Cell Cycle

What is the Cell Theory and how are multicellular organisms organized?

All living things are made of one or more cells. In multicellular organisms, cells work together hierarchically: Cells → Tissues → Organs → Organ Systems. Cells are specialized (e.g., skin, stomach, muscle) and require regulation for proper function.

How does growth occur in multicellular organisms, and what role do mitosis and cytokinesis play?

Growth occurs via cell reproduction, not individual cell size increase. Mitosis and cytokinesis produce new body cells, ensuring proper tissue growth and repair.

What can result from unregulated cell division?

Uncontrolled cell division can lead to cancer, where cells divide uncontrollably, may fail to communicate with healthy cells, not anchor properly (spreading potential), and secrete growth factors that divert nutrients. Causes include genetic predisposition and exposure to toxins, radiation, or UV light.

What are the main phases of the cell cycle, and how much time do cells spend in each?

• Interphase: Cell growth, DNA replication, and normal functions; most of a cell's time is spent here.

• M phase: Mitosis and cytokinesis; actual cell division. Frequency of mitosis varies by cell type (e.g., hair follicle cells divide frequently).

What is the purpose of checkpoints in the cell cycle and what are the main ones?

Checkpoints ensure proper growth, DNA integrity, and sufficient resources before division:

• G1 checkpoint: Checks growth, DNA damage, and resources before S phase.

• G2 checkpoint: Ensures DNA replication is complete and the cell is ready for mitosis.

• M phase (metaphase) checkpoint: Confirms chromosomes are aligned and attached to the spindle to prevent incorrect separation.

How is the cell cycle regulated?

Proteins regulate progression:

• Positive regulators (e.g., cyclins, Cdk) promote cell cycle progression.

• Negative regulators (e.g., p53) halt progression or trigger apoptosis if damage is irreparable.
  Cyclin-Cdk complexes rise and fall in each phase (G1, S, G2, M), determining progression.

What is apoptosis, and why is it important?

Apoptosis is programmed cell death, occurring when cellular damage cannot be repaired, maintaining tissue health and preventing damaged cells from dividing.

What is the G0 phase, and which cells may remain in it permanently?

G0 is a resting phase where cells perform normal functions but do not divide. Some cells enter temporarily due to limited resources, while others, like neurons, remain permanently in G0, limiting regeneration after injury.

What is the overall significance of the cell cycle?

The cell cycle ensures proper growth, DNA replication, and regulated division. Checkpoints and regulatory proteins maintain genomic integrity, uncontrolled division can cause cancer, and the G0 phase identifies non-dividing cells important for tissue repair.