lecture 12

• Module 2 Exam: Scheduled for Monday, 10/27.

  • Students requiring testing accommodations must contact the SAS Testing Center to schedule the exam a week in advance.

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• Complete Homework 16: Due date set to 10/24.

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• Discussion Board Forum: For Module 2 Exam Material, titled "Muddiest Point," where students can post questions and clarify confusing topics.

Lecture 12

12.1 Analyze the eukaryotic cell cycle.

• 12.1.1 Sketch the sequence of stages in the eukaryotic cell cycle, labeling major events:

  • M phase (Mitosis): The period of nuclear division, followed by cytokinesis (cytoplasmic division).

  • G1 phase (Gap 1): A primary growth phase where the cell increases in size and synthesizes proteins and organelles, preparing for DNA replication. Cells can exit the cell cycle here to enter G0.

  • S phase (Synthesis): The phase during which DNA replication occurs, resulting in the duplication of chromosomes. Each chromosome now consists of two identical sister chromatids.

  • G2 phase (Gap 2): The cell continues to grow and synthesize proteins and organelles, specifically preparing for mitosis. Centrosome duplication is completed.

• 12.1.2 Explain the necessity of chromosome replication prior to mitosis during interphase. This replication ensures that each of the two daughter cells receives a complete and identical set of chromosomes, maintaining genetic fidelity across generations.

• 12.1.3 Determine chromosome counts during various cell cycle phases including mitosis. Understanding 'n' (haploid number of chromosomes) and 'C' (DNA content) is crucial. For a diploid human cell ($2n = 46$ chromosomes):

  • G1: $46$ chromosomes, $46$ chromatids ($2C$ DNA content).

  • S (after replication): $46$ chromosomes, $92$ chromatids ($4C$ DNA content).

  • G2 & Prophase & Metaphase: $46$ chromosomes, $92$ chromatids ($4C$ DNA content).

  • Anaphase: $92$ chromosomes (as sister chromatids separate and are now considered individual chromosomes), $92$ chromatids ($4C$ DNA content, transient).

  • Telophase (per forming nucleus): $46$ chromosomes, $46$ chromatids ($2C$ DNA content).

• 12.1.4 Predict structural or fate consequences resulting from alterations in cell cycle stages (M, G1, S, G2). Abnormalities can lead to cell cycle arrest, apoptosis (programmed cell death), or uncontrolled proliferation (e.g., cancer) due to uncorrected DNA damage or improper chromosome segregation.

12.2 Describe nuclear and cytoplasmic division during M phase.

• 12.2.1 Explain the fundamental differences between mitosis and meiosis.

  • Mitosis:

    • Produces two identical diploid (2n) daughter cells from a single parent cell.

    • Involves one round of nuclear and cytoplasmic division.

    • Maintains the chromosome number from parent to daughter cells.

    • Occurs in somatic cells, used for growth, repair, and asexual reproduction.

    • Resulting daughter cells are genetically identical to the parent cell.

  • Meiosis:

    • Produces four genetically diverse haploid (n) gametes (sperm or egg) from a diploid parent cell.

    • Involves two successive rounds of division: Meiosis I (reductional division) and Meiosis II (equational division).

    • Reduces the chromosome number by half.

    • Occurs in germline cells for sexual reproduction.

    • Results in genetically varied cells due to crossing over and independent assortment.

The Eukaryotic Cell Cycle

12.1: Four Regulated Phases

• The cell cycle is sequentially regulated through four main phases: G1, S, G2, and M phase, which collectively manage cell growth and division.

12.1.2: Importance of Chromosome Replication

• Accurate chromosome replication during S-phase before M-phase is critical to ensure that each daughter cell receives a full and precise complement of the genetic material, thus maintaining genome stability.

Stages of Cell Cycle

• M phase: Encompasses both Mitosis (nuclear division) and Cytokinesis (cytoplasmic division).

• Interphase: The longest phase, comprising G1, S, and G2 phases; it is critical for cell growth, DNA replication, and preparation for division.

M-Phase Details

12.2: M-Phase Process (Mitosis and Cytokinesis)

• Mitosis ensures the precise separation of sister chromatids, allocating one copy of each chromosome to each daughter nucleus:

  1. Interphase: Chromosomes are de-condensed, and DNA replication occurs, resulting in each chromosome consisting of two sister chromatids. The centrosomes also duplicate.

  2. Prophase: Chromosomes condense, becoming visible under a light microscope. The mitotic spindle (composed of microtubules) begins to form from the centrosomes, which start to move apart.

  3. Prometaphase: The nuclear envelope breaks down, and spindle microtubules extend into the nuclear area. Specific microtubules attach to kinetochores (protein complexes) located at the centromeres of each sister chromatid.

  4. Metaphase: Chromosomes align along the metaphase plate (the cell's equator), equidistant from the two spindle poles, with kinetochore microtubules ensuring proper tension.

  5. Anaphase: Sister chromatids abruptly separate, becoming individual chromosomes. These new daughter chromosomes are pulled towards opposite spindle poles by the shortening of kinetochore microtubules, while non-kinetochore microtubules lengthen, pushing the poles apart.

  6. Telophase: Daughter chromosomes arrive at the poles and begin to de-condense. New nuclear envelopes re-form around the two sets of chromosomes. The spindle apparatus disassembles.

  7. Cytokinesis: The division of the cytoplasm. In animal cells, a contractile ring of actin and myosin filaments forms a cleavage furrow, pinching the cell into two distinct daughter cells. In plant cells, a cell plate forms, becoming a new cell wall.

12.2.1: Mitosis vs Meiosis

• Mitosis:

  • Produces two genetically identical diploid (2n) daughter cells.

  • Involves one round of DNA replication and one cell division.

  • No homologous chromosome pairing or crossing over occurs.

  • Chromosome number is maintained ($2n <br>ightarrow 2n$).

• Meiosis:

  • Produces four genetically diverse haploid (n) gametes.

  • Involves one round of DNA replication but two successive cell divisions (Meiosis I and Meiosis II).

  • Homologous chromosomes pair up to form bivalents (synapsis) and undergo crossing over in Prophase I, leading to genetic recombination.

  • Chromosome number is halved ($2n <br>ightarrow n$).

Chromosome Counts in the Cell Cycle

12.1.3: Main Learning Points

• To determine chromosome and chromatid counts across the cell cycle, consider the definition of a chromosome (a DNA molecule with its associated proteins) and sister chromatids (identical copies of a chromosome joined together).

• Example question: If a eukaryotic cell has $6$ chromosomes ($2n = 6$) in G1, how many chromosomes and chromatids are present during G2 and telophase?

  • G1: $6$ chromosomes, $6$ chromatids (each chromosome has one chromatid).

  • S phase: DNA replicates. While still $6$ chromosomes, now each chromosome consists of $2$ sister chromatids.

  • G2: $6$ chromosomes, $12$ chromatids (each of the $6$ chromosomes has $2$ sister chromatids).

  • Telophase (per forming nucleus): After sister chromatids separate in anaphase, they are considered individual chromosomes. Thus, each new daughter nucleus will have $6$ chromosomes and $6$ chromatids.

Regulation of the Cell Cycle

12.3: Cyclin-Cdks Functions

• The precise progression through the eukaryotic cell cycle is critically dependent on the controlled activity of a family of enzymes called cyclin-dependent protein kinases (Cdks).

  • Role: Cdks are inactive alone but become active when bound to regulatory proteins called cyclins. Once activated, cyclin-Cdk complexes phosphorylate specific target proteins, thereby triggering or regulating key cell cycle events (e.g., DNA replication, nuclear envelope breakdown, spindle formation).

  • Different cyclin-Cdk complexes become active at different phases:

    • G1-Cdks (e.g., Cyclin D-Cdk4/6) promote passage through the G1 checkpoint.

    • S-Cdks (e.g., Cyclin A-Cdk2) initiate DNA replication.

    • M-Cdks (e.g., Cyclin B-Cdk1), also known as M-phase promoting factor (MPF), trigger and regulate entry into M phase.

  • MPF activity changes dramatically throughout the cycle: Cyclin B levels gradually rise during interphase, activating Cdk1 and forming MPF, which peaks in M phase. MPF is then rapidly de-activated by the degradation of Cyclin B, allowing the cell to exit mitosis.

Check Points of the Cell Cycle

12.3.1 Checkpoints Include:

• G1 Checkpoint (Restriction Point): This is a crucial decision point. The cell checks for adequate cell size, sufficient nutrients, presence of growth factors, and integrity of DNA. If conditions are unfavorable or DNA is damaged, the cell may enter a quiescent G0 state or undergo apoptosis. The p53 protein plays a critical role here.

• G2 Checkpoint: Before entering M phase, the cell ensures that DNA replication is complete and that there is no DNA damage. This checkpoint also monitors cell size and protein reserves.

• M Phase Checkpoints (Spindle Assembly Checkpoint/SAC): These checkpoints ensure that all chromosomes are properly attached to the mitotic spindle microtubules and correctly aligned at the metaphase plate, preventing aneuploidy (abnormal chromosome number) in daughter cells.

Cancer Connection

12.4: Properties of Cancer Cells

• Characteristics: Cancer cells exhibit hallmarks such as:

  • Uncontrolled cell division: They propagate relentlessly, ignoring normal growth-inhibiting signals and often lacking contact inhibition.

  • Invasiveness: They can invade surrounding tissues.

  • Metastasize: They can spread to distant parts of the body through the bloodstream or lymphatic system, forming secondary tumors.

  • Angiogenesis: Many tumors can promote the formation of new blood vessels to supply themselves with nutrients.

12.4.1 Defects in Cell Division:

1. Activation of growth-promoting proteins (oncogenes) when inappropriate: Mutations in proto-oncogenes (normal genes that stimulate cell growth and division) can convert them into oncogenes, leading to hyperactive cell division (e.g., a mutated RAS gene that is always 'on').

2. Deactivation of cell cycle stopping proteins (tumor suppressor genes) when they should remain functional: Mutations that inactivate tumor suppressor genes (which normally inhibit cell division or initiate apoptosis) remove critical brakes on the cell cycle (e.g., mutations in p53 or Rb).

• G1 checkpoint relevance: The G1 checkpoint is often compromised in cancer. For example, if the retinoblastoma protein (Rb) is mutated or inactivated, cells can pass the G1 checkpoint without appropriate external signals, leading to uncontrolled proliferation.

12.4.2 Outcomes of Mutations

• Analyzing conditions such as specific mutations can strikingly illustrate potential pathways to uncontrolled growth:

  • Mutations in tumor suppressors like Rb: The Rb protein normally acts as a tumor suppressor by inhibiting cell cycle progression at the G1 checkpoint. If Rb is mutated and dysfunctional, it fails to block cells with damaged DNA or without growth signals, leading to unregulated entry into S-phase.

  • Overproduction or hyperactivation of proto-oncogenes: For example, a mutation leading to constant activation of a growth factor receptor can continuously signal for cell division.

  • Loss of function of p53: The p53 protein is a crucial tumor suppressor, often called the "guardian of the genome." It can induce cell cycle arrest (e.g., at G1 or G2) or apoptosis in response to DNA damage. Mutations in p53 (common in human cancers) allow cells with damaged DNA to continue dividing, accumulating further mutations and potentially becoming cancerous.

Statistical Data

• Approximately 50% of men and 33% of women in the USA will develop cancer in their lifetime, highlighting the significant impact of cell cycle dysregulation on human health.

Learning Checks and Applications

Learning Check Questions: Frequently encountered queries relate to cell cycle mechanisms, such as:

• Treatment effects of drugs like Taxol on cell cycle progress: Taxol (paclitaxel) is an anticancer drug that stabilizes microtubules, preventing their depolymerization. This inhibits the proper function of the mitotic spindle, arresting cells in metaphase and ultimately leading to apoptosis, thus inhibiting cell proliferation.

• The role of cell cycle checkpoints in arresting the cycle: Checkpoints act as surveillance mechanisms, ensuring that each phase of the cell cycle is completed accurately and that DNA is not damaged before progression to the next phase. They can halt the cycle to allow for repair or trigger programmed cell death if problems are irreparable.

• Identifying when major changes in microtubule dynamics occur during cell cycle phases: Significant assembly of microtubules occurs during prophase to form the mitotic spindle, while rapid shortening/depolymerization of kinetochore microtubules takes place during anaphase to separate sister chromatids. Non-kinetochore microtubules also lengthen to push poles apart.

• Understanding how cyclin-Cdks successfully pass checkpoints and lead cells into subsequent phases of the cell cycle: Active cyclin-Cdk complexes, upon sensing favorable conditions and completion of prior events, phosphorylate specific substrate proteins that then activate the machinery for the next phase, overriding checkpoint blocks.

• Recognizing that conditions like underproduction or inactivation of tumor suppressors (e.g., Rb, p53) are more likely to lead to cancer progression due to unchecked growth stimuli and an inability to halt division in the presence of errors.

Conclusion

• Understanding the intricate details of the cell cycle, its distinct phases, elaborate regulatory mechanisms, and profound implications in cancer biology is essential for grasping fundamental biological processes, developing targeted medical therapies, and advancing our knowledge in cellular and molecular science.