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Questions and Answers
What do G1/S cyclins help initiate in the cell cycle?
What do G1/S cyclins help initiate in the cell cycle?
Cdk inhibitor proteins promote the activity of cyclin-Cdk complexes.
Cdk inhibitor proteins promote the activity of cyclin-Cdk complexes.
False
What process leads to the degradation of M and S cyclins during the cell cycle?
What process leads to the degradation of M and S cyclins during the cell cycle?
Ubiquitylation
G1 cyclins act early in G1 and bind to other Cdk proteins to form _____ that drive cells through G1.
G1 cyclins act early in G1 and bind to other Cdk proteins to form _____ that drive cells through G1.
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Match the following transitions with their corresponding regulatory mechanisms:
Match the following transitions with their corresponding regulatory mechanisms:
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What needs to happen for S-Cdks and M-Cdks to be inactivated by the end of the M phase?
What needs to happen for S-Cdks and M-Cdks to be inactivated by the end of the M phase?
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Mitogens stimulate the production of G1 cyclins that promote cell division.
Mitogens stimulate the production of G1 cyclins that promote cell division.
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What is the role of the p53 protein during the G1 to S phase transition?
What is the role of the p53 protein during the G1 to S phase transition?
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Terminally differentiated cells, such as ______, permanently stop dividing.
Terminally differentiated cells, such as ______, permanently stop dividing.
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Match the following processes with their descriptions:
Match the following processes with their descriptions:
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Study Notes
Cell Division Cycle
- Cell cycle is duplication and division
- Eukaryotic cell cycle usually includes four phases:
- M phase: mitosis and cytokinesis
- Interphase: period between M phases
- S phase: DNA replicates
- G1 and G2: cells grow, monitor internal/external environment for suitable conditions for reproduction
- Interphase: Cell continues transcription of genes, synthesizes proteins, and grows
- Cell cycle control system ensures cell cycle events occur completely and checks critical points with feedback
- G1 to S: control system checks the environment (nutrients and signal molecules) before replicating DNA. Cycle can be arrested in G0 if not suitable.
- G2 to M: undamaged DNA is confirmed to be fully replicated prior to mitosis (if intact DNA, cell enters mitosis). Cell cycle control machinery checks duplicated chromosomes are attached to the mitotic spindle before being pulled apart.
- Cell cycle control system works to turn machinery on and off for new components
- Cyclically activated protein kinases (Cdks) are key to activating and inactivating proteins/complexes for mitosis, based on de/phosphorylation of proteins
- Protein Kinases are involved in transitioning phases G1 to S and another before M to mitosis
- Cyclins are proteins in the control system switching kinases on/off
- Cyclins are not enzymatic but bind to cell cycle kinases, activating them
- Cyclin-dependent kinases (Cdks): kinases of cell cycle control systems
Cyclin-Cdk Complexes
- Different Cyclin-Cdk complexes trigger different steps in the cell cycle
- M-cyclin: acts in G2 and triggers M phase (forms M-Cdk)
- S-cyclin and G1/S cyclin: bind to certain Cdk proteins in late G1 to form S-Cdk
- Cyclin concentrations regulated by transcription and proteolysis
- Increased gradually by transcription of cyclin genes
- Rapidly decreased via targeted destruction of the protein
- Anaphase-promoting complex (APC) degrades M and S cyclins part way through M phase, tagging cyclins with ubiquitin, leading to degradation by proteasomes
- Cdk activity can be blocked by Cdk inhibitor proteins
- Some may help Cdks remain inactive during G1
G1 Phase
- Metabolic activity, cell growth and repair, uses IC and EC signals
- Cdks are stably inactivated in G1
- If S and M-Cdks are active, the cell will immediately replicate DNA and divide without sufficient time in G1 or G2
- Mitogens promote production of cyclins that stimulate cell division
- Mitogens are signals from other cells
S Phase
- S-Cdk initiates DNA replication and blocks re-replication
- Occurs in G1 with preparation for DNA replication: Proteins recruiting to sites along chromosomes (origins of replication) is recruited by ORC
- Origin Recognition Complex (ORC)- top of replication origins
- Replication signal from S-Cdk helps start DNA replication, activating helicases and assembling proteins at replication fork
- S-Cdk inhibits re-replication
M Phase
- Cell recognizes components and distributes equally into 2 daughter cells
- M-Cdk allows for arrangement in early stages of mitosis
- Helps prepare duplicated chromosomes for segregation
- Induces mitotic spindle assembly
- M-Cdk complexes accumulate through G2, then activated when its inhibitory phosphate is removed by Cdc25 at the end of G2. Activated complex can trigger addition M-Cdk complexes
- Cohesins: hold duplicated chromosomes together during DNA replication
- Condensins: condense duplicated chromosomes so they can be easily segregated
- Spindle fibers (microtubules) are part of the mitotic spindle
- Sister chromatids are held together by cohesins
- Mitotic spindles composed of microtubules (MTs) and MT-associated motor proteins
- Pull duplicated chromosome copy to each daughter cell
- Centrosomes in animal cells duplicate and start to assemble mitotic spindles; Duplicated chromosomes are attached to mitotic spindle, align at an equator, to form metaphase plate.
- Proteolysis (separation of sister chromatids at anaphase) is triggered by separase, held inactive by securin, which is targeted by APC (anaphase promoting complex), which allows separase to detach duplicated sister chromatids and distribute them into opposite daughter cells
- Mitotic spindle assembly is dictated by spindle assembly checkpoint, controlling the onset of anaphase and exit from mitosis
Cytokinesis
- Cytoplasm is split between daughter cells
- Mitotic spindle determines the plane of cytoplasmic cleavage
- Contractile ring of animal cells made by actin and myosin filaments
- Cytokinesis in plant cells involves formation of new cell wall (phragmoplast) from remains of interpolar MTs.
- Membrane-enclosed organelles are distributed randomly to daughter cells during division
Apoptosis
- Cell death, regulated by an intracellular proteolytic cascade.
- Caspases (proteases) are important in apoptosis
- Inherent to cell until signaled
- Intrinsic death pathway involves proteins in the Bcl2 family regulating caspase activation
- Bax and Bak induce cell death
- Extracellular signals can induce apoptosis by activating death receptors (like Fas)
- Extracellular signals controlling survival, growth, and division
- Survival factors inhibit apoptosis and promote cell survival (through pathways relating to Bcl2 proteins)
- Mitogens promote cell division, overcoming inhibitory mechanisms, through S phase of cell cycle
- Growth factors stimulate cell growth by increasing protein/macromolecular synthesis rates (and decreasing rates of degradation).
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Description
Explore the intricacies of the cell division cycle, including its phases such as mitosis and interphase. This quiz covers key concepts of the eukaryotic cell cycle, focusing on the crucial control mechanisms that ensure proper cell replication. Test your knowledge on the factors influencing DNA replication and cell growth.
