Seminario 3A Poblaciones celulares y Ciclo celular - Tomas Falzone

Understanding the Cell Cycle and Cellular Populations

Introduction to the Seminar

• The seminar focuses on cell cycle regulations, explaining how cells divide in various ways.

• Tomas Falsone introduces himself as the facilitator of this conceptual synthesis, aiming for simplicity and clarity in explanations.

• Participants are encouraged to ask questions throughout the session to clarify any uncertainties.

Key Concepts of Cell Cycle Regulation

• The discussion will cover essential objectives related to understanding cellular functions and enzyme regulation during the cell cycle.

• Important events such as restriction points or checkpoints will be explained, highlighting their roles in regulating the cell cycle.

• The seminar will address specific regulatory proteins and their relationship with cellular changes, including DNA repair mechanisms.

Mechanisms of Cell Division

• Focus will be placed on molecular mechanisms involved in different phases of cell division (mitosis and meiosis).

• Emphasis on maintaining DNA integrity during cell division is crucial for understanding cellular processes.

• Participants will learn to relate cellular vision events with differentiation mechanisms and abnormal growth patterns like tumors.

Experimental Models for Studying Cell Cycle

• Various experimental models can be utilized to observe different stages of the cell cycle across diverse organisms.

• Techniques involving biochemical methods may help quantify DNA content at various stages of the cell cycle.

Understanding Genetic Material Duplication

• Cells transition from non-dividing states to dividing states, necessitating genetic material duplication for successful division.

• A diploid cell must duplicate its genome from two copies to four before division can occur effectively.

Cell Division and the Cell Cycle

Overview of Cell Division

• The process of cell division involves splitting a single cell into two daughter cells, each containing an identical set of genetic material (diploid).

• Questions are encouraged throughout the discussion to clarify complex concepts related to the cell cycle and its phases.

Phases of the Cell Cycle

• The graph illustrates DNA quantity across different phases of the cell cycle, emphasizing that a diploid cell must duplicate its DNA during synthesis to maintain genomic integrity.

• The cyclical nature of the cell cycle is highlighted, with cells transitioning through various stages including G1, Synthesis (S), G2, and Mitosis.

Stages of DNA Duplication

• In G1 phase, cells prepare for division by responding to internal and external signals before entering S phase where DNA duplication occurs.

• By the end of G2 phase, duplicated DNA is compacted in preparation for mitosis, ensuring that two genetically identical daughter cells will be produced.

Observations on Cell Behavior

• Different rates of division among cells can be observed in a culture plate; some may be in G1 while others are duplicating their genomes or preparing for mitosis.

• A population analysis reveals varying numbers of cells at different stages: some in G1, others actively synthesizing DNA (G2), and those ready for mitosis.

Molecular Regulation Mechanisms

• Key regulatory proteins known as cyclins and cyclin-dependent kinases (CDKs) play crucial roles in advancing through the cell cycle's checkpoints.

• CDKs function as enzymes that phosphorylate target proteins; their activity is contingent upon binding with specific cyclins which activate them.

Activation Process of CDKs

• For a cyclin-dependent kinase to become active, it must bind with a cyclin. This binding induces conformational changes that expose the active site necessary for phosphorylation.

• Cyclins fluctuate throughout the cell cycle; they are present only at certain times to regulate CDK activity effectively.

Role of Cyclins in Cell Cycle Progression

• Cyclins regulate CDK activity by being present or absent at specific points within the cycle. Their degradation after fulfilling their role ensures proper timing for subsequent phases.

• The interplay between CDKs and cyclins is essential for cellular progression through critical checkpoints like G1/S transition and mitosis initiation.

Cell Cycle Regulation and Cyclin-Dependent Kinases

Importance of Cyclins in Cell Cycle Initiation

• The discussion begins with the role of cyclins as crucial components for initiating the cell cycle, particularly at the end of the G1 phase.

• It is emphasized that while cyclins are unstable and vary in presence due to synthesis and degradation, cyclin-dependent kinases (CDKs) remain stable throughout different phases.

Understanding Cell Division Phases

• The cell division process includes distinct phases: G1, Synthesis (S), G2, and Mitosis. At the end of G1, cells assess their readiness to enter division.

• A critical control point known as the restriction point determines whether a cell can proceed to synthesis based on internal conditions and external signals.

Role of CDKs and Cyclins

• Active CDKs require specific cyclins to function; this interaction is essential for progressing through the cell cycle when environmental conditions are favorable.

• The interactions between proteins involved in this process are non-covalent; they rely on affinity rather than permanent bonds, allowing for dynamic regulation.

Activation Mechanisms of CDKs

• Once activated by cyclins, CDKs can initiate necessary processes for DNA synthesis. However, after entering synthesis, certain cyclins degrade.

• For mitosis initiation, specific cyclin-CDK complexes like CDK1 with Cyclin B become crucial for regulating mitotic processes.

Control Points in Cell Cycle Progression

• Different checkpoints exist within the cell cycle where active kinases regulate progression based on protein associations; once cyclins degrade, these kinases become inactive.

• Each checkpoint's significance will be explored further as understanding these regulatory mechanisms is vital for comprehending cellular division.

Transition Sites Between Phases

• The activation of CDKs involves multiple steps: association with a cyclin followed by phosphorylation which activates them while other factors may inhibit this activation.

• Proper regulation ensures that kinases activate only when needed during specific moments in the cycle to coordinate cellular activities effectively.

Key Checkpoints During Cell Division

• Important checkpoints occur at various stages such as at G2 where cells decide if they can proceed with division based on size and component availability.

Cell Cycle Regulation and Control Mechanisms

Overview of Cell Cycle Control

• The discussion begins with the role of cyclins and CDK4 in cell cycle regulation, emphasizing the need for signals to activate master regulators that facilitate progression into synthesis.

• Once DNA is replicated, it must be compacted; the cell checks if replication is complete and intact before proceeding to mitosis. Any damage can halt the cell cycle.

Transition to Mitosis

• Master regulators allow cells to transition into mitosis after confirming that all DNA is properly replicated and compacted, which is crucial for chromosome separation.

• The process involves checkpoints where the cell assesses whether it can proceed with division based on its internal conditions.

Checkpoints in Cell Division

• Key checkpoints (e.g., G1 phase) ensure that cells have all necessary components for division; significant DNA damage triggers a halt in synthesis processes.

• During G2 phase, another checkpoint evaluates if DNA is compacted correctly without damage before entering mitosis.

Consequences of Cell Cycle Arrest

• If issues are detected during these checkpoints, such as improper DNA replication or environmental factors, the cell may arrest until conditions improve or initiate programmed cell death.

• Cells must assess their size and environment before entering synthesis; favorable conditions lead to progression through the cycle.

Implications of Dysregulation

• When regulatory mechanisms fail, it can result in uncontrolled cellular growth associated with tumor development and metastasis.

• The discussion highlights how dysregulation leads to excessive proliferation linked primarily to cancerous growth patterns.

Growth Factors and Signaling Pathways

• Growth factors play a critical role in signaling pathways that determine whether a cell enters division; they can be proteins or lipid-derived molecules activating membrane receptors.

• Upon activation by growth factors, intracellular signaling cascades are initiated leading to gene expression changes necessary for cell division.

Role of Cyclins in Cell Division

• Specific genes activated by growth factor signaling often code for cyclins, which associate with kinases essential for advancing through the cell cycle phases.

Cell Cycle Regulation and Mechanisms

Key Concepts in Cell Division

• The role of specific targets, such as membrane receptors, is crucial for transitioning into the cell division cycle. These receptors are involved in signaling pathways that regulate gene transcription to increase cyclin levels, which activate the cell cycle phases.

• Recognition of replication origin sites is essential during the G1 to synthesis phase transition. This recognition activates cellular machinery necessary for cell division.

• Activation of complexes that bind to replication origins is critical. These complexes recruit helicases, enabling the formation of the replication complex necessary for DNA synthesis.

• Phosphorylation of retinoblastoma protein (Rb) inhibits its function, allowing gene expression related to cyclins. This process illustrates how Rb regulates transcription factors by sequestering them when active.

• When Rb is phosphorylated by cyclin-dependent kinases (CDKs), it releases E2F transcription factors, promoting the transcription of genes required for DNA synthesis.

Mechanisms of Transcription Activation

• The phosphorylation state of retinoblastoma influences its interaction with E2F; when phosphorylated, Rb no longer inhibits E2F, facilitating gene activation necessary for S phase progression.

• Cyclins and their associated kinases play a pivotal role in activating transcription factors that lead to increased expression of genes vital for DNA synthesis and cell cycle progression.

• CDK-cyclin complexes are essential for recognizing replication origins and initiating DNA replication processes effectively.

Chromatin Compaction and Mitosis Preparation

• After DNA duplication, chromatin must be compacted properly to ensure accurate segregation during mitosis. Various CDK complexes facilitate this compaction through ATP-driven mechanisms.

• Different types of CDK complexes are responsible for compacting newly duplicated chromatin efficiently so that it can be separated correctly during mitosis.

Transition into Mitosis

• Understanding how kinases control both initiation and completion phases is crucial as they prepare genomic material for proper distribution during cell division.

• During late G2 phase leading into mitosis, degradation processes occur alongside kinase activity ensuring correct genome organization before division begins.

Role of Mitosis Promoting Factor (MPF)

• MPF plays a significant role in regulating entry into mitosis; understanding its function helps clarify differences between mitotic and meiotic divisions within cellular processes.

Understanding Mitosis and Cellular Division

The Role of Cytoplasm in Cell Division

• Discussion on the significance of cytoplasm during fertilization, highlighting its role in advancing cellular division.

• Experiment where cytoplasm from a cell in mitosis was introduced to an embryo, demonstrating that it could progress to the next phase of mitosis.

Identification of Key Factors

• Introduction of the "promoter factor" for mitosis, which was isolated to understand its function in cellular processes.

• Experiments revealed that factors from cells in synthesis or G2 phase were crucial for achieving maximum compaction during cell division.

Mechanisms Involved in Mitosis

• Explanation of cyclin-dependent kinases (CDKs) and their regulatory roles during mitosis, particularly regarding phosphorylation events necessary for nuclear membrane breakdown.

• Importance of phosphorylation for organizing the nuclear membrane and facilitating microtubule attachment to chromosomes.

Microtubule Dynamics During Cell Division

• Description of how phosphorylated proteins contribute to the controlled disorganization of the endoplasmic reticulum and other structures essential for mitosis.

• Emphasis on how specific proteins stabilize microtubules required for chromosome separation.

Stages and Structures Critical to Mitosis

• Overview of prophase's importance in duplicating centrosomes, which are vital for organizing microtubules during cell division.

• Clarification on how duplicated centrosomes facilitate proper alignment and separation of chromosomes through microtubule formation.

Types of Microtubules Essential for Chromosome Separation

• Identification of three types of microtubules: kinetochore, polar, and astral, each playing distinct roles during cell division.

• Explanation that these microtubules generate forces necessary for effective separation and organization within dividing cells.

Understanding Microtubules and Their Role in Cell Division

The Function of Microtubules

• Microtubules are essential for separating cells and chromosomes during cell division, generating the necessary forces for this process.

• They provide traction and separation forces that facilitate the correct alignment and movement of chromosomes.

Phases of Mitosis

• During prophase, centrosomes duplicate, leading to nuclear membrane disorganization as intermediate filaments are phosphorylated by mitosis-promoting factors.

• Phosphorylation regulates the disassembly of the nuclear envelope, allowing for controlled reorganization post-mitosis.

Centrosome Dynamics

• Duplicated centrosomes produce microtubules that connect to kinetochores on chromosomes, crucial for their proper orientation during cell division.

• Kinetochores have specific protein regions recognized by microtubules, enabling them to pull chromosomes toward the cell equator.

Mechanisms of Chromosome Movement

• Microtubules exert force to orient chromosomes correctly; polar microtubules push centrosomes apart while astral microtubules anchor them to the plasma membrane.

• This dynamic interaction between different types of microtubules is vital for effective cellular separation during mitosis.

Regulation by Mitosis-Promoting Factors

• A cyclin-dependent kinase (CDK), known as the mitosis-promoting factor (MPF), plays a critical role in regulating these processes throughout mitosis.

• MPF activity is tightly controlled through phosphorylation events that can both activate and inhibit its function at different stages of cell division.

Questions on Microtube Functions

• The formation of kinetochores is regulated by MPF, which also influences microtube assembly and chromosome compaction during mitosis.

Understanding the Role of the Promoter Complex in Mitosis

Mechanisms of Mitosis Activation

• The promoter factor for active mitosis plays a crucial role by inactivating the mitotic promoter, which is essential for progressing through successful cell division and separating cells.

• During metaphase, proper orientation of chromosomes at the equator is vital; orderly separation of these chromosomes ensures that each daughter cell receives an identical set.

• To facilitate chromosome separation, strong proteins known as cohesins must be cleaved. This requires activation of the phase-promoting complex (APC), which phosphorylates specific proteins to initiate this process.

Role of Securin and Separase

• Once securin is degraded, separase becomes active. This enzyme is responsible for cutting cohesins, allowing sister chromatids to separate during mitosis.

• The APC marks proteins for degradation through ubiquitination. When securin is tagged and subsequently degraded, it releases separase to perform its function effectively.

Regulation of Cyclins and Kinases

• The APC can also inactivate cyclin-dependent kinases (CDKs). By targeting cyclins for degradation via ubiquitination, CDK activity is inhibited, allowing progression to subsequent phases of cell division.

• Active APC facilitates transition from metaphase to anaphase by degrading cyclins and freeing up inactive CDKs necessary for further cellular processes.

Feedback Mechanisms in Cell Division

• A complex regulatory mechanism exists where once APC activates and degrades cyclins, it leads to a cascade effect that allows cells to advance through the cell cycle efficiently.

• Questions arise regarding understanding these mechanisms; clarifying how factors like CDKs interact with APC can enhance comprehension of mitotic regulation.

Importance of Protein Marking and Degradation

• Ubiquitination serves as a critical signal for protein degradation within the proteasome system. This process ensures timely removal of regulatory proteins like cyclins after their function has been fulfilled.

• As separase becomes active post-degradation of securin, it cleaves cohesins enabling chromatid separation—an essential step in ensuring accurate genetic distribution during cell division.

Clarifications on Mitosis Processes

• Students are encouraged to note down any questions or uncertainties regarding these processes as they continue learning about protein marking and degradation mechanisms involved in mitosis.

• Understanding how specific regions within proteins are targeted for degradation helps clarify how cells regulate their internal environment during division cycles effectively.

Understanding the Role of Ubiquitin in Cell Division

Mechanisms of Ubiquitin and Cyclins

• The ubiquitin system is crucial for marking proteins for degradation, particularly cyclins, which regulate the cell cycle. When cyclins are degraded, it leads to the inactivation of mitotic factors.

• The mitotic checkpoint involves a kinase that activates complexes necessary for cell division. This activation facilitates protein tagging for degradation, advancing cellular processes.

Microtubule Dynamics in Chromosome Separation

• Microtubules play a vital role in chromosome movement during cell division. They generate forces that ensure chromosomes are correctly aligned and separated towards centrosomes.

• Proteins depolymerize microtubules to facilitate the movement of sister chromatids toward opposite poles during anaphase.

Forces Involved in Cell Division

• Polar microtubules connect opposite poles and utilize motor proteins to slide apart, contributing to the separation of centrosomes during cell division.

• Astral microtubules anchor at the plasma membrane, providing traction to prevent centrosomes from moving closer together as cells divide.

Nuclear Envelope Reorganization

• During prophase, nuclear lamina undergoes phosphorylation by mitotic promoters leading to its disassembly. This reorganization is essential for proper chromosomal segregation.

• As chromosomes separate, structures like the endoplasmic reticulum reform around newly organized nuclei after silencing mitotic kinases.

Actin Filaments and Cytokinesis

• After chromosome separation, actin filaments contribute significantly to cytokinesis by facilitating membrane contraction through interactions with motor proteins like myosin.

• The contraction of the plasma membrane is driven by actin filaments associating with myosin motors, enabling successful cell division into two daughter cells.

Final Stages of Cell Division: Cytokinesis

• The final phase involves actin filament contraction leading to membrane pinching and ultimately separating two cells. This process is regulated through specific phosphorylation events on proteins involved.

• Understanding how actin and myosin work together highlights their importance in completing cytokinesis effectively after chromosomal segregation has occurred.

Control Processes in Cell Division

Overview of Control Mechanisms

• Discussion on the complexity of control processes in cell division, emphasizing the importance of DNA integrity and chromosome orientation for coordinated immunity.

• Introduction to the concept of cell cycle arrest, particularly during meiosis, where regulated halting occurs before fertilization.

Arrest Mechanisms During Meiosis

• Explanation of how specific factors inhibit further separation of chromosomes until fertilization activates mitotic promoters.

• Description of regulatory processes that facilitate necessary transitions for cellular division, leading to a final mitotic division with half genomic content.

Calcium's Role in Fertilization

• Notable influxes of calcium during fertilization are highlighted as crucial for activating specific phosphorylation targets and promoting cell cycle progression.

Cellular Arrest Due to Damage

• Examination of cellular arrests caused by damage, which can lead to programmed cell death or excessive activation of the cell cycle depending on protein interactions.

DNA Damage Response Mechanisms

• Insights into how external agents or replication errors can cause double-strand breaks in DNA, prompting repair mechanisms within cells.

• The initial response involves attempts at repair; failure leads to a halt in the cell cycle due to unresolved damage.

Activation of Repair Pathways

• Discussion on ATM (Ataxia Telangiectasia Mutated), a key protein recognizing DNA strand breaks and activating downstream kinases involved in repair pathways.

p53 Protein Functionality

• p53 is identified as a critical protein regulating programmed cell death; its overactivation can trigger apoptosis or influence transcriptional activity related to cell cycle arrest.

Consequences of DNA Breakage

• Breakdown events activate checkpoints that prevent progression through the cell cycle while allowing time for repair mechanisms to function effectively.

Balancing Repair and Apoptosis

• The balance between activation levels determines whether cells undergo repair or enter programmed death; high levels may push towards apoptosis if damage remains unrepaired.

Importance During Critical Cycle Phases

• Emphasis on monitoring mechanisms throughout the entire cell cycle but especially during synthesis and replication phases when risks are heightened.

Understanding Key Proteins

• Encouragement not just to memorize proteins like p53 but rather grasp their roles within broader cellular processes responding to DNA integrity issues.

Cell Cycle Regulation and Tumor Suppressor Genes

Overview of Cell Cycle Control Mechanisms

• The discussion begins with the importance of DNA integrity during the cell cycle, particularly in the synthesis (S) phase and G2 phase, where ATM (Ataxia Telangiectasia Mutated) plays a crucial role in monitoring DNA replication.

• ATM is highlighted as a key regulator of DNA replication processes, emphasizing its significance throughout the cell cycle due to its role in repairing double-strand breaks.

• The mechanisms of cell cycle arrest are primarily active during S and G2 phases, with proteins like p53 being activated to halt the cycle or induce apoptosis when issues arise.

Tumor Suppressor Genes vs. Proto-Oncogenes

• Tumor suppressor genes prevent uncontrolled cell division; for instance, retinoblastoma acts as a master controller that inhibits transcription initiation when active.

• p53 is identified as a tumor suppressor gene that inhibits tumor development by preventing excessive cellular division; however, it does not encode proteins that promote cell growth directly.

• Proto-oncogenes are normal genes that support regulated cell growth; mutations can convert them into oncogenes, leading to unregulated tumor growth.

Mechanisms Leading to Tumorigenesis

• For tumor development, loss of function in tumor suppressor genes is necessary; both alleles must be affected for significant impact on cancer progression.

• Activation of p53 by ATM leads to cellular arrest and programmed death mechanisms; understanding this pathway is critical for grasping how cells respond to DNA damage.

Role of p53 in Cellular Response

• Under normal conditions, p53 is rapidly degraded but becomes stabilized upon DNA damage through phosphorylation, allowing it to function effectively as a transcription factor.

• Once stabilized, p53 can activate target genes such as p21 which inhibit replication processes and contribute to maintaining genomic stability during stress conditions.

Importance of Controlled Cell Division

• The discussion transitions into how controlled cell division generates various tissue types from multipotent stem cells capable of self-renewal and differentiation into specialized cells.

• Asymmetric division allows stem cells to produce different types while maintaining their population; this process is essential for tissue homeostasis and repair mechanisms following cellular damage or death.

Understanding Cell Division and Differentiation

Properties of Different Cells in Tissues

• Various cells within tissues possess the ability to divide, enter phases 0, 1, and 2, or remain in a quiescent state. Some populations can divide while others cannot.

• Specific regions contain stem cells that are the only ones capable of entering cell division cycles. Once differentiated, these cells become quiescent but maintain metabolic functions.

Tissue Renewal and Stem Cells

• In certain tissues like muscle, there is limited or no renewal; however, muscle damage can lead to massive proliferation of muscle cells.

• The nervous tissue primarily consists of stable neurons that must persist for long periods. Controlled niches exist in areas such as the subventricular zone and hippocampus for neural stem cells.

Cellular Equilibrium and Apoptosis

• Tissue recovery relies on a balance between cell division and programmed cell death (apoptosis). This balance is crucial for maintaining constant organism size.

• Disruptions in this balance can lead to neoplasia. Factors like DNA damage or replication stalling may trigger programmed cell death pathways regulated by molecules such as p53.

Implications of Imbalance in Cell Processes

• An imbalance between cellular proliferation and apoptosis can result in diseases characterized by excessive neuronal death without recovery.

• Conditions like neurodegenerative diseases illustrate how unregulated apoptosis leads to significant neuronal loss.

Future Discussions on Programmed Cell Death

• Upcoming seminars will delve into apoptosis mechanisms, including pathways dependent on p53 versus those that are independent.

• Understanding checkpoint regulation during mitosis is essential for grasping cellular biology concepts relevant to cancer treatment strategies.

Importance of Cellular Mechanisms

• Knowledge about control points during the cell cycle is vital not just academically but also for practical applications in oncology.

PCR Techniques and Workshop Engagement

Importance of Workshops

• Emphasis on utilizing workshops for clarifying doubts related to the concepts discussed, particularly focusing on replication techniques.

• Introduction of PCR (Polymerase Chain Reaction) as a revolutionary technique in DNA study, highlighting its relevance in current diagnostics, especially for COVID-19.

Participation Encouragement

• A call for active participation during workshops, noting that many attendees have their cameras and microphones off, which hinders engagement.

• Reminder that participation is not only beneficial but also essential for effective training and learning outcomes.

Upcoming Sessions