Lecture 17 - Cell Cycle

relative DNA content and mass of a cell G1 phase

During the G1 phase, the cell grows and prepares for DNA replication. A cell in G1 has a certain amount of DNA (let's call it X) and a certain mass.

relative dna content s phase

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The S phase is when DNA synthesis (replication) occurs. The DNA content doubles during this phase, so a cell in S phase will have between X and 2X amount of DNA. The total cell mass also increases.

relative dna content g2 phase

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In the G2 phase, the cell continues to grow and prepares for chromosomal segregation. A cell in G2 has twice as much DNA as in G1 (2X). The total cell mass also continues to increase.

relative dna content m phase

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The M phase involves mitosis (nuclear division) and cytokinesis (cell division). During mitosis, the duplicated chromosomes are separated, and in cytokinesis, the cell divides into two daughter cells. Each daughter cell returns to the G1 phase with the original amount of DNA (X) and roughly half the mass of the mother cell before division.

cells in g0 phase

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Cells in the G0 phase have exited the cell cycle, usually from G1, and are not actively dividing. Their DNA content is the same as cells in G1 (X), and their mass is generally maintained.

Describe how cell cycle control mechanisms respond to internal and external signals.

Cell cycle control mechanisms rely on signals that trigger key reactions for the correct order of events and on checkpoints that block progression if it's unsafe for the cell to proceed.

Internal signals indicate if key processes within the cell have been completed correctly and whether it is safe to move forward. An example of an internal signal is the M phase (spindle) checkpoint. This checkpoint monitors whether all chromosomes are properly attached to the kinetochore microtubules. If some kinetochores are not attached, they generate a "wait" signal that delays anaphase, ensuring daughter cells do not end up with missing or extra chromosomes.

External signals come from outside the cell and can either stimulate or inhibit cell division.

Explain the roles of checkpoints, in cell cycle control

Checkpoints are critical points in the cell cycle where progression is halted unless a "go-ahead" signal is received. They act as molecular brakes to detect errors in processes like DNA integrity or spindle attachment and allow time for error correction. If the damage is too severe, checkpoints can even signal apoptosis (cell suicide). The three major checkpoints are the G1 checkpoint (START or Restriction Point), the G2 checkpoint, and the M checkpoint (metaphase or spindle checkpoint).

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The G1 checkpoint determines whether the cell will divide, delay division, or enter G0. Once a cell passes this point and enters the S phase, cell division is usually irreversible.

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The G2 checkpoint checks if the cell is ready for mitosis by assessing DNA damage and the completion of DNA replication.

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The M checkpoint ensures that all chromosomes are properly attached to the mitotic spindle before they are separated.

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Describe how the cell regulates the activity of CDKs

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The primary way the cell regulates CDK activity is through the binding of cyclins. Without the appropriate cyclin, the CDK is inactive because its active site is not fully exposed. The cyclic production and degradation of cyclins lead to rhythmic fluctuations in the activity of their associated CDKs, pacing the cell cycle. For example, MPF activity peaks at the M phase due to increased cyclin concentration and then drops as the cyclin is degraded at the end of mitosis.

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CDK activity can also be regulated by phosphorylation and dephosphorylation. While cyclin binding is required for initial activation, the CDK-cyclin complex can be further regulated by the addition or removal of phosphate groups at specific sites.

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Inhibitory proteins can also regulate CDK activity. For example, p21 is a protein whose transcription is initiated by the tumor suppressor protein p53 in response to DNA damage. p21 acts as a cyclin inhibitor, blocking the cell cycle at the G1 checkpoint and preventing DNA replication, providing time for DNA repair.

Growth factors

• pgdf

• growth inhibitors

Growth factors are an example of external signals that stimulate cell division. Platelet-derived growth factor (PDGF), released by platelets at the site of a wound, binds to tyrosine-kinase receptors on fibroblasts, triggering them to divide and aid in wound healing. The arrival of growth factors can lead to an increase in G1/S cyclin concentration, activating cyclin-dependent kinases and pushing the cell through the G1 checkpoint to divide.

Growth inhibitors are external signals that can inhibit cell division. Myostatin is a protein that inhibits the growth of myoblasts, preventing excessive muscle cell division. It acts by continuously inhibiting muscle growth and promotes arrest of cells in G1 by inducing the degradation of a G1 cyclin.

Explain how loss of cell cycle control can result in growth defects or cancer

Cancer cells are characterized by their ability to escape normal cell cycle controls, leading to unregulated growth. This loss of control can arise through several mechanisms:

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Cancer cells may make required growth factors themselves, reducing their dependence on external signals.

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They can have abnormal signaling pathways that are constitutively active or fail to convey growth factor signals properly, leading to uncontrolled proliferation signals. For example, mutations in the Ras gene, an oncogene, can result in a hyperactive Ras protein that continuously signals for cell division even without the presence of growth factors. This can lead to the upregulation of cyclin D, promoting progression through the G1/S checkpoint.

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Defects in CDKs or other cell cycle signaling machinery can also lead to a loss of proper regulation, allowing DNA replication and mitosis to occur under unfavorable conditions.

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Cancer cells often have defects in checkpoints, causing them to proceed through the cell cycle prematurely, even with DNA damage or incomplete chromosome attachment.

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Mutations in tumor suppressor genes, such as p53, can also contribute to cancer. Normally, p53 prevents the proliferation of damaged cells by halting the cell cycle, initiating DNA repair, or promoting apoptosis. If p53 is mutated and non-functional, cells with DNA damage can continue to divide, accumulating further mutations that can lead to cancer.

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Growth defects can also result from loss of cell cycle control. For instance, mutations affecting growth inhibitors like myostatin can lead to excessive cell division and tissue growth, as seen in the increased muscle mass in animals with myostatin gene mutations. Conversely, defects that prevent necessary cell division can lead to insufficient growth or tissue repair.

G1, S, G2, M, G0 phases

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G1 phase: This is the cell growth and preparation for DNA replication phase of the cell cycle. It is part of interphase, which accounts for about 90% of the cell cycle in most cells. Cells can exit G1 into a "quiescent" phase called G0. The G1 checkpoint (also known as START or Restriction Point) is where the cell decides whether to divide, delay division, or enter G0. Once a cell passes from G1 to S, cell division is usually irreversible. Any condition unfavorable for cell division or DNA replication causes arrest of the cell cycle at G1. Cells that do not divide often arrest in G1 and enter G0.

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S phase: This is the Synthesis of DNA (replication) phase of the cell cycle. It occurs between the G1 and G2 phases, resulting in twice as much DNA per cell in G2 than in G1.

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G2 phase: This is the phase of growth and preparation for chromosomal segregation. It is part of interphase. The G2 checkpoint assesses if the cell is ready for mitosis by checking for DNA damage and completed replication.

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M phase: This phase involves Mitosis (nuclear division) and cytokinesis (cell division).

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G0 phase: This is a “Quiescent” phase where cells exit at G1. Some cells in G0 have the ability to divide if necessary (like liver cells in response to growth factors), while others rarely or never divide after maturity (like nerve and muscle cells). G0 cells are considered "quiescent" but not "quiet".

Checkpoint

Checkpoint: A checkpoint is a point at which the cell cycle stops (“arrested”) unless it gets a “go-ahead” signal. Signals registered at checkpoints indicate if key processes have been completed correctly and whether or not it is safe to move forward. Checkpoints are molecular brakes that can halt the cell cycle. They provide a mechanism to detect errors in processes such as spindle attachment or DNA integrity and can slow down or arrest the cell cycle so the cell can correct these errors. If damage is too great, checkpoints can signal apoptosis (cell suicide). The three major checkpoints are the G1, G2, and M (a.k.a. Metaphase or spindle) checkpoints.

Cyclin

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Cyclin: Cyclins are proteins that bind to and activate cyclin-dependent kinases (CDKs). The amount of cyclin fluctuates during the cell cycle, unlike CDKs, which are present in constant amounts. Rhythmic fluctuations in cyclin levels lead to rhythmic fluctuations in the activity of CDKs, pacing the cell cycle. Different cyclins act with different CDKs to control different checkpoints.

Cyclin-dependent kinase (CDK)

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Cyclin-dependent kinase (CDK): CDKs are cyclically activated protein kinases. They are present in constant amounts throughout the cell cycle but are inactive until they bind to a cyclin. Binding to a cyclin causes a conformational change in the CDK, exposing its active site. Different cyclins activate different CDKs to control different phases and checkpoints of the cell cycle. The go-ahead signal at each checkpoint comes from the activity of one or more CDKs that act at different checkpoints.

Protein kinase

Protein kinase: A protein kinase is an enzyme that catalyzes the addition of a phosphate group to a target protein. This transfer is called phosphorylation. Negatively charged phosphate groups can change a protein’s conformation, potentially activating or de-activating the phosphorylated target protein. The go-ahead signal at each checkpoint comes from the activity of one or more protein kinases (cyclin-dependent kinases). CDKs are a type of protein kinase.

Protein phosphorylation

Protein phosphorylation: This is the process where a protein kinase catalyzes the addition of a phosphate group to a target protein. This can change the protein's conformation and activity.

MPF (M-phase promoting factor) Growth factor

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MPF (M-phase promoting factor): MPF is an example of a cyclin-dependent kinase that triggers the transition from G2 to mitosis. It was the first cyclin-CDK complex discovered. MPF acts at the G2->M checkpoint to trigger mitosis and controls when to enter mitosis. MPF activity peaks at the M phase and then drops by G1 due to the degradation of its cyclin component. Activated MPF phosphorylates proteins that initiate M phase events like nuclear envelope breakdown, chromosome condensation, and activation of the mitotic spindle. Notably, MPF also phosphorylates an enzyme that degrades cyclin at the end of mitosis, leading to its own inactivation.

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PDGF

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PDGF (Platelet-derived growth factor): PDGF is an example of an external cue that stimulates cell division. It is produced by platelet blood cells and is released in the vicinity of a wound in mammals. PDGF binds to tyrosine-kinase receptors on fibroblasts (e.g., skin), triggering them to divide and aid in wound healing. This process involves PDGF triggering an increase in G1/S cyclin concentration, activating CDKs and leading to cell division.

p53

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p53: p53 is a tumor suppressor protein. It is often referred to as "the guardian of the genome" because it normally prevents the proliferation of damaged cells. In response to DNA damage, p53 can stop cell cycle progression (halting at the G1 checkpoint), initiate the DNA repair pathway, or promote programmed cell death (apoptosis). p53 is a transcription factor that binds to and activates specific genes that encode proteins that arrest the cell cycle, promote DNA repair, and apoptosis. One of the proteins p53 initiates the transcription of is p21, a cyclin inhibitor that blocks the cell cycle at the G1 checkpoint and prevents DNA replication. Mutations in p53 are very frequent in human cancers.

cyclins and cdks

Cyclins are regulatory proteins whose concentration fluctuates cyclically during the cell cycle. They bind to and activate cyclin-dependent kinases (CDKs). Different cyclins are produced at different stages of the cell cycle and activate specific CDKs to control progression through those stages.

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Cyclin-dependent kinases (CDKs) are protein kinases that catalyze the addition of a phosphate group to target proteins, a process called phosphorylation. This phosphorylation can change the conformation and activity of the target protein, regulating various steps of the cell cycle. CDKs are present in constant amounts throughout the cell cycle but are inactive unless bound to a cyclin. The activity of a specific CDK is determined by the availability of its specific cyclin partner. MPF (M-phase promoting factor) is an example of a cyclin-CDK complex that triggers the transition from G2 to mitosis.