5 The Cell Cycle and Dividing to Multiply - Copy

The Cell Cycle

The cell cycle can be visualized similarly to a human life cycle, with stages involving growth, development, and eventual division into new cells. This cycle is crucial for organismal development, tissue repair, and maintaining cellular homeostasis.

Encourage students to draw the stages of the animal cell cycle to reinforce understanding and help visualize the transitions between phases. Consider discussing the differences and similarities between cellular and human life cycles; both involve growth, development, and division, but cellular cycles are strictly biological processes governed by molecular mechanisms, while human cycles include social and environmental factors influencing development.

Learning Objectives

1. Describe the life history of a cell:

   • The life history consists of several stages:

     • G1 Phase (Gap 1): Lasts 6-12 hours; focusing on growth, organelle duplication, and preparation for DNA synthesis.

     • S Phase (Synthesis): Lasts 6-8 hours; DNA is replicated, chromosome number temporarily doubles.

     • G2 Phase (Gap 2): Lasts 3-4 hours; final preparations for mitosis occur, ensuring all elements are in place.

     • M Phase (Mitosis): Lasts about 1 hour; the cell divides, resulting in two identical daughter cells.Overall, these phases are crucial for growth, repair, and maintaining cellular homeostasis.

2. Detail the events of the cell cycle:

   • Events include:

     • G1 Phase: Cellular metabolism, organelle duplication, environment assessment.

     • S Phase: DNA synthesis with enzyme action ensuring accurate replication.

     • G2 Phase: Further growth, synthesis of necessary proteins and organelles for division.

     • M Phase: Mitosis (chromosomal alignment and separation) and cytokinesis (cytoplasmic division), yielding two genetically identical cells.

3. Identify major checkpoints within the cell cycle:

   • Major checkpoints include:

     • G1 Checkpoint: Assesses cell size, energy reserves, and DNA integrity.

     • G2 Checkpoint: Checks for DNA damage and completeness of DNA replication.

     • M Checkpoint: Ensures proper chromosome alignment and attachment to spindle fibers before anaphase.Disruptions can lead to mutations, cell cycle arrest, or uncontrolled cell division, contributing to diseases like cancer.

4. Describe the process of DNA replication:

   • DNA replication occurs during the S phase and involves several key enzymes:

     • DNA Polymerase: Synthesizes new DNA strands.

     • DNA Ligase: Joins Okazaki fragments to form continuous strands.

     • Helicase: Unwinds the DNA double helix.

   • Replication is crucial for genetic fidelity; errors can result in genetic disorders or cell dysfunction, emphasizing its importance in the development and maintenance of organisms.

Introduction to the Cell Cycle

Overview of the concept of cell division: cells undergo metabolic and functional division to create new cells, emphasizing the importance of this process in growth, repair, and reproduction in multicellular organisms. Understanding the cell cycle is key to grasping how organisms develop and respond to internal and external signals.

Explore - Cell Cycle Stages

Interphase: Consists of three distinct phases:

• G₁ Phase (Gap 1): This initial growth stage is characterized by cellular metabolism, organelle duplication, and preparation for DNA synthesis.

• S Phase (Synthesis): During this phase, DNA synthesis occurs, where genetic material is replicated, ensuring that each daughter cell receives an exact copy of the genetic information.

• G₂ Phase (Gap 2): In this phase, further growth occurs, and the cell prepares for mitosis, checking that all cellular components are ready and intact.

Mitotic Phase: Involves the processes of mitosis and cytokinesis, leading to the formation of two genetically identical daughter cells, crucial for growth and tissue repair.

Phases of the Cell Cycle

Phases:

• G1 (Gap 1): During this growth phase, the cell doubles its organelles, increases its size, and assesses the environment, preparing itself for DNA replication.

• S (Synthesis): In this crucial phase, DNA is replicated; thus, the chromosome number temporarily doubles, ensuring that each new cell will have a complete set of chromosomes.

• G2 (Gap 2): The cell prepares for mitosis by synthesizing proteins and organelles required for division, ensuring that all cellular contents are ready for division.

• M (Mitosis): This phase represents the actual division of the cell, involving spindle formation, chromosomal alignment, and their distribution to daughter cells.

Explore - Mitosis

Stages of Mitosis:

1. Prophase: Chromosomes condense and become visible; spindle fibers emerge from the centrosomes; the nuclear envelope begins to break down, allowing spindle fibers to interact with chromosomes.

2. Prometaphase: The chromosomes condense further while kinetochores appear at the centromeres, facilitating attachment to spindle fibers.

3. Metaphase: Chromosomes align at the metaphase plate, ensuring that each chromatid is attached to its respective spindle fiber to facilitate equal distribution.

4. Anaphase: Sister chromatids separate and move toward opposite poles of the cell, ensuring each daughter cell will inherit an identical set of chromosomes.

5. Telophase: Chromosomes arrive at the poles and de-condense; new nuclear membranes form around each set of chromosomes, signaling the near end of cell division.

6. Cytokinesis: The final stage where the cytoplasm divides, usually resulting in two distinct daughter cells. In animal cells, this forms a cleavage furrow, whereas in plant cells, a cell plate is formed.

Practice - The Cell Cycle

Engage with the material by drawing each stage of the cell cycle. Label the drawings and describe key events occurring during each phase to solidify understanding and retention of the process.

Cell Cycle Regulation – Quality Assurance

Emphasizes the necessity of cell cycle checkpoints to prevent errors in cell division. These checkpoints verify whether the processes at each phase have been accurately completed, preventing mutations and ensuring integrity during cell division. Link to resources (Khan Academy) for more depth on cell checkpoints and regulation mechanisms, offering students a broader understanding of cellular function and implications of cell cycle dysregulation.

Explore - DNA Replication

S Phase Details: Focus on the process of DNA replication, which is critical for maintaining genetic fidelity. In-depth understanding includes:

• DNA Polymerase: The enzyme responsible for synthesizing new DNA strands by adding nucleotides complementary to the template strand.

• DNA Ligase: Joins Okazaki fragments formed on the lagging strand, sealing nicks between DNA fragments to produce a continuous strand.

• Helicase: Unwinds the DNA double helix, separating the two strands to allow for replication.

• Leading and Lagging Strands: Differentiates the continuous leading strand from the discontinuous lagging strand; highlights the role of RNA primers in initiating replication, ensuring new strands are synthesized accurately.

Phases of the Cycle - Review

Duration of Phases:

• G1 (6-12 hours)

• S (6-8 hours)

• G2 (3-4 hours)

• M (1 hour)

GO Phase: A resting phase where the cell is not actively dividing; cells may exit the cycle temporarily for functions like differentiation or rest.

Summarizes key functions:

• G1: Growth and metabolism, preparing for DNA replication.

• S: DNA replication ensuring genetic fidelity and integrity for daughter cells.

• G2: Continued growth check and preparation for mitosis.

• Mitosis: Actual division ensuring accurate division of genetic material.

Review Objectives

Reflect on the learning objectives:

• Are you closer to understanding the life history of a cell and its stages?

• What areas need further review? Reinforce confidence in the knowledge about the cell cycle, mitosis, checkpoints, and DNA replication, understanding their critical roles in cell function and organismal development.