cell cycle

Cell Cycle Regulation and Cell Division

Learning Objectives

• Understand cell cycle regulation and checkpoints:

  • Identify major steps and checkpoints in the cell cycle.

  • Explain processes checked at each checkpoint and what initiates after passing them.

  • Understand cyclin-Cdk complexes and their regulation, including:

  • How Cdks regulate cell progression.

  • How cyclins regulate Cdks.

  • Discuss changes in cyclin levels during the cell cycle.

  • Investigate the role of Xenopus eggs in studying cyclin-Cdk complexes.

• Familiarity with three major checkpoints:

  • Entry into S phase.

  • Entry into Mitosis.

  • Exit from Mitosis.

  • Identify the activated cyclin-Cdk complex and mechanisms behind it.

  • Recognize internal and external signals serving as checkpoints.

  • Understand how activated cyclin-Cdk affects cell cycle progression and consequences if checkpoints fail.

  • Define mitogens and their role in cell division.

  • Explain Rb function and p53 involvement in the DNA damage checkpoint.

  • Discuss S-Cdk and M-Cdk roles in regulating DNA replication and cell division.

  • Understand how DNA damage inhibits M-Cdk activity and the activation process of APC/C by M-Cdk.

Overview of the Cell Cycle

• Eukaryotic cells must coordinate:

  1. Chromosome replication.

  2. Segregation of DNA into identical daughter cells.

  3. Provisioning daughter cells with cytoplasmic components.

  4. Error checking processes.

  5. Regulation of growth and division influenced by external signals.

Cell Cycle Phases and Checkpoints

• The cell cycle consists of four phases:

  • M phase (takes approximately 1 hour in mammalian cells).

  • Interphase length varies with internal and external conditions.

• Major checkpoints include:

  • G1 to S checkpoint.

  • G2 to M checkpoint.

  • M to G1 checkpoint.

• The control system is managed by regulatory proteins monitoring growth and replication, using feedback from cellular processes.

Cell Cycle Control Mechanisms

• The cell cycle control system comprises:

  • Feedback mechanisms for regulating progression through key cycle stages.

  • Molecular checkpoints that pause the cycle until appropriate signals confirm readiness for the next step

  • Integration of internal and external signals into the regulatory system.

Role of Kinases and Cyclic Activation

• Proteins in the control system act as molecular switches:

  • Regulated by phosphorylation from kinases and deactivation by phosphatases.

  • Cyclins activate cyclin-dependent kinases (Cdks), leading to cell cycle progression.

Cyclin-CDK Complexes

• Cyclin-dependent kinases (Cdks):

  • Present in the cell but inactive without cyclin binding.

  • Cyclin levels vary throughout the cell cycle, coordinating with checkpoint signals.

  • Active complexes push the cell cycle forward.

• Discovery:

  • Conducted through experiments with Xenopus eggs, revealing synchronous division patterns and identification of mitotic cyclins and Cdks.

Discovery of MPF

• M-phase promoting factor (MPF) is composed of an M-cyclin and a Cdk.

• High levels are noted in M-phase extracts, inducing mitosis when injected into G2-phase cells.

Checkpoints and Regulatory Processes

• Various Cyclin-Cdk complexes control distinct phases of the cell cycle:

  • M-Cdk: Activates mitosis.

  • S-Cdk: Regulates DNA synthesis.

  • G1/S-Cdk and G1-Cdk: Respond to external stimuli.

• Cyclin levels and their degradation via APC/C reset the cycle post-mitosis.

Key Regulatory Pathways

• The phosphorylation of histones and regulatory proteins like Rb and p53 ensures cell cycle integrity:

  • Rb inhibits E2F transcription factor; this interaction must be overcome for S phase initiation.

  • p53 halts the cell cycle in G1 via activation of p21, a Cdk inhibitor, in response to DNA damage.

S Phase and DNA Replication Control

• S-Cdk ensures single genome replication:

  • Activated at G1's end to trigger DNA helicase recruitment.

  • Prevents premature re-replication through phosphorylation of cdc6 and ORC (origin recognition complex).

G2 Phase Regulation and M-Cdk Activation

• Incomplete DNA replication stalls entry into M phase:

  • M-Cdk requires dephosphorylation by Cdc25.

  • DNA damage responses inhibit activations that lead to M phase entry until integrity is assured.

M-Cdk Functions

• M-Cdk initiates mitosis by regulating:

  • Chromosome preparation.

  • Mitotic spindle assembly.

  • Inducing a positive feedback loop of Cdc25 activation.

  • Inhibiting Wee1 to ensure rapid progression into mitosis.

Cytokinesis and Mitosis

• Mitosis consists of:

  • Prophase, where chromosomes condense and spindle forms.

  • Prometaphase, nuclear envelope breaks down; kinetochores attach to spindle fibers.

  • Metaphase alignment at the spindle equator.

  • Anaphase, separation of sister chromatids, aided by microtubule dynamics and regulatory proteins like APC/C and separase.

• Cytokinesis occurs through actin and myosin filaments forming a contractile ring, ensuring daughter cell separation post-mitosis.