The Cell Cycle: Mechanisms and Regulation

Chapter 9: The Cell Cycle

Overview: The Key Roles of Cell Division

Cell division is fundamental to the continuity of life, enabling organisms to reproduce, grow, and repair tissues. This process ensures that genetic information is faithfully transmitted from one generation of cells to the next.

• Cell division produces identical copies of cells.

• Functions in reproduction, growth, and repair.

• Involves copying the cell's DNA (genome).

Genetic Material and Chromosome Structure

Most cell division results in genetically identical daughter cells due to precise DNA replication and distribution.

• Genome: All the genes and chromosomes of an organism.

• Gene: A segment of DNA that codes for a protein.

• Chromosome: Packaged DNA molecule.

• Chromatin: DNA-protein complex in a supercoiled state.

• Sister chromatids: Identical copies of a chromosome connected at the centromere.

Example: Human somatic cells have 46 chromosomes (23 pairs), while gametes have 23 chromosomes.

Types of Cells and Chromosome Number

• Somatic cells: Nonreproductive cells; diploid (2n), containing two sets of chromosomes.

• Gametes: Reproductive cells (sperm and eggs); haploid (n), containing one set of chromosomes.

• Germ cells: Diploid cells that give rise to gametes.

Karyotype

A karyotype is an organized profile of an individual's chromosomes, used to detect chromosomal abnormalities.

Mechanisms of Eukaryotic Cell Division

• Mitosis: Division of the genetic material in the nucleus, producing genetically identical daughter cells.

• Cytokinesis: Division of the cytoplasm, resulting in two separate cells.

• Meiosis: Specialized division producing gametes with half the chromosome number (genetically unique).

The Cell Cycle: Phases and Regulation

The cell cycle is a series of events that cells go through as they grow and divide. It consists of alternating periods of growth (interphase) and division (mitotic phase).

• Mitotic (M) phase: Includes mitosis and cytokinesis.

• Interphase: Period of cell growth and DNA replication; subdivided into:

  • 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.

Interphase accounts for about 90% of the cell cycle.

Phases of Mitosis

Mitosis is conventionally divided into five phases, followed by cytokinesis:

• Prophase: Chromosomes condense, spindle apparatus begins to form.

• Prometaphase: Nuclear envelope breaks down, spindle fibers attach to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate and move toward opposite poles.

• Telophase: Nuclear envelopes reform, chromosomes decondense.

• Cytokinesis: Division of the cytoplasm, producing two daughter cells.

The Mitotic Spindle

The mitotic spindle is a structure composed of microtubules and proteins that orchestrates chromosome movement during mitosis.

• In animal cells, spindle assembly begins at the centrosome (microtubule organizing center).

• Spindle fibers attach to chromosomes at the kinetochore.

Cytokinesis: Animal vs. Plant Cells

• In animal cells, cytokinesis occurs by cleavage, forming a cleavage furrow that pinches the cell in two.

• In plant cells, a cell plate forms, eventually developing into a new cell wall between daughter cells.

Binary Fission in Prokaryotes

Prokaryotes (bacteria and archaea) reproduce by binary fission, a simpler process than mitosis.

1. Chromosome replication begins at the origin of replication.

2. Two copies of the origin move to opposite ends of the cell.

3. Replication finishes and the cell elongates.

4. The plasma membrane pinches inward, dividing the cell into two.

The Evolution of Mitosis

• Prokaryotes evolved before eukaryotes; mitosis likely evolved from binary fission.

• Certain protists (e.g., dinoflagellates, diatoms, some yeasts) show intermediate forms of cell division.

Regulation of the Eukaryotic Cell Cycle

The frequency of cell division varies by cell type and is tightly regulated by molecular mechanisms.

• Cancer cells can escape normal cell cycle controls, leading to uncontrolled division.

Cytoplasmic Signals and Cell Cycle Control

• The cell cycle is driven by specific signaling molecules in the cytoplasm.

• Experiments show that fusing cells at different stages can induce progression to S or M phase, indicating the presence of regulatory molecules.

Cell Cycle Checkpoints

The cell cycle is regulated at specific checkpoints (G1, G2, M) where progression is halted until conditions are favorable.

• The G1 checkpoint is often the most important; without a go-ahead signal, cells enter a nondividing state called the G0 phase.

• Other checkpoints ensure proper DNA replication and chromosome attachment before division proceeds.

Internal Regulation: Protein Kinases and Cyclins

• Protein kinases: Enzymes that add phosphate groups to proteins, altering their activity (phosphorylation).

• Cyclin-dependent kinases (Cdks): Kinases that must bind to a regulatory protein called cyclin to be active.

• The concentration of cyclins fluctuates during the cell cycle, controlling Cdk activity and cell cycle progression.

External Regulation: Growth Factors and Inhibition

• Growth factors: Proteins released by cells to stimulate division in other cells (e.g., PDGF for fibroblasts).

• Density-dependent inhibition: Crowded cells stop dividing.

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

• Cancer cells lack both density-dependent inhibition and anchorage dependence.

Loss of Cell Cycle Controls in Cancer

• Cancer cells do not respond to normal regulatory signals.

• They may produce their own growth factors, signal without external growth factors, or have abnormal control systems.

Transformation and Tumor Formation

• Transformation: Conversion of a normal cell to a cancerous cell.

• Benign tumors: Abnormal cells remain at the original site.

• Malignant tumors: Invade surrounding tissues and can metastasize (spread to other parts of the body).

Statistical Analysis: The Null Hypothesis and Chi-Square Test

The null hypothesis states there is no significant difference between observed and expected values. The chi-square test is used to evaluate this hypothesis.

• If the calculated chi-square value ($\chi^2$) is greater than the critical value at a given probability (e.g., $p = 0.05$), the null hypothesis is rejected.

Formula:

$\chi^2 = \sum \frac{(O - E)^2}{E}$

Where $O$ = observed value, $E$ = expected value.

Term	Definition
Genome	All genetic material of an organism
Gene	Segment of DNA coding for a protein
Chromosome	Packaged DNA molecule
Chromatin	DNA-protein complex
Sister chromatid	Identical copy of a chromosome joined at the centromere
Centromere	Region where sister chromatids are joined
Mitotic spindle	Microtubule structure that separates chromosomes
Centrosome	Microtubule organizing center
Kinetochore	Protein structure on chromosome where spindle fibers attach
Cleavage furrow	Indentation that begins the process of cytokinesis in animal cells
Cell plate	Structure that forms during cytokinesis in plant cells

Additional info: This guide integrates both textual and visual content from the provided slides and notes, expanding on definitions, mechanisms, and regulatory processes for clarity and exam preparation.