15-Cell Cycle and Cell Division: Structure, Phases, and Regulation

Cell Cycle

Overview

The cell cycle is the series of events that cells go through as they grow and divide. It is fundamental to both prokaryotic and eukaryotic life, ensuring genetic material is accurately replicated and distributed to daughter cells.

• Prokaryotes divide by binary fission.

• Eukaryotes divide by mitosis (somatic cells) or meiosis (germ cells).

Cell Division

Prokaryotic Cell Division: Binary Fission

Prokaryotic cells, such as bacteria, reproduce by binary fission, a simple and rapid process.

• One bacterium divides into two identical daughter cells.

• Replication of a single, circular chromosome precedes division.

• Process can take as little as 15 minutes under optimal conditions.

Steps of Binary Fission

1. Chromosome replication begins at the origin of replication.

2. Replication continues as the cell elongates.

3. Replication finishes; the cell membrane grows inward.

4. Two daughter cells are formed, each with an identical chromosome.

Eukaryotic Cell Division

Eukaryotic cells have a more complex division process due to their multiple linear chromosomes and organelles.

• Somatic cells undergo mitosis to produce two identical daughter cells.

• Germ cells undergo meiosis to produce gametes (sperm and eggs), which fuse to form a zygote.

• DNA is replicated before cell division.

• Cell division in eukaryotes takes longer than in prokaryotes.

Eukaryotes: DNA and Chromosomes

Chromosome Structure

Eukaryotic cells contain much more DNA than prokaryotic cells, organized into several linear chromosomes.

• Human cells have 46 chromosomes (23 pairs).

• Chromosomes are composed of DNA and associated proteins (chromatin).

• Organization is more complex than the single, circular DNA molecule in prokaryotes.

Phases of the Cell Cycle

Major Phases

The cell cycle consists of distinct phases that coordinate cell growth, DNA replication, and division.

• Interphase: Normal cell activity, growth, and DNA replication.

• Mitotic phase (M phase): Nuclear division (mitosis).

• Cytokinesis: Division of the cytoplasm.

Five Phases of the Cell Cycle

• G1 – Primary growth phase

• S – Synthesis; DNA replicated

• G2 – Secondary growth phase

• M – Mitosis

• C – Cytokinesis

G1, S, and G2 collectively are called interphase.

Interphase

G1 Stage

• First growth stage after cell division.

• Cells mature by making more cytoplasm and organelles.

• Cell carries on its normal metabolic activities.

S Stage

• Synthesis stage.

• Chromosomes are replicated.

Chromosome Duplication

• Each chromosome is duplicated to form two sister chromatids joined at the centromere.

• During mitosis, sister chromatids are separated and distributed to daughter cells.

G2 Stage

• Second growth stage.

• Occurs after DNA has been copied.

• All cell structures needed for division are made (e.g., centrioles).

• Both organelles and proteins are synthesized.

Summary of Interphase Activities

• DNA replicates.

• Centrioles (in animal cells) replicate.

• Cell prepares for division.

Mitosis and Cytokinesis

• Mitosis: The process during cell division in which the nucleus divides into two nuclei, producing identical copies.

• Cytokinesis: The division of the cytoplasm, resulting in two separate daughter cells.

Control of the Cell Cycle

Checkpoints

The cell cycle is tightly regulated by checkpoints, which act as control points to ensure proper division and prevent errors.

• Checkpoints function as "red light, green light" signals.

• If conditions are favorable, the cell proceeds; if not, the cycle halts.

Major Checkpoints

Checkpoint	Location	Function
G1 checkpoint	End of G1 phase	Checks for cell size, nutrients, growth factors, and DNA damage. If conditions are good, moves to S phase; if not, cell may enter G0 (resting state).
G2 checkpoint	End of G2 phase	Checks for DNA replication completion and DNA damage. Repair enzymes correct errors; if passed, triggers mitosis.
M checkpoint	During mitosis (metaphase)	Checks for proper chromosome attachment to spindle fibers; if passed, allows cell to proceed to cytokinesis.

Checkpoint Regulation

• Cell cycle control system integrates signals from inside and outside the cell.

• Growth factors and tumor suppressor proteins (e.g., Rb) play key roles in checkpoint regulation.

• Many mature cells in the human body are in G0, a non-dividing state.

Summary Table: Cell Cycle Phases and Key Events

Phase	Main Events
G1	Cell growth, organelle synthesis, normal metabolism
S	DNA replication, chromosome duplication
G2	Preparation for division, synthesis of division structures
M	Nuclear division (mitosis)
C	Cytoplasmic division (cytokinesis)

Key Terms and Definitions

• Binary fission: A method of asexual reproduction in prokaryotes resulting in two identical cells.

• Mitosis: Eukaryotic cell division producing two genetically identical daughter cells.

• Meiosis: Eukaryotic cell division producing gametes with half the chromosome number.

• Interphase: The phase of the cell cycle where the cell grows, replicates DNA, and prepares for division.

• Checkpoint: A control point in the cell cycle where stop and go-ahead signals regulate progression.

• G0: A resting state where cells exit the cell cycle and do not divide.

Relevant Equations

• DNA replication (S phase): $\text{DNA}_{\text{original}} \rightarrow 2 \times \text{DNA}_{\text{identical copies}}$

• Cell division (binary fission): $\text{Cell}_{\text{parent}} \rightarrow 2 \times \text{Cell}_{\text{daughter}}$

Example

Example: In human cells, the cell cycle ensures that each daughter cell receives 46 chromosomes after mitosis, maintaining genetic stability across generations.

Additional info: The notes also reference the role of tumor suppressor proteins (e.g., Rb) in cell cycle control, which is crucial for preventing uncontrolled cell division and cancer.