depolymerization, which results in the loss of tubulin subunits at the plus end of the kinetochore as it moves toward the pole 2) microtubule flux, which is the poleward movement of the microtubules toward the spindle pole, where minus-end depolymerization occurs - Anaphase B: the two spindle poles move apart. - Depends on kinesin-5 motor proteins and dynein motors - Segregated Chromosomes Are then Packaged in Daughter Nuclei at Telophase
3 Unique Features of Meiosis I: Compare behavior of chromosome behavior to meiosis II and mitosis
- Chromosomes behave similarly in mitosis and meiosis II, but they behave very differently in meiosis I. - Homolog Segregation Depends on 3 Unique Features of Meiosis I: 1) Both sister kinetochores in a homolog must attach stably to microtubules from the same spindle pole - Mitosis: both sister kinetochores attach to microtubules from different spindle poles 2) Crossovers form linkage needed for bi-orientation of duplicated homologs at the equator (meiosis I) - The proteolytic cleavage of cohesin along chromosome arms unglues the arms and ends the crossovers (process APC/C dependent) - this allows the duplicated homologs to separate at anaphase I, while the centromeric cohesins keep the sister chromatids together - Mitosis/meiosis II: cohesin is needed for bi-orientation 3) Cleavage of centromeric cohesin allows the sister chromatids to separate at anaphase II (APC/C dependent). Mitosis: two sister chromatids come apart at the start of anaphase and segregate into separate daughter nuclei (APC/C dependent)
Cohesin: include composition, its purpose, and mention a requirement for chromosome duplication
- Cohesin is a protein complex with four subunits - Two subunits, Smc1 and Smc3, are coiled-coil proteins with an ATPase domain at one end - two additional subunits, Scc1 and Scc3, connect the ATPase head domains, forming a ring structure that may encircle the sister chromatids - The ATPase domains are required for cohesin loading on the DNA - Chromosome Duplication Requires Duplication of Chromatin Structure
Condensin: composition and what it does
- Condensin is a five-subunit protein complex that resembles cohesin - The ATPase head domains of its two major subunits, Smc2 and Smc4, are held together by three additional subunits (CAP-G, -H, and D2) - forms a ringlike structure that uses the energy from ATP hydrolysis to promote the compaction and resolution of sister chromatids - Condensin Helps Configure Duplicated Chromosomes for Separation
Crossovers between homologs: how its regulated, the avg. amount, where it typically occurs, and discuss meiosis errors (e.g. where it's usually common and associated phenomenon)
- Crossing-Over Is Highly Regulated (e.g. by crossover interference- presence of one inhibits another forming close by) - on avg., all of the bivalents are linked by 2-3 crossovers (no more than four) - typ. CO occurs at "hot spots" where DNA is accessible and not "cold spots" (e.g. heterochromatin near centromeres and telomeres) - Meiosis Frequently Goes Wrong, esp. in human female meiosis, which arrests for years after diplotene: meiosis I completed at ovulation and meiosis II only after egg is fertilized - nondisjunction: homologs fail to separate properly ( common cause of Down syndrome)
4 Major classes of motor proteins of the spindle: include the general purpose of these classes together and then discuss in detail the function of each individual class
- Four major classes of microtubule-dependent motor proteins govern spindle assembly and function 1) Kinesin-5: contains two motor domains that interact with the plus ends of antiparallel microtublules in the spindle midzone - they slide the two antiparallel microtubules past each other, pushing the poles apart 2) Kinesin-14: has minus-end directed motors - can cross-link antiparallel interpolar microtubules at the spindle midzone and tend to pull the poles together 3) Kinesin-4 & 10 (chromokinesins): plus-end directed motors that associate with chromosome arms and push the attached chromosome away from the pole 4) Dyneins: minus-end directed motors that link the plus ends of astral microtubules to components of the actin cytoskeleton at the cell cortex - dynein motors pull the spindle poles toward the cell cortex and away from each other
How does the cell escape from this stable G1 state to initiate a new cell cycle?
- G1/S- Cdk activity, which rises in late G1
synaptonemal complex: assembly/composition
- Homolog Pairing Culminates in the Formation of a Synaptonemal Complex - Each homolog is organized around a protein axial core, and the synaptonemal complex forms when these homolog axes are linked by rod-shaped transverse filaments. - The axial core of each homolog also interacts with the cohesin complexes that hold the sister chromatids together
Alternative forms of kinetochore attachment to the spindle poles
- Initially, a single microtubule from a spindle pole binds to one kinetochore in a sister-chromatid pair - Additional microtubules can then bind to the chromosome in various ways. 1) A microtubule from the same spindle pole can attach to the other sister kinetochore 2) microtubules from both spindle poles can attach to the same kinetochore. - These incorrect attachments are unstable and one of the two microtubules tends to dissociate - When a microtubule from the opposite pole binds to the second kinetochore, the sister kinetochores are thought to sense tension (mechanism depends on Aurora-B kinase) - This triggers an increase in microtubule binding affinity, thereby locking the correct attachment in place
Kinetochores: what they do and how they do it
- Kinetochores Attach Sister Chromatids to the Spindle - Each microtubule is attached to the kinetochore by interactions with multiple copies of the Ndc80 complex - This complex binds to the sides of the microtubule near its plus end, allowing polymerization and depolymerization to occur while the microtubule remains attached to the kinetochore.
Spindle self-organization by motor proteins: what initiates spindle assembly (and when), what assembly requires, and describe acentrosomal spindle assembly (include the downside of ASA)
- M-Cdk Initiates Spindle Assembly in Prophase - The Completion of Spindle Assembly in Animal Cells Requires Nuclear-Envelope Breakdown - Microtubule Instability Increases Greatly in Mitosis - Mitotic Chromosomes Promote Bipolar Spindle Assembly 1) Nucleation: Mitotic chromosomes stimulate the local activation of proteins that nucleate and promote the formation of microtubules in the vicinity of the chromosomes. 2) Antiparallel cross-linking by kinesin-5 3) Plus-end directed kinesin-4,10 pushes minus ends outward away from chromosomes 4) Focusing of the poles/minus ends by dynein and kinesin-14 - CON: spindle lacks astral microtubules, resulting in a mispositioned spindle in the cell
Comparison of meiosis and mitosis.
- Meiosis is a form of nuclear division in which a single round of chromosome duplication (meiotic S phase) is followed by two rounds of chromosome segregation. 1) Meiosis 1: Pairing and genetic recombination (crossing over) of duplicated homologs - homolog pairs long up on the spindle - segregation of homologs at anaphase I - duplicated homologs are segregated into different daughter nuclei 2) Meiosis 2: Segregation of sister chromatids at anaphase II - Each diploid cell that enters meiosis produces four genetically different haploid nuclei, which are distributed by cytokinesis into haploid cells that differentiate into gametes. 3) Mitosis: homologs do not pair up, and the sister chromatids are segregated during the single division. - Each diploid cell that divides by mitosis produces two genetically identical diploid daughter nuclei, which are distributed by cytokinesis into a pair of daughter cells.
Mitosis without cytokinesis in the early Drosophila embryo (include example of mammalian cells that undergo this process)
- Mitosis Can Occur Without Cytokinesis - In Drosphila embryo, the first 13 nuclear divisions occur synchronously and without cytoplasmic division to create a large syncytium (A cell in which multiple nuclei share the same cytoplasm) - Most of the nuclei migrate to the cortex, and the plasma membrane extends inward and pinches off to surround each nucleus to form individual cells in a process called cellularization. - types of mammalian cells that undergo this process: megakaryocytes, hepatocytes, and heart muscle cells
The control of the cell cycle: purpose of the control system and the 3 major control points
- Purpose: to trigger the major events of the cell cycle (e.g. DNA replication, mitosis, and cytokinesis) 1) The Restriction Point (or Start) in late G1: the cell commits to cell-cycle entry and chromosome duplication - if the environment is favorable, the cell enters into the S phase 2) G2/M Transition Point: the control system triggers the early mitotic spindle in metaphase - if the environment is favorable and DNA is replicated, cell enters mitosis 3) Metaphase-to-anaphase Transition Point: the control system stimulates sister-chromatid separation, leading to the completion of mitosis and cytokinesis - if all of the chromosomes are attached to the spindle, anaphase is triggered and can proceed to cytokinesis
Regulation of the contractile ring by the GTPase RhoA
- RhoA is activated by a RhoGEF and inactivated by RhoGAP - RhoA-GTP is focused at the future cleavage site. - By binding formins, RhoA-GTP promotes the assembly of actin filaments in the contractile ring. - By activating Rho-activated protein kinases, such as Rock, it stimulates myosin II filament formation and activity (Rock phosphorylates regulatory myosin light-chain) and inihibits myosin phosphatase, thereby promoting contraction of the ring
The Two major events of the Cell Cycle
- S phase: when the chromosomes are duplicated - M phase: when the duplicated chromosomes are segregated into a pair of daughter nuclei (in mitosis), after which the cell itself divides into two (cytokinesis)
Control of chromosome duplication: two general steps in the initiation phase
- S-Cdk Initiates DNA Replication Once Per Cycle 1) Preparations for DNA replication begin in late mitosis and early G1 when the DNA helicases are loaded by multiple proteins at the replication origin, forming the prereplicative complex (preRC). 2) S-Cdk activation leads to activation of the DNA helicases, which unwind the DNA at origins to initiate DNA replication. - Two replication forks move out from each origin until the entire chromosome is duplicated. - Duplicated chromosomes are then segregated in M phase - S-Cdk activation in S phase also prevents assembly of new preRCs at any origin until the following G1- thereby ensuring that each origin is activated only once in each cell cycle
The initiation of sister chromatid separation by the APC/C (also describe the negative feedback system)
- The APC/C Triggers Sister-Chromatid Separation and the Completion of Mitosis - The activation of APC/C by Cdc20 and M-Cdk leads to the ubiquitylation and destruction of securin, which normally holds separase in an inactive state. - The destruction of securin allows separase to cleave Scc1, a subunit of the cohesin complex holding the sister chromatids together - The pulling forces of the mitotic spindle then pull the sister chromatids apart. - Neg. Feedback: M-Cdk phosphorylates APC/C, which helps Cdc20 bind to APC/C and activate it - APC/C also targets S- and M-cyclins for destruction, leading to the loss of M-Cdk activity in anaphase
Complexes of the cell-cycle control system (comment on the conc. of cyclins and Cdks as well)
- The Cell-Cycle Control System Depends on Cyclically Activated Cdks - The conc. of the three major cyclin types oscillate during the cell cycle, while the conc. of Cdks doesn't change and exceeds cyclin amounts 1) In late G1, rising G1/S-cyclin levels lead to the formation of G1/S-Cdk complexes that trigger progression through the Start transition. 2) S-Cdk complexes form at the start of S phase and trigger DNA replication, as well as some early mitotic events. 3) M-Cdk complexes form during G2 but are held in an inactive state - they are activated at the end of G2 and trigger entry into mitosis at the G2/M transition 4) APC/C initiates the metaphase-to anaphase transition
An overview of the cell-cycle control system
- The Cell-Cycle Control System Functions as a Network of Biochemical Switches - The system consists of a series of cyclin-Cdk complexes (G1-Cdk, G1/S-Cdk, S-Cdk, M-Cdk) - The activity of each complex is influenced by various inhibitory mechanisms (e.g. DNA damage, unreplicated DNA, or chromosome unattached to spindle)
What happens in cell w/ and w/o a G1 phase
- The G1 Phase Is a Stable State of Cdk Inactivity 1) In early embryonic cell cycles, Cdc20-APC/C activity rises at the end of metaphase, triggering M-cyclin destruction. - Because M-Cdk activity stimulates Cdc20-APC/C activity, the loss of M-cyclin leads to APC/C inactivation after mitosis, which allows M-cyclins to begin accumulating again. 2) In cells that have a G1 phase, the drop in M-Cdk activity in late mitosis leads to the activation of Cdh1-APC/C and the accumulation of Cdk inhibitor proteins (e.g. p27) - Additionally, there is a decrease in M- cyclin gene expression - This ensures a continued suppression of Cdk activity after mitosis, as required for a G1 phase
Metaphase mitotic spindle: composition of spindle and the types
- The Mitotic Spindle Is a Microtubule-Based Machine - The plus ends of the microtubules project away from the spindle pole, while the minus ends are anchored at the spindle poles, which are organized by centrosomes. - Kinetochore microtubules: connect the spindle poles with the kinetochores of sister chromatids - Interpolar microtubules: overlap of the two plus ends of microtubules from each spindle pole, resulting in an antiparallel array in the spindle midzone - Astral microtubules: radiate out from the poles into the cytoplasm
Detailed explanation of the Control of the initiation of DNA replication
- The replication origin is bound by the origin recognition complex (ORC) throughout the cell cycle - In early G1, Cdc6 associates with the ORC, and these proteins bind the DNA helicase, which contains six closely related subunits called Mcm proteins. - The helicase also associates with a protein called Cdt1. - ORC and Cdc6 proteins load two copies of the DNA helicase, in an inactive form, around the DNA next to the origin, thereby forming the preRC - At the onset of S phase, S-Cdk stimulates the assembly of several initiator proteins on each DNA helicase, while another protein kinase, DDK, phosphorylates subunits of the DNA helicase - As a result, the DNA helicases are activated and unwinds the DNA. - DNA polymerase and other replication proteins are recruited to the origin, and DNA replication begins - The ORC is displaced by the replication machinery and then rebinds. - S-Cdk inactivates the preRC components (ORC, Cdc6, and Cdt1) thereby preventing formation of new preRCs at the origins until the end of mitosis
Spindle assembly checkpoint
- Unattached Chromosomes Block Sister Chromatid Separation - if it is not properly attached, kinetochore sends out a neg. signal that blocks Cdc20-APC/C activation throughout the cell and thus blocks the metaphase-to-anaphase transition - the negative signal depends on Mad2, which is recruited to unattached kinetochores - unattached kinetochore acts like an enzyme that catalyzes a change in the conformation of Mad2, so that Mad2 can bind and inhibit Cdc20-APC/C
How DNA damage arrests the cell cycle in G1
- When DNA is damaged, various protein kinases are recruited to the site of damage and initiate a signaling pathway that causes cell-cycle arrest. 1) The first kinase at the damage site is either ATM or ATR 2) Additional protein kinases, called Chk1 and Chk2, are then recruited and activated, resulting in the phosphorylation of the transcription regulatory protein p53. - Mdm2 normally binds to p53 and promotes its ubiquitylation and destruction in proteasomes 3) Phosphorylation of p53 blocks its binding to Mdm2; as a result, p53 accumulates to high levels and stimulates transcription of numerous genes, including the gene that encodes the CKI protein p21. 4) The p21 binds and inactivates G1/S-Cdk and S-Cdk complexes, arresting the cell in G1. - In some cases, DNA damage also induces either the phosphorylation of Mdm2 or a decrease in Mdm2 production, which causes a further increase in p53
High Tension in Microtubule Attachment to the Kinetochore
- When bi-orientation is achieved, the forces pulling the kinetochore toward the spindle pole are resisted by forces pulling the other sister kinetochore toward the opposite pole - The resulting tension pulls the outer kinetochore away from the inner kinetochore. - As a result, Aurora-B is unable to reach the outer kinetochore, and Ndc80 complexes aren't phosphorylated. - Microtubule binding affinity is therefore increased, resulting in the stable attachment of multiple microtubules to both kinetochores. - The dephosphorylation of outer kinetochore proteins depends on a phosphatase
Key component of cell-cycle control system
- When cyclin forms a complex with Cdk, the protein kinase is activated to trigger specific cell-cycle events. - Without cyclin, Cdk is inactive.
Cytokinesis: what it is, the first sign of it, when the sign starts to form/assemble and the cause of the first sign
- the division of the cytoplasm in two - first visible change of cytokinesis is the sudden appearance of a cleavage furrow - the structure underlying this process is the contractile ring - during anaphase, the ring assembles just beneath the PM - The actin-myosin bundles of the contractile ring are oriented so that their contraction pulls the membrane inward.
How does the mitotic spindle specify the site of division?
- the microtubules of the mitotic spindle determine the plane of animal cell division 1) astral stimulation model: astral microtubules carry furrow-inducing signals, which are somehow focused in a ring on the cell cortex, halfway between the spindle poles 2) central spindle stimulation model: the spindle midzone (central spindle) generates a furrow-inducing signal that specifies the site of furrow formation at the cell cortex - the overlapping interpolar microtubules of the central spindle associate w/ signaling proteins, including proteins that may stimulate RhoA 3) astral relaxation model: astral microtubules promote the local relaxation of actin-myosin bundles at the cell cortex - the cortical relaxation is minimal at the spindle equator, thus promoting cortical contraction at that site
1) Homolog pairing are described as being___?____(include basic composition) (2) crossing-over (how it occurs and any subsequent structures formed because of this process)
1) Bivalent: four-chromatid structure formed by two closely aligned duplicated homologs - As in mitosis, the sister chromatids in each homolog are tightly connected along their entire lengths, as well as at their centromeres. - At this stage, the homologs are usually joined by a protein complex called the synaptonemal complex 2) A single crossover occurs between non-sister chromatids. - It is only when the synaptonemal complex disassembles and the paired homologs separate a little at the end of prophase I that the crossover is seen as a thin connection between the homologs called a chiasma.
Mechanisms Responsible for the Assembly of a Bipolar Mitotic Spindle
1) Centrosomes 2) the other mechanisms depend on the ability of mitotic chromosomes to nucleate and stabilize microtubules and on the ability of motor proteins to organize microtubules into a bipolar array
The Major Cyclins and Cdks of Vertebrates: include the Cdk, the cyclin it assoc. w/, and the Cdk partner(s)
1) G1-Cdk - Cyclin: D - Cdk partners: 4, 6 2) G1/S-Cdk - Cyclin: E - Cdk partners: 2 3) S-Cdk - Cyclin: A - Cdk partners: 2, 1 4) M-Cdk - Cyclin: B - Cdk partners: 1
Chromosome attachment to the mitotic spindle: how bi-orientation is achieved
1) In late prophase, the mitotic spindle poles have moved to opposite sides of the nuclear envelope, with an array of overlapping microtubules between them. 2) Early prometaphase: Following nuclear envelope breakdown, the kinetochores are first attached to the sides of the microtubules - At the same time, the arms of the chromosomes are pushed outward from the spindle interior - laterally-attached sister chromatids are arranged in a ring around the outside of the spindle. 3) Mid-prometaphase: plus end of microtubules eventually achieve end-on orientation and kinetochores are captured and stabilized. 4) Metaphase: Stable end-on attachment to both poles results in bi-orientation. - Additional microtubules are attached to the kinetochore, resulting in a kinetochore fiber containing 10-40 microtubules
Homolog synapsis and desynapsis during the different stages of prophase I (five different stages in meiotic prophase)
1) Leptotene: when homologs condense and pair and genetic recombination begins 2) Zygotene: the synaptonemal complex begins to assemble at sites where the homologs are closely associated and recombination events are occurring 3) Pachytene: the assembly process is complete, and the homologs are synapsed along their entire length 4) Diplotene: desynapsis begins; disassembly of the synaptonemal complexes and concomitant condensation and shortening of the chormosomes 5) Diakinesis: when separation and crossing over are complete
The four phases of the eukaryotic cell cycle
1) M phase: mitosis (nuclear division) and cytokinesis (cytoplasmic division) *Remaining phases make up the Interphase*- occupies 23/24 hour cycle 2) G1 phase: GAP/growth phase 3) S phase: DNA replication 4) G2 phase: GAP/growth phase
Mitogen stimulation of cell-cycle entry
1) Mitogen binds to cell-surface receptors to initiate Ras/Raf intracellular signaling pathway, leading to increased expression of immediate early genes, including the gene encoding the transcription regulatory protein Myc 2) Myc increases the expression of many delayed response genes, including some that lead to increased G1-Cdk activity (cyclin D-Cdk4), which triggers the phosphorylation of Rb 3) This inactivates the Rb proteins, freeing the gene regulatory protein E2F to activate the transcription of G1/S genes, including the genes for a G1/S-cyclin (cyclin E) and S-cyclin (cyclin A). 4) The resulting G1/S-Cdk and S-Cdk activities further increase Rb protein phosphorylation, forming a positive feedback loop. - E2F proteins also stimulate the transcription of their own genes, forming another positive feedback loop.
Extracellular Signal molecules that regulate cell growth, division, and survival
1) Mitogens: stimulate cell division - triggers G1/S- Cdk activity that then inhibits neg. controls that otherwise block progress through the cell cycle 2) Growth factors: stimulate cell growth (cell mass) - promotes synthesis of proteins by inhibiting their degradation 3) Survival factors: promote cell survival - suppresses apoptosis
Six Principle Stages of M Phase
1) Prophase: the replicated chromosomes, each consisting of two sister chromatids held together by a kinetochore, condense - outside the nucleus, the mitotic spindle assembles between the two centrosomes, which have replicated and move apart - in diploid cells, there would be two copies of each chromosome 2) Prometaphase: starts with the abrupt breakdown of the nuclear envelope - chromosomes can now attach to spindle microtubules via their kinetochores and undergo active movement 3) Metaphase: the chromosomes are aligned at the equator of the spindle - the kinetochore microtubules attach sister chromatids to opposite poles of the spindle 4) Anaphase: the sister chromatids separate to form two daughter chromosomes, and each is pulled toward the spindle pole it faces - the kinetochore microtubules get shorter, and the spindle poles also move apart 5) Telophase: two sets of daughter chromosomes arrive at the poles of the spindle and decondense - a new nuclear envelope reassembles around each set, completing the formation of two nuclei and marking the end of mitosis 6) Cytokinesis: the cytoplasm is divided in two by a contractile ring of actin and myosin filaments, which pinches the cell in two to create two daughters, each with one nucleus
The regulation of Cdk activity
1) Wee1 kinase: the active cyclin-Cdk complex is turned off when the two closely spaced sites above the active site are phosphorylated 2) Cdc25 phosphatase: Removal of phosphates activates the cyclin-Cdk complex. 3) Cdk-activating Kinase (CAK): phosphorylates the AA near the entrance of the Cdk active site - this causes a conformational change that further increases the activity of the Cdk 4) Cdk inhibitor proteins (CKIs): binding of these proteins inactivates cyclin-Cdk complexes (e.g. p27)
Detailed step-by-step cell-cycle control system
1) When conditions for cell proliferation are right, various external and internal signals stimulate the activation of G1-Cdk, which in turn stimulates the expression of gene encoding G1/S- and S-cyclins 2) The resulting activation of the G1-/S-Cdk then drives progression through the Start transition 3) G1/S- Cdks unleash a wave of S-Cdk activity, which initiates chromosome duplication in S phase and also contributes to some early events of mitosis 4) M-Cdk activation then triggers progression through G2/M-transition and the events of early mitosis, leading to the alignment of sister-chromatid pairs at the equator of the mitotic spindle 5) Finally, APC/C (plus Cdc20) triggers the destruction of securin and cyclins, unleashing the sister-chromatid separation and segregation (completion of mitosis) 6) When mitosis is complete, Cdk activity is suppressed, resulting in a stable G1 period