The Cell Cycle: Structure, Regulation, and Division

Cell Cycle and Cell Division

Overview of the Cell Cycle

The cell cycle is the series of events that cells undergo to grow and divide. It is essential for growth, development, tissue repair, and reproduction in multicellular organisms. The cycle consists of two main phases: interphase and the mitotic (M) phase.

• Interphase: Period of cell growth and DNA replication, divided into G1 (first gap), S (synthesis), and G2 (second gap) phases.

• Mitotic Phase: Includes mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm).

• Cell Division: Produces two genetically identical daughter cells, ensuring each receives an exact copy of the parent cell's DNA.

Key Concepts in Cell Division

• Genetic Consistency: Most cell division results in genetically identical daughter cells.

• Alternation of Phases: The mitotic phase alternates with interphase.

• Molecular Control: The eukaryotic cell cycle is regulated by a molecular control system involving cyclins and cyclin-dependent kinases (Cdks).

Cellular Organization of Genetic Material

Chromosomes and Chromatin

Genetic material is organized into chromosomes, which are structures made of DNA and proteins. The combination of DNA and proteins is called chromatin.

• Chromosome: Structure containing genetic information.

• Chromatid: Each of the two identical halves of a duplicated chromosome.

• Chromatin: Material that makes up chromosomes, consisting of DNA and proteins.

• Genome: The complete set of genetic material in a cell.

• Centromere: Region where sister chromatids are most closely attached.

• Arm: Portion of a chromatid on either side of the centromere.

Eukaryotic species have a characteristic number of chromosomes. For example, human somatic cells have 46 chromosomes (two sets of 23), while gametes have 23.

Distribution of Chromosomes During Cell Division

• Chromosomes exist as long, thin chromatin fibers when not dividing.

• After DNA replication, chromosomes condense and become visible.

• Each duplicated chromosome consists of two sister chromatids joined by cohesins.

• During cell division, sister chromatids separate and become individual chromosomes.

• Mitosis is followed by cytokinesis, resulting in two genetically identical daughter cells.

• Meiosis produces gametes with half the chromosome number.

Phases of the Cell Cycle

Interphase

Interphase is divided into three stages:

• G1 Phase: Cell growth and normal functions.

• S Phase: DNA replication.

• G2 Phase: Preparation for mitosis.

Mitotic Phase

The mitotic phase includes mitosis and cytokinesis. Mitosis is divided into five stages:

• Prophase: Chromatin condenses, nucleoli disappear, spindle forms, centrosomes move apart.

• Prometaphase: Nuclear envelope fragments, kinetochores form, microtubules attach to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Cohesins are cleaved, sister chromatids separate and move to opposite poles.

• Telophase: Daughter nuclei form, chromosomes decondense, spindle disassembles.

Cytokinesis divides the cytoplasm, forming two daughter cells. In animal cells, this involves a cleavage furrow; in plant cells, a cell plate forms.

The Mitotic Spindle and Chromosome Movement

Mitotic Spindle Structure

The mitotic spindle is composed of microtubules and associated proteins, forming during prophase. It is essential for chromosome movement and segregation.

• Microtubules: Elongate by polymerizing tubulin subunits; shorten by depolymerizing.

• Centrosome: Microtubule organizing center; duplicates during interphase.

• Aster: Radial array of short microtubules from each centrosome.

• Kinetochore: Protein structure at the centromere; attaches to spindle microtubules.

Chromosome Movement During Anaphase

• Kinetochore microtubules shorten at the kinetochore end, moving chromosomes toward spindle poles.

• Nonkinetochore microtubules elongate, pushing spindle poles apart and elongating the cell.

• Motor proteins facilitate chromosome movement via "Pacman" and "reel-in" mechanisms.

Cytokinesis: Animal vs. Plant Cells

Animal Cell Cytokinesis

Occurs via cleavage:

• Formation of a cleavage furrow near the metaphase plate.

• Contractile ring of actin microfilaments and myosin contracts, pinching the cell in two.

Plant Cell Cytokinesis

Occurs via cell plate formation:

• Vesicles from the Golgi apparatus coalesce at the cell center, forming a cell plate.

• Cell plate grows and fuses with the plasma membrane.

• A new cell wall forms, separating the daughter cells.

Binary Fission in Bacteria

Process of Binary Fission

Binary fission is the method by which prokaryotes reproduce, involving chromosome replication and cell division.

• Bacterial chromosomes are circular DNA molecules.

• Replication begins at the origin of replication, producing two origins.

• Origins move to opposite ends as replication continues.

• Cell elongates, plasma membrane pinches inward, and two daughter cells form.

Steps of Bacterial Cell Division

1. Chromosome replication begins; one origin moves to the other end.

2. Replication continues; cell elongates.

3. Replication finishes; membrane pinches inward; new cell wall forms.

4. Two daughter cells result, each with a complete genome.

Proteins resembling actin and tubulin play roles in chromosome movement and cell division.

Evolution of Mitosis

Origins and Intermediate Stages

Mitosis likely evolved from simpler prokaryotic mechanisms. Certain unicellular eukaryotes exhibit intermediate nuclear division processes.

• Dinoflagellates: Chromosomes attach to the nuclear envelope, which remains intact; microtubules pass through the nucleus.

• Diatoms and Yeasts: Nuclear envelope remains intact; spindle forms within the nucleus.

• Most Eukaryotes: Spindle forms outside the nucleus; nuclear envelope breaks down during mitosis.

Regulation of the Cell Cycle

Cell Cycle Control System

The cell cycle is regulated by signaling molecules in the cytoplasm, including cyclins and cyclin-dependent kinases (Cdks).

• Cyclins: Proteins whose concentrations fluctuate cyclically.

• Cyclin-Dependent Kinases (Cdks): Enzymes that activate/inactivate proteins by phosphorylation.

• MPF (Maturation-Promoting Factor): Cyclin-Cdk complex that triggers mitosis.

MPF activity peaks during metaphase and is regulated by cyclin concentration.

Cell Cycle Checkpoints

• G1 Checkpoint: Ensures cell is ready for DNA synthesis; most crucial.

• G2 Checkpoint: Ensures cell is ready to enter mitosis.

• M Checkpoint: Ensures all chromosomes are properly attached to spindle fibers before anaphase.

If a cell does not receive a go-ahead signal at the G1 checkpoint, it may enter the non-dividing G0 phase.

External and Internal Signals

• Internal signals: Report completion of crucial processes.

• External signals: Growth factors (e.g., PDGF) stimulate cell division.

• Density-dependent inhibition: Crowded cells stop dividing.

• Anchorage dependence: Cells must be attached to a substratum to divide.

Loss of Cell Cycle Controls in Cancer Cells

Cancer Cell Characteristics

• Cancer cells ignore normal cell cycle signals and continue dividing.

• They may produce their own growth factors or have mutations in cell cycle control genes.

• Do not exhibit density-dependent inhibition or anchorage dependence.

• Can divide indefinitely in culture (e.g., HeLa cells).

• Evade apoptosis (programmed cell death).

• Form tumors: Benign (localized) or malignant (spread and impair organ function).

• Metastasis: Malignant tumors spread to other body parts via blood and lymph.

• May have abnormal chromosome numbers, altered metabolism, and lose cell attachments.

• Promote angiogenesis (blood vessel growth toward tumor).

• Treatment: Radiation for localized tumors; chemotherapy for metastatic tumors.

Scientific Skills Exercise: Cell Cycle Inhibition

Histograms and Cell Cycle Analysis

Histograms compare DNA content of cells in control and treated samples, indicating the phase where inhibitors act.

• Control Sample: Peaks represent G1 (low DNA), S (DNA synthesis), and G2 (high DNA).

• Treated Sample: Accumulation in G1 phase, decrease in S phase, similar G2 phase.

• Interpretation: Inhibitor blocks cell cycle at G1, likely by down regulating cyclin D1.

Personalized Cancer Treatment

Advances in cell-signaling and molecular techniques allow for targeted therapies (e.g., HER2, estrogen receptors), improving survival and reducing recurrence.

Summary Table: Cell Cycle Phases and Key Features

Phase	Main Events	Key Molecules
G1	Cell growth, normal functions	Cyclin D, Cdk
S	DNA replication	Cyclin E, Cdk
G2	Preparation for mitosis	Cyclin A, Cdk
M (Mitosis)	Nuclear division, chromosome segregation	MPF (Cyclin B-Cdk)
Cytokinesis	Cytoplasmic division	Actin, myosin

Key Equations and Concepts

• DNA Content: If a cell has n chromosomes, after DNA replication it has 2n chromatids.

• MPF Activity: MPF triggers mitosis when cyclin concentration is high.

Equation for MPF activity:

$\text{MPF Activity} \propto \text{Cyclin Concentration} \times \text{Cdk Activity}$

Equation for DNA content during cell cycle:

$\text{DNA Content (G1)} = 2n$ $\text{DNA Content (G2)} = 4n$

Review and Applications

• Cell division is essential for reproduction, growth, and repair.

• Genetic material is organized into chromosomes, duplicated before division.

• Cell cycle regulation is crucial for normal function; loss of control leads to cancer.

• Binary fission is the prokaryotic method of cell division; mitosis evolved from simpler mechanisms.

• Personalized medicine uses molecular knowledge to target cancer treatments.

Example: HeLa cells are immortal cancer cells used in research, illustrating loss of cell cycle control.

Example: Platelet-derived growth factor (PDGF) stimulates fibroblast division for wound healing.

Additional info: The notes expand on the original content by providing definitions, examples, and equations for clarity and completeness.