The Cell Cycle and Cell Division: Regulation, Mechanisms, and Implications

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The Cell Cycle and Cell Division

Key Roles of Cell Division

Cell division is fundamental to the life cycle of all organisms, enabling reproduction, growth, and regeneration. It ensures the continuity of life by producing new cells from existing ones.

Reproduction: Single-celled organisms reproduce by cell division; multicellular organisms use cell division for gamete production.
Growth: Cell division increases the number of cells, contributing to organismal growth.
Regeneration: Damaged tissues are repaired through cell division.
Key Events: Cell division signals, DNA replication, DNA segregation, and cytokinesis.

Cell division roles: reproduction, growth, regeneration

Cell Division in Prokaryotes

Prokaryotic cell division occurs via binary fission, a process that results in the reproduction of the entire organism. Division is typically triggered by external signals such as nutrient availability.

Chromosome Structure: Most prokaryotes have a single, circular DNA molecule.
Replication Origins: ori (origin of replication) and ter (terminus).
Segregation: After replication, ori regions move to opposite ends, segregating daughter chromosomes.
Cytokinesis: Cell membrane pinches in, forming a ring of protein fibers; new cell wall materials are synthesized.

Binary fission in a bacterium

Binary Fission Steps

DNA replication begins at ori.
Chromosomal DNA moves through the replication complex.
Segregation of daughter chromosomes.
Formation of Z ring and cytokinesis.

Segregation and cytokinesis in prokaryotes

Cell Division in Eukaryotes

Eukaryotic cell division is more complex, regulated by internal and external signals, and involves multiple chromosomes. Division is often linked to the needs of the organism.

DNA Replication: Occurs at multiple origins; restricted to a specific cell cycle phase.
DNA Segregation: Mitosis separates replicated chromosomes into two nuclei.
Cytokinesis: Differs between animal (membrane pinching) and plant cells (cell plate formation).

Regulation of the Eukaryotic Cell Cycle

Phases of the Cell Cycle

The eukaryotic cell cycle consists of interphase (G1, S, G2) and M phase (mitosis and cytokinesis). Interphase is when the cell grows and DNA is replicated; M phase is when nuclear and cytoplasmic division occur.

G1: Cell grows; chromosomes are unreplicated .
S: DNA synthesis; chromosomes replicate.
G2: Cell prepares for mitosis.
M: Mitosis and cytokinesis.

Eukaryotic cell cycle phases

Cell Cycle Regulation

Progression through the cell cycle is controlled by cyclin-dependent kinases (CDKs) , which are activated by binding to cyclins. CDKs phosphorylate target proteins, regulating cell cycle events.

CDK Activation: Cyclin binding exposes CDK's active site.
Phosphorylation: CDKs transfer phosphate groups from ATP to proteins, altering their function.
Checkpoints: Cyclin-CDK complexes act at specific cell cycle checkpoints.

Cyclin binding activates CDK

Restriction Point (R)

The G1-to-S transition is a critical control point. The retinoblastoma protein (RB) inhibits cell cycle progression; phosphorylation by cyclin-CDK inactivates RB, allowing the cycle to proceed.

Phosphorylation of RB protein controls cell cycle progression

Cyclin Regulation

Cyclins are synthesized and degraded cyclically, ensuring CDK activity is tightly regulated.

Cyclins are transient in the cell cycle

External Regulation

Growth factors and other external signals can stimulate cell division by activating signal transduction pathways that lead to cyclin synthesis and CDK activation.

Platelet-derived growth factor: Stimulates skin cell division for wound healing.
Interleukins and erythropoietin: Stimulate blood cell division and specialization.

Eukaryotic Cell Division: Mitosis

Chromatin Structure and Chromosome Packing

DNA is bound to histone proteins, forming chromatin. During mitosis, chromatin is highly compacted, making chromosomes inaccessible to replication and transcription factors.

Nucleosomes: Beadlike units formed by DNA wrapping around histones.
Condensins: Proteins that further compact DNA during mitosis.

DNA packing into mitotic chromosome

Phases of Mitosis

Mitosis is subdivided into prophase, prometaphase, metaphase, anaphase, and telophase. It produces two genetically identical nuclei.

Prophase: Chromosomes condense; spindle forms.
Prometaphase: Nuclear envelope breaks down; kinetochores form.
Metaphase: Chromosomes align at metaphase plate.
Anaphase: Sister chromatids separate.
Telophase: Nuclear envelopes reform.

Phases of mitosis in an animal cell

Mitotic Spindle and Chromosome Movement

The spindle apparatus, composed of microtubules, moves sister chromatids apart. Centrosomes, containing centrioles, determine the division plane.

Mitotic spindle consists of microtubules

Chromatid Separation

During anaphase, the anaphase-promoting complex (APC) is activated, leading to the hydrolysis of cohesin by separase and separation of chromatids.

Chromatid attachment and separation

Cytokinesis

Cytokinesis divides the cytoplasm. In animal cells, a contractile ring pinches the cell in two; in plant cells, vesicles form a cell plate that becomes the new cell wall.

- Cytokinesis differs in animal and plant cells

Cell Death: Necrosis and Apoptosis

Necrosis

Necrosis is accidental cell death due to damage or starvation. The cell swells, bursts, and releases contents, causing inflammation.

Apoptosis

Apoptosis is programmed cell death, essential for development and preventing cancer. Cells detach, chromatin is digested, and the cell forms blebs that are ingested by neighbouring cells.

Initiation: Triggered by hormones, growth factors, toxins, or DNA damage.
Caspases: Proteases that hydrolyze membrane proteins and nucleosomes.

F Apoptosis: programmed cell death Apoptosis: caspase activation and cell breakdown

Unregulated Cell Division and Cancer

Cancer Cell Characteristics

Cancer cells lose control over division, often dividing continuously and forming tumors. They may migrate (metastasize) to other parts of the body.

Benign Tumors: Localized, slow-growing, resemble parent tissue.
Malignant Tumors: Irregular, invasive, do not resemble parent tissue.

Cancer cell with normal neighbors

Regulation and Molecular Changes in Cancer

Oncogenes: Mutated positive regulators, overactive or in excess (e.g., HER2 in breast cancer).
Tumor Suppressors: Negative regulators (e.g., RB, p53) are inactive in cancer cells.
Multiple Mutations: Tumors often involve several oncogenes and tumor suppressor genes (e.g., Myc and Ras).

Molecular changes in cancer cells: HER2 and RB Multiple events trigger cancer cell cycle

Cancer Treatments

Treatments target cell cycle processes, including drugs that block DNA replication, spindle function, or growth factor stimulation. Radiation induces DNA damage and apoptosis.

5-fluorouracil: Blocks thymine synthesis.
Paclitaxel: Disrupts mitotic spindle.
Trastuzumab: Targets HER2 receptor in breast cancer.

Cancer treatment and the cell cycle

Summary Table: Comparison of Cell Division in Prokaryotes and Eukaryotes

Feature	Prokaryotes	Eukaryotes
Division Process	Binary fission	Mitosis (and meiosis)
Chromosome Structure	Single, circular	Multiple, linear
Division Signals	External (nutrients, environment)	Internal & external (growth factors, organismal needs)
DNA Replication	Single origin	Multiple origins
Segregation	Simple, via ori movement	Complex, via mitotic spindle
Cytokinesis	Protein ring, cell wall formation	Contractile ring (animals), cell plate (plants)

Key Terms

Binary fission: Prokaryotic cell division.
Mitosis: Eukaryotic nuclear division.
Cytokinesis: Division of cytoplasm.
CDK: Cyclin-dependent kinase, cell cycle regulator.
Oncogene: Mutated gene promoting cancer.
Tumor suppressor: Gene inhibiting cell division.
Apoptosis: Programmed cell death.