The Cell Cycle and Interphase

Key Concepts of Growth and Cell Cycle

Introduction to Growth

• Growth and development are fundamental characteristics of life.

• Organisms grow by increasing cell size and/or increasing the number of cells.

• The surface area-to-volume ratio limits how large individual cells can grow.

• Multicellular organisms grow by cell division, which is tightly regulated by the cell cycle.

The Cell Cycle Overview

• The cell cycle is a repeating sequence of growth and division.

• It consists of four main stages:

  1. G1 Phase (Gap 1): Cell grows, performs normal functions, and prepares for DNA replication.

  2. S Phase (Synthesis): DNA replication occurs, producing two identical copies.

  3. G2 Phase (Gap 2): Cell prepares for division by making proteins and organelles.

  4. M Phase (Mitotic Phase): Active division occurs, consisting of mitosis (nuclear division) and cytokinesis (cell division).

• Most cells spend the majority of their lifespan in interphase (G1, S, and G2 phases).

Detailed Breakdown of Interphase

G1 Phase (Gap 1)

• Cells grow and carry out their typical metabolic functions.

• Some cells enter G0 phase, a non-dividing state where they perform normal functions indefinitely.

• If conditions are suitable for division, cells proceed to S phase.

S Phase (Synthesis)

• DNA replication takes place, ensuring that each daughter cell receives an identical set of DNA.

• The cell now contains twice the original amount of DNA.

G2 Phase (Gap 2)

• Final preparations for mitosis.

• The cell synthesizes additional proteins and organelles.

• It increases in size to ensure both daughter cells receive sufficient cellular components.

Mitotic (M) Phase: Cell Division

Mitosis (Division of Nucleus)

1. Prophase: Chromosomes condense, nuclear envelope begins to break down.

2. Metaphase: Chromosomes align at the cell's equator.

3. Anaphase: Sister chromatids separate and move to opposite poles.

4. Telophase: Nuclear envelopes reform, and chromosomes decondense.

Cytokinesis (Division of Cytoplasm)

• The cytoplasm divides, forming two identical daughter cells.

• In animal cells, the membrane pinches inward; in plant cells, a cell plate forms.

Regulation of the Cell Cycle

• The cycle is controlled by regulatory proteins that ensure accurate progression.

• Three major checkpoints:

  1. G1 Checkpoint: Determines if the cell is ready to proceed to S phase.

  2. S Checkpoint: Ensures proper DNA replication.

  3. M Checkpoint: Verifies that chromosomes are correctly aligned before division.

• If errors are detected, the cell may attempt repair or undergo apoptosis (programmed cell death).

Cancer and the Cell Cycle

• Cancer occurs when cell cycle regulation fails.

• Mutations in checkpoint regulatory genes can lead to uncontrolled cell division.

• Carcinogens (e.g., radiation, chemicals) damage DNA, increasing mutation rates.

• Cancer cells divide uncontrollably, ignoring normal growth signals and checkpoints.