Cell Cycle Control Lecture

BIOL 151 Lecture Notes on Cell Cycle Control

Starter Clicker Question of the Day

  • How often do students review the lecture slides after class? Options include:

    • A. After every class.

    • B. Most of the time.

    • C. Occasionally (sort of).

    • D. Rarely (never).

    • E. Uncertain (do not know how to access them).

Announcements

  • Grades Update:

    • Exam, quiz, and discussion grades have been posted.

    • Discussion grades only account for the bubble sheet part of the grade.

    • If errors or missing grades exist, students must use the Grade Error Report Form.

  • Makeup Exams:

    • Instructions for makeup exams to be discussed shortly.

  • Bubble Sheet Instructions:

    • Use a #2 pencil for marking.

    • Write clearly, include first and last names.

    • Ensure ID number is accurate; retrieve ID when needed.

    • Properly bubble in responses without crossing out or making additional marks.

Nobel Prize in Physiology or Medicine 2001

  • Awarded to:

    • Leland H. Hartwell

    • Tim Hunt

    • Sir Paul Nurse

    • Includes a photo of Hartwell receiving the Nobel Prize from the King of Sweden.

Genetic Screens

  • Definition:

    • A method to identify genes that influence a specific trait or phenotype.

  • Methodology:

    • Induce random mutations throughout the genome.

    • Each individual may acquire a few mutations.

    • Assess if mutant individuals exhibit changes in the trait of interest.

  • Saturation Screens:

    • Conducted to ensure every gene in the genome is mutated and tested several times.

    • Genetically map mutations to identify involved genes.

    • Confirm findings by either repeating induced mutations or rescuing mutations by reintroducing normal gene copies.

Identification of Cell Division Mutants

  • A test identified mutations preventing cells from blocking division when DNA is damaged:

    • Wild type shows normal response to damage.

    • Repair mutants arrest cells but cannot recover.

    • Checkpoint mutants cannot arrest correctly and revert to inviable microcolonies.

Isolation of Temperature-Sensitive Mutants

  • Mutants isolated that prevent normal cell division under temperature changes:

    • Categories based on common phenotypes were established.

    • Included research from Leland H. Hartwell and others, published in PNAS.

    • Experimental temperatures:

    • Permissive: 23°C

    • Restrictive: 36°C

Functional Analysis of Mutant Phenotypes

  • Use of double and triple mutants to elucidate gene interactions during normal cell division leading to established order of gene functionality.

Key Findings in Cell Cycle Control Studies

  • Hartwell and colleagues identified:

    • Genes encoding proteins functioning in cell division mechanisms:

    • Cyclins, Cyclin-dependent Kinases (CDKs), Cyclin-dependent Kinase Inhibitors (CDKIs).

    • Key proteins include Rb/E2F, p53, and those involved in DNA repair.

    • Control over:

    • S phase entry

    • DNA synthesis regulation

    • Mitosis execution and chromosome segregation accuracy.

    • Mechanisms by which external/internal signals affect cell division:

    • Growth factor signaling

    • Nutritional and cellular conditions

    • DNA damage detection and repair mechanisms.

EGF Pathway and Protein Interaction Graphic

  • Overview of the EGF pathway illustrating the intricate signaling process involving Ras and downstream signaling components leading to cell division.

Overview of the Cell Cycle Phases

  • The eukaryotic cell cycle comprises:

    • Interphase:

    • G1 Phase (Gap 1)

    • S Phase (DNA Synthesis)

    • G2 Phase (Gap 2)

    • M Phase (Mitosis and Cytokinesis)

Checkpoints in the Cell Cycle

  • Importance of checkpoints:

    • Regulation of cell division through three key checkpoints:

    1. G1 checkpoint (Restriction point): Fundamental decision point for cell division.

    2. G2 checkpoint: Monitors DNA integrity before mitosis.

    3. M checkpoint: Ensures correct chromosome segregation.

  • Cells can become arrested at checkpoints if conditions are not suitable for division.

Cyclins and CDKs Mechanism

  • Cyclins and CDK interaction:

    • Cyclins bind and activate CDKs, controlling the progression through the cell cycle.

    • Cyclin-dependent kinase inhibitors (CDKIs) can block CDK activity.

  • Cyclins:

    • Levels vary throughout the cycle, promoting division when high and stopping when low.

  • CDKIs:

    • Responsible for blocking cell cycle progression during unfavorable conditions.

Regulation of Cyclin-CDK Interactions

  • Conditions for checkpoint progression rely on:

    • High cyclin concentrations

    • CDK activation through cyclin binding.

    • High CDKIs blocking interaction leading to halted cell cycle.

Roles of Cyclins, CDKs, and CDKIs in Cell Cycle Control

  • Cyclins:

    • Activate CDKs influencing cell cycle checkpoints and progression based on concentration changes.

  • CDKs:

    • Maintain constant levels and become active when associated with cyclins, targeting genes necessary for each cycling stage.

  • CDKIs:

    • Actively inhibit cyclin-CDK complexes when DNA damage is detected.

Signaling Pathways and Cell Cycle Progression

  • Steps in cyclin functionality:

    1. Signaling pathways promote cyclin gene expression.

    2. Cyclin levels rise within the cytoplasm.

    3. Cyclin binds and activates CDKs.

    4. Activated cyclin-CDK complexes translocate to the nucleus.

    5. Phosphorylation of Rb occurs, allowing E2F to activate S phase gene expression.

p53 Role in DNA Damage Response

  • p53 acts as a pivotal protein for detecting DNA damage:

    • In the presence of damage, it blocks cell cycle progression.

    • Activation process involves kinase-mediated phosphorylation that prevents MDM2 from degrading p53.

  • p53 promotes the expression of:

    1. Cell cycle inhibitors (CDKIs).

    2. Genes involved in DNA repair.

    3. Cell death genes, if damage is irreparable.

Responses to DNA Damage and Cell Cycle Blockage

  • Activation of DNA damage responses leads to the induction of:

    • Increased expression of CDKI genes preventing progression through the cycle.

  • Potential situations that can affect the cell division cycle include:

    • Blocking signaling pathways that lead to cyclin expression, thus inhibiting cyclin-CDK interactions.

    • Preventing Rb phosphorylation to hinder S phase initiation.

Questions for Interpretation and Understanding

  • Pose hypothetical scenarios regarding the G1 checkpoint leading to S phase initiation based on signal activations:

    1. Presence of growth factors activating cyclin gene expression.

    2. Consequences of high rates of ubiquitin-dependent proteolysis of cyclins.

    3. Effects of mutations affecting Rb phosphorylation.

Conclusion of Key Concepts

  • Overall understanding of the mechanisms governing cell cycle control positions students to comprehend the integrative role of genetic components in cellular function and potential implications in cancer therapy and genetics.