The Cell Cycle and Eukaryotic Cell Division (Chapter 12) – Study Notes

The Cell Cycle and Eukaryotic Cell Division

Introduction

The cell cycle is the series of events that cells go through as they grow and divide. Eukaryotic cell division involves the division of the nucleus and cytoplasm, ensuring the faithful transmission of genetic material to daughter cells. This process is essential for growth, development, and maintenance in multicellular organisms.

Overview of Cell Division

Cell Division in Unicellular and Multicellular Organisms

• Unicellular organisms: Cell division reproduces the entire organism.

• Multicellular organisms: Cell division is necessary for growth (from a fertilized cell), normal size maintenance, and tissue repair.

Cell division is a part of the cell cycle, which is the life of a cell from its formation to its next division.

Daughter Cells and Genetic Information

• Most cell division produces daughter cells with identical DNA (clones).

• DNA instructions are copied so each cell receives the complete genetic information.

• A special type of division (meiosis) produces nonidentical daughter cells (gametes), each with half the genetic information.

Genetic Material Organization

DNA Structure and Packaging

• DNA molecules are typically double-stranded and held together by hydrogen bonds.

• Genome: All the DNA in a cell. In prokaryotes, usually a single DNA molecule; in eukaryotes, multiple DNA molecules.

• DNA is packaged into chromosomes (DNA + protein).

Chromosomes and Chromatin

• Each eukaryotic species has a characteristic number of chromosomes in its nucleus.

• Somatic cells (nonreproductive) have two sets of chromosomes (diploid).

• Gametes (sperm and eggs) have one set (haploid).

• Eukaryotic chromosomes are made of chromatin (DNA + protein).

Chromosome Duplication and Structure

• Before cell division, DNA is replicated and chromosomes condense (become visible).

• Each duplicated chromosome consists of two sister chromatids joined at a centromere (the narrow "waist").

Phases of the Cell Cycle

Main Phases

• Interphase (about 90% of the cycle): Cell growth and DNA replication.

• Mitotic (M) phase: Mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Subphases of Interphase

• G1 phase ("first gap"): Cell grows and carries out normal functions.

• S phase ("synthesis"): DNA is replicated.

• G2 phase ("second gap"): Cell prepares for division, now with two copies of DNA.

Chromosomes are duplicated only during the S phase.

Mitosis and Cytokinesis

Overview of Mitosis and Cytokinesis

1. Copy DNA (S phase of Interphase)

2. Break down the nucleus

3. Move DNA copies to opposite sides of the cell

4. Rebuild the nucleus around each DNA copy

5. Divide the cell into two (cytokinesis)

Phases of Mitosis

1. Prophase: Chromatin condenses into visible chromosomes; nucleoli disappear; mitotic spindle begins to form; centrosomes move apart.

2. Prometaphase: Nuclear envelope fragments; spindle microtubules attach to kinetochores on chromosomes.

3. Metaphase: Chromosomes align at the metaphase plate (cell equator); centrosomes at opposite poles.

4. Anaphase: Cohesin proteins are cleaved; sister chromatids separate and move to opposite poles; cell elongates.

5. Telophase: Two genetically identical nuclei form; chromosomes decondense; nuclear envelopes reform.

Cytokinesis usually overlaps with telophase, resulting in two daughter cells.

The Mitotic Spindle

• The mitotic spindle is a structure made of microtubules that orchestrates chromosome movement.

• It forms from the centrosomes (microtubule organizing centers), which replicate and move to opposite cell poles.

• The spindle includes centrosomes, spindle microtubules, and asters (short microtubules radiating from centrosomes).

• Kinetochores are protein complexes on chromatids where spindle fibers attach.

Cytokinesis in Animal and Plant Cells

Animal Cells	Plant Cells
Occurs by cleavage, forming a cleavage furrow (contractile ring of microfilaments pinches the cell in two)	Occurs by formation of a cell plate (vesicles coalesce at the center, forming a new cell wall between daughter cells)

Regulation of the Eukaryotic Cell Cycle

Cell Cycle Control System

• The cell cycle is regulated by a cell cycle control system, functioning like a clock with checkpoints.

• Checkpoints are control points where the cell cycle can be stopped until certain conditions are met.

• The G1 checkpoint is the most important; if a cell passes it, it usually completes the cycle.

• If a cell does not receive the go-ahead signal at G1, it enters a nondividing state called G0 phase.

Regulatory Molecules

• Cyclins and Cyclin-dependent kinases (Cdks) are proteins that regulate the cell cycle.

• Their concentrations and activity fluctuate during the cycle.

• MPF (Maturation-Promoting Factor) is a cyclin-Cdk complex that triggers passage from G2 to M phase.

Internal and External Signals

• Internal signals: For example, unattached kinetochores send signals to prevent anaphase until all chromosomes are properly attached.

• External signals:

  • Growth factors: Proteins that stimulate cell division.

  • Density-dependent inhibition: Crowded cells stop dividing.

  • Anchorage dependence: Most animal cells must be attached to a substrate to divide.

Loss of Cell Cycle Control and Cancer

Cancer Cells

• Cancer cells do not respond to normal cell cycle controls.

• They do not exhibit density-dependent inhibition or anchorage dependence and may not require growth factors to divide.

• Cancer cells divide uncontrollably, forming tumors (masses of abnormal cells).

• Benign tumors remain at the original site; malignant tumors invade tissues and can metastasize (spread to other parts of the body).

• Transformation (accumulation of mutations) converts a normal cell to a cancerous one.

Summary Table: Key Terms and Concepts

Term	Definition
Genome	All the DNA in a cell
Chromosome	DNA molecule packaged with proteins
Chromatin	Complex of DNA and protein in chromosomes
Sister Chromatids	Identical copies of a duplicated chromosome, joined at the centromere
Centromere	Region where sister chromatids are most closely attached
Mitosis	Division of the nucleus
Cytokinesis	Division of the cytoplasm
Interphase	Cell growth and DNA replication phase
Mitotic Spindle	Microtubule structure that separates chromosomes
Checkpoint	Control point in the cell cycle
Cyclin/Cdk	Proteins that regulate the cell cycle
MPF	Cyclin-Cdk complex that triggers mitosis
G0 phase	Nondividing state
Growth Factor	Protein that stimulates cell division
Density-dependent inhibition	Cells stop dividing when crowded
Anchorage dependence	Cells must be attached to divide
Transformation	Conversion of a normal cell to a cancer cell
Metastasis	Spread of cancer cells to other tissues

Key Questions for Review

1. Describe the structural organization of the prokaryotic genome and the eukaryotic genome.

2. List the phases of the cell cycle; describe the sequence of events during each phase.

3. List the phases of mitosis and describe the events characteristic of each phase.

4. Draw or describe the mitotic spindle, including centrosomes, kinetochore microtubules, nonkinetochore microtubules, and asters.

5. Compare cytokinesis in animals and plants.

6. Describe the normal cell cycle control system.

7. Describe cancer as an example of loss of cell cycle control.

Additional info: This guide integrates foundational concepts from molecular biology (DNA structure, genome organization), cell biology (chromosome structure, mitotic spindle), and cancer biology (loss of cell cycle control), providing a comprehensive overview suitable for introductory college biology students.