Cell Cycle, DNA Replication, and Mitosis: Structured Study Notes

Cell Cycle and Cell Division

Overview of the Eukaryotic Cell Cycle

The eukaryotic cell cycle is a series of events that cells go through as they grow and divide. It ensures the accurate replication and segregation of genetic material, followed by division of the cytoplasm.

• G1 Phase: Normal cellular function and growth.

• S Phase: DNA replication occurs, resulting in two sister chromatids per chromosome.

• G2 Phase: Preparation for mitosis, including synthesis of microtubules.

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

Interphase includes G1, S, and G2 phases, during which the cell grows and prepares for division.

DNA Replication

DNA replication is the process by which a cell duplicates its DNA, ensuring that each daughter cell receives an identical set of chromosomes.

• Occurs only when a cell needs to divide.

• DNA replication is independent of transcription and translation.

• Replication takes place during the S phase of the cell cycle.

• Results in the formation of two sister chromatids for each chromosome.

Example: In eukaryotic cells, DNA replication is tightly regulated and occurs before mitosis.

Cell Theory and Requirements for New Cells

Cell theory states that all organisms are made of cells, all cells arise from preexisting cells, and the cell is the basic unit of life.

• To create a new cell, a pre-existing cell must provide:

  • DNA (nucleus/nucleoid)

  • Mitochondria

  • Ribosomes

  • Endoplasmic Reticulum (ER)

  • Lysosomes

  • Cytoplasm

  • Cell membrane

  • Chloroplasts (in plants)

  • Golgi apparatus

  • Centrosomes

Example: During cell division, organelles and cytoplasmic contents are distributed to daughter cells.

Mechanisms of Cell Division

Cells divide by different mechanisms depending on their type.

• Prokaryotic cells: Divide by binary fission, which includes DNA replication, segregation, and cytokinesis.

• Eukaryotic cells: Divide by mitosis (for growth, repair, and asexual reproduction) or meiosis (for sexual reproduction), followed by cytokinesis.

Example: Binary fission in bacteria is a rapid and constant process, while mitosis in eukaryotes is more complex and regulated.

Mitosis and Chromosome Segregation

Steps of Mitosis

Mitosis is the process by which a eukaryotic cell divides its nucleus and distributes chromosomes equally to two daughter cells.

• Prophase: Chromatin condenses into visible chromosomes; spindle apparatus begins to form.

• Metaphase: Chromosomes align at the cell's equatorial plate.

• Anaphase: Sister chromatids are pulled apart to opposite poles by the spindle fibers.

• Telophase: Nuclear envelopes reform around the separated chromosomes.

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

Example: In animal cells, cytokinesis occurs via cleavage furrow formation; in plant cells, a cell plate forms.

Chromatin and Chromosome Structure

DNA is organized into chromatin, which condenses into chromosomes during cell division.

• Chromatin: DNA-protein complex that is less condensed during interphase.

• Condensed chromosomes: Highly compacted structures visible during mitosis.

Example: Chromatin undergoes several levels of packing, from nucleosomes to fully condensed chromosomes.

Spindle Apparatus and Chromatid Separation

The spindle apparatus is a microtubule-based structure that ensures accurate segregation of sister chromatids.

• Centrosome: Microtubule organizing center containing two pairs of centrioles.

• Spindle fibers: Formed in G2 phase, attach to kinetochores on chromosomes.

• Kinetochore: Protein complex at the centromere, site of spindle attachment.

Example: Spindle fibers pull sister chromatids apart during anaphase.

Cell Cycle Regulation and Checkpoints

Cell Cycle Checkpoints

Checkpoints are control mechanisms that ensure the cell cycle progresses only when certain conditions are met.

• G1 Checkpoint: Checks for cell size, nutrients, and DNA integrity.

• G2 Checkpoint: Ensures DNA replication is complete and checks for DNA damage.

• M Checkpoint: Verifies that all chromosomes are properly attached to the spindle before anaphase.

Example: If DNA damage is detected, the cell cycle may pause for repair or trigger apoptosis.

Cyclin-Dependent Kinases (CDKs) and Cyclins

Cyclin-dependent kinases (CDKs) are enzymes that, when bound to cyclins, regulate progression through the cell cycle.

• CDKs: Phosphorylate target proteins to drive cell cycle transitions.

• Cyclins: Regulatory proteins whose levels fluctuate during the cell cycle.

• CDK-cyclin complexes: Promote entry into mitosis and other phases.

Example: The rise and fall of cyclin levels ensure that CDKs are active only at appropriate times.

Mitosis and Cancer

Relationship Between Mitosis and Cancer

Cancer is characterized by uncontrolled cell division, often resulting from mutations in genes regulating the cell cycle.

• Loss of checkpoint control: Cells divide without proper regulation.

• Mutations in CDKs or cyclins: Can lead to unregulated progression through the cell cycle.

• Oncogenes and tumor suppressors: Genes that, when mutated, contribute to cancer development.

Example: Defective p53 protein fails to halt the cell cycle in response to DNA damage, promoting tumor formation.

Table: Comparison of Prokaryotic and Eukaryotic Cell Division

Feature	Prokaryotic Cells	Eukaryotic Cells
Division Mechanism	Binary Fission	Mitosis or Meiosis
DNA Segregation	Simple, single chromosome	Complex, multiple chromosomes
Cytokinesis	Occurs after DNA segregation	Occurs after nuclear division
Cell Cycle Phases	No distinct phases	G1, S, G2, M

Additional info:

• These notes expand on the provided lecture slides and activities, offering definitions, examples, and structured explanations suitable for introductory cell biology or genetics, not organic chemistry.