The Cell Cycle and Cell Division

The Cell Cycle and Cell Division

Key Concepts

  • Different Life Cycles Use Different Modes of Cell Reproduction

  • Both Binary Fission and Mitosis Produce Genetically Identical Cells

  • Cell Reproduction Is Under Precise Control

Opening Question

  • How does infection with HPV result in uncontrolled cell reproduction?

Different Life Cycles Use Different Modes of Cell Reproduction

  • The lifespan of an organism is linked to cell reproduction, also called cell division.

  • Organisms primarily have two basic strategies for reproducing themselves:   - Asexual Reproduction   - Sexual Reproduction

  • Cell division is important for:   - Growth   - Repair of tissues

The Importance of Cell Division

Part 1: Reproduction

  • Fundamental for any organism's lifecycle.

Part 2: Growth

  • Necessary for increasing organism size through the production of new cells.

Part 3: Regeneration

  • Important for tissue repair after injury.

Asexual Reproduction

  • Offspring produced via asexual reproduction are clones, genetically identical to the parent organism.

  • Any genetic variations arise from mutations.

  • Unicellular prokaryotes reproduce by binary fission, while single-cell eukaryotes use mitosis.

  • Other eukaryotes can reproduce through both asexual and sexual means.

DNA in Eukaryotic Cells

  • In eukaryotic cells, DNA is organized into chromosomes.   - A chromosome consists of a single molecule of DNA and histone proteins (forming nucleosomes).   - Somatic Cells: Body cells not specialized for reproduction (e.g., not eggs or sperm) contain two sets of chromosomes (homologues).

Review: Chromosomes

  • Chromosome: Threadlike structure within the nucleus containing genetic information (DNA) passed to daughter cells.

  • Each chromosome consists of two identical sister chromatids joined at a centromere.

Key Terms in Cell Division

  • Genome: Cell's genetic information.

  • Chromosomes: Structures made of DNA molecules and proteins.

  • Chromatin: DNA-protein complex.

  • Chromatids: Replicated strands of a chromosome.

  • Centromere: Narrowing waist of sister chromatids.

  • Mitosis: Process of nuclear division.

  • Cytokinesis: Division of cytoplasm.

Cell Division Events

  1. Reproductive signals initiate cell division.

  2. Replication of DNA occurs.

  3. Segregation of DNA into two new cells takes place.

  4. Cytokinesis divides the cytoplasm and separates the two new cells.

Prokaryotic Cell Division

  • In prokaryotes, cell division leads to the reproduction of the entire organism.

  • Steps in Binary Fission:   - Cell grows in size.   - DNA replication occurs.   - DNA and cytoplasm separate into two cells through binary fission.

Prokaryotic Chromosome Structure

  • Most prokaryotes have one chromosome, consisting of a single circular DNA molecule (also known as plasmid).

  • Important regions:   - ori: Origin of replication.   - ter: Termination of replication.

DNA Replication in Prokaryotes

  • Replication begins at the ori site and proceeds toward the ter site, threaded through a replication complex.

  • The ori complexes move to opposite ends of the cell, with proteins hydrolyzing ATP to aid in segregation.

Eukaryotic Cell Division Process

  • Eukaryotic cells divide by mitosis followed by cytokinesis.

  • Replication occurs in specific stages of the cell cycle.

  • DNA becomes highly condensed into chromatids for segregation during mitosis.

The Cell Cycle Duration

  • The cell cycle: the period during which a cell grows, prepares for division, and divides to form two genetically identical daughter cells.

  • Interphase: Long phase where cellular activities occur:   - G1 Phase: Cell carries out specialized functions and prepares for division.   - S Phase: DNA replication occurs, creating identical copies of each chromosome.   - G2 Phase: Final preparations for mitosis occur, including cell growth and organelle production.

  • M Phase (Mitotic Phase): Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Phases of the Cell Cycle

  1. G1 Phase (Gap 1): Cell growth and production of organelles; 90% of cell cycle time spent here.

  2. S Phase (Synthesis): DNA replicates, forming attached copies (chromatids).

  3. G2 Phase (Gap 2): Preparation for mitosis, final growth phase, and beginning of DNA condensation.

  4. M Phase: Includes prophase, metaphase, anaphase, telophase, and cytokinesis.

The Process of Mitosis

  • Mitosis is a continuous process but can be divided into distinct phases:   - Prophase: Chromatin condenses, spindle forms, and nuclear envelope dissolves.   - Prometaphase: Chromosomes attach to spindle microtubules; nuclear envelope fully breaks down.   - Metaphase: Chromosomes line up at the metaphase plate; centrosomes at opposite poles.   - Anaphase: Sister chromatids separate and move to opposite poles; controlled by spindle microtubules.   - Telophase: Nuclear membranes reform around the separated chromosomes, and the spindle apparatus breaks down.

Cytokinesis Details

  • Occurs after mitosis and involves division of the cytoplasm:   - Animal Cells: Inward pinching of plasma membrane forming a cleavage furrow due to actin and myosin.   - Plant Cells: Form a cell plate that develops into new cell walls through vesicle fusion.

Regulation of Cell Division

  • Cyclins: Proteins regulating timing of cell cycle.   - Internal Regulators: Respond to internal cell events; allow progression based on internal conditions.   - External Regulators: Respond to environmental signals, such as growth factors, to regulate cell cycle speed.

Cell Cycle Phases Summary

  • The eukaryotic cell cycle has four stages, each tightly regulated:   - G1 Phase: Restriction point (R), where a cell may exit to a non-dividing state (G0) or proceed to S Phase.   - Restriction Point: Determines if a cell will divide or enter G0.

Consequences of Unregulated Cell Division

  • Uncontrolled cell division can lead to cancer.

  • Cancer cells often lack regulation and invade surrounding tissues, leading to tumors.

  • Important genes involved in regulating cell division include the p53 gene, which can control apoptosis and cell cycle checkpoints.

Programmed Cell Death (Apoptosis)

  • Two forms of cell death:   - Necrosis: Uncontrolled cell death due to damage, leading to inflammation.   - Apoptosis: Genetically programmed cell death essential for normal development and cellular homeostasis.

  • Apoptosis is controlled by signaling pathways that can recognize damage and regulate cellular function and survival, often involving s, caspase which are enzymes that hydrolyze target proteins.

Events of Apoptosis

  1. Cell detaches from neighbors.

  2. Cut chromatin into nucleosomes.

  3. Forms membrane blebs that fragment.

  4. Surrounding cells ingest the remains of dead cells.

Conclusion

  • Cell division is a vital biological process regulated by complex mechanisms, ensuring organisms develop correctly and repair effectively while preventing uncontrolled growth, reminiscent of cancerous behavior.