Cell Cycle Regulation

• Module 5 discusses the cell cycle in depth
• The module focuses on regulation and the molecular details of DNA replication in the S phase of interphase
• Reviews are provided to recap previous learnings and connect them to current topics

Motivation Question

• Explore how one cell can divide and differentiate into millions of cells with specific functions

Lesson 5.1: Cell Cycle Regulation Overview

• This lesson reviews the basic principles of the cell cycle and its regulation
• It provides an overview of the cell cycle control system and checkpoints
• Molecular components that drive these processes are examined

Learning Outcomes

• Review the general cell cycle process
• Contrast the main regulatory changes in the cell cycle control system
• Identify cell cycle control checkpoints
• Identify cell cycle control system components
• Identify unregulated cell cycle consequences

Motivation question:

• Match the stage of cell cycle with image: interphase, prophase, metaphase, anaphase, and telophase
• How cells regulate these phases to make identical daughter cells is explored

Eukaryotic Cell Cycle Review

• A human cell in culture divides roughly every 24 hours
• Mitosis and interphase are the two basic parts of the cell cycle
• Mitosis or nuclear division, is the most dramatic cell cycle stage
• Daughter chromosomes separate, ending in cell division, or cytokinesis, during mitosis
• Mitosis and cytokinesis last about an hour, and interphase, the period between mitoses, takes up about 95% of the cell cycle
• Chromosomes are decondensed and spread throughout the nucleus during interphase, making the nucleus appear morphologically uniform
• Interphase is when DNA is replicated and cell growth occurs as the cell gets ready to divide
• The cell grows at a consistent rate throughout interphase
• Most dividing cells double between one mitosis and the next
• DNA is synthesized during only a portion of interphase
• DNA synthesis divides eukaryotic cells into four phases
  • Namely, M phase, G1 phase (gap 1), S phase, and G2 phase (gap 2)
• M phase is when mitosis happens, generally followed by cytokinesis
• The mitotic phase can be further divided into prophase, metaphase, anaphase, and telophase
• G1 phase follows M phase and marks the gap between mitosis and the start of DNA replication
• S phase is the synthesis phase after G1, where DNA replication happens
• G2 phase is after DNA synthesis, and proteins are made to prepare for mitosis

Cell Cycle Regulation

• The division cycle is regulated by extracellular signals from the environment
• Internal signals monitor and coordinate the various processes that happen during different phases
• Growth factors' effect on animal cell proliferation exemplifies cell cycle regulation by extracellular cues
• Coordination of different processes like growth, DNA replication, and mitosis happen during the cell cycle
• The cell cycle progresses through its phases via a series of control points

Cell Cycle Control System

• The cell-cycle control system in eukaryotes regulates cell-cycle progression at three key regulatory transitions
  • Start (or the restriction point) happens in late G1
  • G2/M transition
  • Metaphase-to-anaphase transition
• Late G1 "Start" point
  • The cell decides to enter the cell cycle and duplicate its chromosomes
  • External cues, like nutrient availability and cell size, regulate "Start"
• G2/M transition
  • The cell cycle control system checks if DNA has been fully replicated
  • It checks for favourable environmental conditions for nuclear division. Triggers early mitotic activities, leading to chromosome alignment on the mitotic spindle in metaphase
  • Checks if all DNA has been replicated, and if the environment will allow the next phase to happen
• Metaphase-to-anaphase transition
  • An integrated control system checks for chromosomal attachment to the spindle
  • It stimulates sister-chromatid separation, causing mitosis and cytokinesis to finish correctly
• The control system stops progression at each transition if issues are recognised inside or outside the cell
• If DNA replication is incomplete, the cell cycle control system prevents G2/M transition until the problems are solved
• The control system blocks progression, preventing cell division until conditions become favorable, if extracellular circumstances are unsuitable for cell proliferation

Cell Control Checkpoints

• Appropriate staging depends on proper sequencing and coordination of different cell cycle stages
• Cell cycle checkpoints prevent the next phase of the cell cycle from happening until the prior phase activities are finished
• Complete genomes are passed to daughter cells via two eukaryotic cell checkpoints
  • DNA damage checkpoints
  • Spindle assembly checkpoint
• DNA damage checkpoints
  • These checkpoints prevent damaged DNA from being replicated and passed to daughter cells
  • Helps sense if DNA damaged or only partly replicated and coordinates the cell cycle to allow DNA replication or repair
  • DNA damage checkpoints work in G1, S, and G2, causing cell cycle arrest in response to damaged or unreplicated DNA
• Quiescent Stage (G0):
  • G1 progression stops at the restriction point if growth factors aren't available
  • Paused cells enter G0 to discontinue multiplying and remain for a longer period
  • Although they stop developing and produce less protein, Go cells are still metabolically active
• Spindle assembly checkpoint
  • Maintains integrity of the genome towards the end of mitosis
  • Sees that chromosomes are aligned properly on the mitotic spindle
  • Distributes full chromosome sets precisely to daughter cells
• Mitosis does not proceed when chromosomes fail to align correctly on the spindle at metaphase
• The chromosomes are distributed to each daughter cell when the the spindle assembly checkpoint ensures they're organised appropriately

Spindle Assembly Checkpoint Mechanisms

• The spindle assembly checkpoint activates when unattached or misattached kinetochores are there
• Kinetochore proteins BUB3, BUBR1, MAD2, and CDC20 aggregate at the cytosol to form the mitotic checkpoint complex (MCC)
• Once the MCC is generated, CDC20 does not bind to the anaphase-promoting complex/cyclosome causing arrest in metaphase to anaphase transition
• SAC deactivation occurs with the proper connection of kinetochore with microtubules in the spindle
• Dynein/SPINDLY removes BUB3, BUBR1, MAD2, and CDC20 of fixed kinetochores
• Assembly of new MCC and existing MCC breakdown is prevented, which in turn releases CDC20 to connect and activate with APC/C
• Mitosis can be exited

Cell Cycle Control System Components

• Cyclin and cyclin-dependent kinases (Cdks) are the two main parts of the cell cycle control system
• Binding cyclin-Cdk activates the protein kinase, which triggers certain processes
• Cdk is inactive without cyclin
• Cyclins are categorised into four types based on function at what stage Cdks bind
  • G1/S cyclins
  • S-cyclins
  • M-cyclins
  • G1 Cyclins
• A single Cdk protein in yeasts couples to all classes of cyclins, causing cell-cycle events by modifying cyclin partners
• Vertebrates have four Cdks that:
  • Two for G1 cyclins
  • One for G1/S and S cyclins
  • One for S and M cyclins
• Cyclin-Cdk complexes can be referred to as G1-Cdk, G1/S-Cdk, S-Cdk, and M-Cdk in this lesson
• These cyclin-Cdk trigger different cell-cycle events
• Cyclin proteins activate Cdk partners and guides it to target proteins
• Each cyclin-Cdk complex phosphorylates different substrate proteins
• Access to certain Cdk substrates change through the cell cycle and the same cyclin-Cdk complex can create different effects
• Proteins that function in mitosis become available for phosphorylation in G2

Cancer: Unregulated Cell Cycle Consequence

• Anticancer medicines target cycle progression
• Cell cycle phases, G0, G1, S, G2, and mitosis, are shown
• The G0 Restriction Point illustrates the reversible nature of cell cycle entry and quiescence
• Exogenous disturbances activate checkpoints that arrest cells during phase transitions
• Cellular proliferation measures mark the cell cycle phase presence
• Drugs inhibiting cell cycle progression have targets and actions in parentheses
• Major regulatory pathways activating each checkpoint are listed near the checkpoint's function
• Accurate DNA replication and cell division require precise control of cell cycle regulation