T9: Cell Cycle Regulation
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389 Terms
What is the cell cycle?
Reproduction of a cell through an orderly sequence of events in which the cell duplicates its contents and divides into two cells. The cell cycle is the cycle of cell division and replication.
Why is the cell cycle essential?
It is essential for the reproduction of all living things.
Do the details of the cell cycle vary?
The details of the cell cycle vary from organism to organism and at different stages of an organism's life.
What fundamental task must the cell accomplish during the cell cycle?
During the cell cycle, the cell must accomplish the fundamental task of passing genetic information to the next generation.
What is required to produce two identical daughter cells?
DNA must be replicated faithfully and accurately, then properly segregated so that each daughter cell receives a complete copy of the entire genome.
What is the main purpose of the cell cycle?
The main purpose of the cell cycle is to copy the cell's DNA and divide it equally into two new cells. This ensures that each daughter cell receives the same genetic information as the original cell.
What are the three steps listed in the cell cycle overview?
cell growth and chromosome replication
chromosome segregation
cell division
What happens in the S phase?
One important part of the cell cycle is the S phase, where the DNA in the chromosomes is copied. chromosome duplication occurs and usually requires about 10-12 hours.
What happens in the M phase?
The M phase includes mitosis, where the copied chromosomes are separated into two new nuclei, and cytokinesis, where the cytoplasm divides and the cell splits into two daughter cells. chromosome segregation and cell division occur in less than an hour.
What are the two major phases of the cell cycle?
S phase (Synthesis phase) - chromosome duplication occurs and usually requires about 10-12 hours.
M phase (Mitotic phase) - chromosome segregation and cell division occur in less than an hour.
What happens during mitosis?
Mitosis (nuclear division) - duplicated chromosomes are distributed into two daughter nuclei.
What happens during cytokinesis?
Cytokinesis - the cell divides into two daughter cells.
What happens at the end of the S phase?
At the end of the S phase, the DNA molecules of each chromosome pair are intertwined and held together by specialized protein linkages.
What happens during prophase?
During early mitosis, in a stage called prophase, the chromosomes become disentangled and condense into rigid, compact rods called sister chromatids.
What happens when the nuclear envelope disassembles?
When the nuclear envelope disassembles during later mitosis, the sister chromatid pairs attach to the mitotic spindle, an array of microtubules.
What happens during metaphase?
The chromatids are then pulled toward opposite poles of the spindle and align at the spindle equator during metaphase.
What happens at the start of anaphase?
At the start of anaphase, the cohesion between sister chromatids is destroyed, allowing them to separate.
What happens during telophase?
The separated chromatids are then segregated and packaged into several nuclei during telophase.
What are the G2 checkpoint questions and outcome?
- is all dna replicated? 2. is all dna damage repaired? - enter mitosis
What is checked in the checkpoint in mitosis?
are all chromosomes properly attached to the mitotic spindle? - pull duplicated chromosomes apart
What is the G1 checkpoint question and outcome?
is environment favorable? - enter s phase
What are the four main phases of the eukaryotic cell cycle?
G1, S, G2, and M.
What is interphase?
The G1, S, and G2 phases together are called interphase, which takes up most of the cell cycle.
How long does interphase take compared to M phase in a typical human cell?
in a typical human cell with a 24-hour cycle, interphase may take about 23 hours, while M phase takes about 1 hour.
When does cell growth happen?
Cell growth happens throughout most of the cell cycle, except during mitosis.
What is the role of gap phases?
The gap phases are not just waiting periods. They allow the cell to assess its internal state and external environment before proceeding to the next stage.
Why does the cell cycle include gap phases?
Most cells need more time to grow and increase their protein and organelle content than to copy their chromosomes and divide.
What is special about the G1 phase?
The G1 phase is especially important because its length can vary depending on external conditions and signals from other cells.
What happens if conditions are unfavorable in G1?
If conditions are unfavorable, the cell may slow down in Gl or enter a resting stage called GO phase. Cells can remain in GO for days, weeks, years, or even permanently.
What happens if conditions are favorable in G1?
If conditions are favorable and growth signals are present, the cell can move through a commitment point near the end of G1.
What is the commitment point in G1 called?
This point is called Start in yeast cells and the restriction point in mammalian cells.
What happens after passing the commitment point?
After passing this point, the cell is committed to DNA replication even if growth signals are removed.
What does the cell-cycle control system regulate?
A cell-cycle control system regulates the main events of the cell cycle such as DNA replication, mitosis, and cytokinesis.
What is the function of the cell-cycle control system?
It works like a controller that moves through the cell cycle and triggers important processes at the right time.
What does the control system do as the cycle progresses?
As the cycle progresses, the control system checks whether each step has been completed properly. It also responds to signals from the cell's environment.
What happens if there is a problem in the cell cycle?
If there is a problem such as unfinished DNA replication or unfavorable conditions, the control system can pause the cycle at specific points until the cell is ready to continue.
What are the 4 phases of the cell cycle?
G1, S, G2, and M phase.
Where are gap phases located in the cell cycle?
G1 phase is between M phase and S phase, while G2 phase is between S phase and M phase.
What is the function of the cell cycle control system?
The cell cycle control system operates like a timer and triggers the events of the cell cycle in a set sequence. It is rigidly programmed to provide enough time for the completion of each cell cycle event. It triggers the essential processes of the cell cycle.
How is the control system represented in the slide?
It is represented as a central arm rotating clockwise, triggering essential processes when it reaches specific transition points in the cycle.
What can signals from the environment do to the control system?
Signals from the environment can cause the control system to arrest or stop the cell cycle at certain transitions.
What are the 3 major regulatory transitions?
Start/restriction point in late G1 phase, where the cell commits to cell cycle entry and chromosome duplication.
G2/M transition, where the control system triggers early mitotic events leading to chromosome alignment on the mitotic spindle during metaphase.
Metaphase-to-anaphase transition, which stimulates sister chromatid separation leading to completion of mitosis.
What happens if a problem is detected in the cell cycle?
The control system blocks the cell cycle if it detects problems inside or outside the cell.
What happens at the G2/M transition if there is a problem?
If a problem is detected, the system can hold the cell at the G2/M transition until the problem is solved.
What happens at the Start checkpoint if conditions are not favorable?
If extracellular conditions are not favorable, the control system also blocks the cell cycle at the Start checkpoint until conditions become favorable.
What are some examples of eukaryotic cell-cycle times?
Early frog embryo cells 30 minutes;
Yeast cells 1.5-3 hours;
Mammalian intestinal epithelial cells ~12 hours;
Mammalian fibroblasts in culture ~20 hours;
Human liver cells ~1 year.
What does the timing of the cell cycle depend on?
The timing of the cell cycle varies among different eukaryotic cells. The length of the cell cycle depends on the type of cell and the organism.
What are Cdks and what do they do?
Cdks (cyclin-dependent kinases) are the main control proteins of the cell cycle. They help the cell move from one phase to the next.
What do Cdks need to become active?
Caks cannot work alone and need to bind with proteins called cyclins to become active.
What is formed when a cyclin binds to a Cdk?
When a cyclin binds to a Cak, they form a cyclin-Cak complex.
What does the cyclin-Cdk complex trigger?
The cyclin-Cak complex triggers important cell-cycle events such as DNA replication, mitosis, and cell division.
What makes the cyclin-Cdk complex active?
The complex becomes active only after certain phosphate groups are added or removed.
What do protein kinases and protein phosphatases do?
Protein kinases add phosphate groups. Protein phosphatases remove inhibitory phosphate groups.
What happens once correct phosphate changes occur?
Once the correct phosphate changes occur, the cyclin-Cdk complex becomes active and signals the cell to continue through the cell cycle.
Why are Cdks called “cyclin-dependent”?
Cdks are called "cyclin-dependent" because their activity depends on cyclins.
What happens to Cdk and cyclin levels during the cell cycle?
The amount of Cak usually remains constant in the cell. Cyclin levels rise and fall during the cell cycle.
What happens when cyclins are present or destroyed?
When cyclins are present, Caks become active. When cyclins are destroyed, Caks become inactive again.
What happens during cyclin destruction in the diagram?
In the diagram, the cyclin is tagged by ubiquitination and then destroyed, leaving the Cak inactive.
How do Cdk complexes act like switches?
The Cak family acts like a group of cell-cycle switches. They turn on when they bind to cyclins and receive the correct phosphate signals. They turn off when cyclins are destroyed.
What is the function of Cdk regulation?
This regulation ensures that each stage of the cell cycle happens at the correct time and in the proper order.
What are the major cyclin-Cdk complexes in vertebrates?
G1-Cdk: cyclin D, Cdk4, Cdk6;
G1/S-Cdk: cyclin E, Cak2;
S-Cdk: cyclin A, Cdlk2;
M-Cdk: cyclin B, Cdk1*.
What does each cyclin-Cdk complex do during the cell cycle?
Each complex becomes active at a specific stage and helps the cell move to the next phase.
What forms the G1-Cdk complex and what does it do?
In the G1 phase, cyclin D binds with Cak4 or Cdk6 to form the G1-Cdk complex. The G1-Cdk complex helps the cell grow and respond to signals that determine whether the cell will continue dividing.
What are the types of cyclin D in mammals?
Cyclin D1, Cyclin D2, Cyclin D3
What forms the G1/S-Cdk complex and what is its function?
Near the end of G1 phase, cyclin E binds with Cdk2 to form the G1/S-Cdk complex. This complex helps the cell pass the restriction point and prepares it to enter the S phase.
What forms the S-Cdk complex and what does it do?
During the S phase, cyclin A binds with Cak2 to form the S-Cdk complex. This complex helps initiate and regulate DNA replication. It ensures that chromosomes are copied properly.
What forms the M-Cdk complex and what does it control?
During the M phase, cyclin B binds with Cak1 to form the M-Cak complex. This complex helps the cell enter mitosis. It controls events such as:
Chromosome condensation
Spindle formation
Preparation for cell division
What is the role of cyclin-Cdk complexes in the cell cycle order?
Different cyclins bind with specific Caks at different stages of the cell cycle. These cyclin-Cdk complexes act like switches that guide the cell through the G1, S, G2, and M phases in the correct order.
What activates G1-Cdk when conditions are favorable?
When conditions are favorable for a cell to divide, signals from inside and outside the cell activate G1-Cdk.
What does active G1-Cdk do?
Once G1-Cdk is active, it helps turn on genes that produce G1/S-cyclins and S-cyclins.
What happens after G1/S-cyclins and S-cyclins are produced?
These cyclins then bind to their Cdk partners and activate G1/S-Cdk and S-Cak.
What is the role of G1/S-Cdk activation?
Activation of G1/S-Cdk helps the cell pass the Start point or restriction point. This means the cell is now committed to entering the cell cycle and preparing for DNA replication.
What does S-Cdk do?
After this, S-Cdk becomes active and starts DNA replication during the S phase, where the chromosomes are copied.
What does M-Cdk do?
Later, M-Cak becomes active and pushes the cell from G2 phase into M phase. This begins mitosis.
What happens during early mitosis with M-Cdk?
During early mitosis, M-Ck helps prepare the chromosomes and mitotic spindle so sister chromatids can line up at the center of the cell.
What happens near the end of mitosis involving APC/C?
Near the end of mitosis, a protein complex called APC/C, together with its activator Cdc20, helps destroy securin and cyclins. The destruction of securin allows sister chromatids to separate. The destruction of cyclins turns off Cdk activity. This allows mitosis to finish properly.
What is the sequence of cell cycle signals in simple terms?
G-Cak prepares the cell
G1/S-Cdk commits the cell to divide
S-Cdk starts DNA replication
M-Cdk begins mitosis
APC/C helps finish mitosis by separating chromosomes and turning off Cdks.
What role do cyclins play in the cell cycle?
Cyclins act as checkpoints before the cell cycle can proceed to the next phase.
What is checked during the transition from G1 to S phase?
The transition from G1 to S phase is checked based on the activation of Caks through cyclins present at that specific point.
What happens if cyclins are not present?
If cyclins are not present, there is no entry, and the cell cycle will not proceed.
What regulatory questions does the cell cycle ask?
Can I proceed to S phase?
What cyclins are present?
Is it appropriate to continue?
Should the cell pause or withdraw because some cyclins are not intended for that stage?
What exists for each transition in the cell cycle?
For each transition in the cell cycle, there is always a checkpoint that controls and regulates progression.
What are the phases shown in the cell cycle?
- G₁ 2. S 3. G₂ 4. M
What can stop progression at the G₁ checkpoint?
- damaged DNA 2. unfavorable extracellular environment
What can stop progression during S phase?
damaged or incompletely replicated DNA
What can stop progression at the G₂ checkpoint?
damaged or incompletely replicated DNA
What can stop progression at the M checkpoint?
chromosome improperly attached to mitotic spindle
What do checkpoints in the cell cycle act like?
The cell cycle has several checkpoints that act like safety inspections.
What do checkpoints do before the cell moves to the next stage?
Before the cell moves to the next stage, it checks if everything is ready and if there are any problems.
What is checked in G1 phase?
In Gl phase, the cell checks if the DNA is damaged and if the outside environment is good enough for growth. If conditions are poor, such as lack of nutrients or growth signals, the cell may pause instead of continuing to divide.
What is checked in S phase?
In S phase, the cell checks whether the DNA is being copied properly. If the DNA is damaged or not completely copied, the cell cycle stops for a while so the cell can try to fix the problem.
What is checked in G2 phase?
In G2 phase, the cell checks again to make sure DNA replication is complete before entering mitosis. This prevents the cell from dividing with incomplete or damaged genetic material.
What is checked in M phase?
In M phase, the cell checks if the chromosomes are properly attached to the mitotic spindle. This is important because the spindle helps pull the chromosomes apart into the two daughter cells.
What is the purpose of cell cycle checkpoints?
These checkpoints help the cell avoid mistakes. They ensure that the cell does not divide unless the DNA is healthy, fully copied, and ready to be separated correctly.
What forms the pre-replicative complex?
- origin recognition complex (ORC) 2. Cdc6 3. other proteins of pre-RC
What triggers S phase?
S-Cdk TRIGGERS S PHASE
What happens to phosphorylated Cdc6?
DEGRADATION OF PHOSPHORYLATED Cdc6
What occurs after S-Cdk triggers S phase?
ASSEMBLY OF REPLICATION FORK
COMPLETION OF DNA REPLICATION
What is the site shown for the pre-replicative complex?
origin of replication
What is ORC and what does it do?
Molecular control of DNA replication initiation involves a key multiprotein complex called the Origin Recognition Complex (ORC), which binds to replication origins throughout the cell cycle.