IndietroThe Cell Cycle and Molecular Biology of Cancer: Study Notes
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The Cell Cycle
Overview of Cell Division
Cell division is a fundamental process in biology, enabling reproduction, growth, and repair in organisms. It ensures the continuity of life by accurately passing genetic material from one generation to the next. - Key Point 1: Cell division produces genetically identical daughter cells in most cases. - Key Point 2: Multicellular organisms begin as a single cell that divides repeatedly. - Example: Cell division is essential for embryonic development, tissue renewal, and asexual reproduction. 
Cellular Organization of Genetic Material
The genetic material in cells is organized into chromosomes, which are composed of DNA and associated proteins. - Genome: All the DNA in a cell; prokaryotes typically have a single DNA molecule, while eukaryotes have multiple. - Chromatin: The complex of DNA and protein in eukaryotic chromosomes, which condenses during cell division. - Somatic Cells: Non-reproductive cells with two sets of chromosomes. - Gametes: Reproductive cells (sperm and eggs) with half the chromosome number of somatic cells. 
Chromosome Structure and Replication
Chromosomes are duplicated before cell division, resulting in two sister chromatids joined at the centromere. - Chromatid: Each of the two identical halves of a duplicated chromosome. - Centromere: The region where sister chromatids are most closely attached. - Chromosome Number: Changes during the human life cycle, with somatic cells having two sets and gametes one set. 
Phases of the Cell Cycle
Interphase and M Phase
The cell cycle consists of interphase (cell growth and DNA replication) and the mitotic (M) phase (mitosis and cytokinesis). - Interphase: Divided into G1 (growth), S (DNA synthesis), and G2 (preparation for division). - M Phase: Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division). 
Stages of Mitosis
Mitosis is conventionally divided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase. - Prophase: Chromosomes condense, spindle apparatus forms, nuclear envelope breaks down. - Prometaphase: Microtubules attach to kinetochores, chromosomes begin to move. - Metaphase: Chromosomes align at the metaphase plate. - Anaphase: Sister chromatids separate and move to opposite poles. - Telophase: Nuclear envelope reforms, chromosomes decondense. 
Cytokinesis
Cytokinesis divides the cytoplasm, resulting in two daughter cells. - Animal Cells: Occurs by cleavage, forming a cleavage furrow. - Plant Cells: Occurs by formation of a cell plate.
Binary Fission in Prokaryotes
Prokaryotes reproduce by binary fission, a simpler process than mitosis. - Key Steps: Chromosome replication, movement of daughter chromosomes, division of the cell.
Regulation of the Cell Cycle
Cell Cycle Control System
The cell cycle is regulated by a molecular control system involving checkpoints, cyclins, and cyclin-dependent kinases (Cdks). - Checkpoints: G1, G2, and M checkpoints ensure proper progression and integrity. - Cyclins: Proteins with cyclically fluctuating concentrations. - Cdk: Cyclin-dependent kinases, active only when bound to cyclin. - MPF: Maturation-promoting factor, a cyclin-Cdk complex that triggers entry into M phase. 
Internal and External Signals
- Internal Signals: Ensure chromosomes are properly attached before anaphase. - External Signals: Growth factors stimulate cell division; platelet-derived growth factor (PDGF) is an example.
Loss of Cell Cycle Controls in Cancer
Cancer cells escape normal cell cycle controls, leading to uncontrolled division. - Transformation: Cells acquire the ability to divide indefinitely. - Tumors: Benign (localized) or malignant (invasive and metastatic).
Molecular Biology of Cancer
Proto-oncogenes and Oncogenes
- Proto-oncogenes: Genes that stimulate normal cell growth and division. - Oncogenes: Mutated proto-oncogenes that lead to excessive cell division. - Genetic Changes: Mutations, gene amplification, and translocation can convert proto-oncogenes to oncogenes.
Tumor-Suppressor Genes
- Function: Inhibit cell division, repair DNA, control cell adhesion, and act in cell-signaling pathways. - Mutations: Loss of function can contribute to cancer onset.
Key Genes in Cancer
- Ras Proto-oncogene: Mutations lead to hyperactive Ras protein and increased cell division. - p53 Tumor Suppressor: Mutations prevent suppression of the cell cycle, allowing cells with DNA damage to proliferate.
Multistep Model of Cancer Development
Multiple mutations are required for cancer development, often involving both oncogenes and tumor-suppressor genes. - Inherited Predisposition: Individuals can inherit mutant alleles, increasing cancer risk (e.g., BRCA1/BRCA2 in breast cancer).
Summary Table: Cell Cycle and Cancer Genes
Component | Function | Role in Cancer |
|---|---|---|
Proto-oncogene | Stimulates cell growth | Mutation leads to oncogene, causing excessive division |
Tumor-suppressor gene | Inhibits cell division, repairs DNA | Mutation leads to loss of inhibition, allowing uncontrolled growth |
Cyclin/Cdk | Regulates cell cycle progression | Dysregulation can contribute to abnormal division |
Checkpoints | Ensure proper cell cycle progression | Failure allows cells with errors to divide |
Key Equations and Concepts
Chromosome Replication
Cell Cycle Phases
MPF Activation
Summary
Understanding the cell cycle and its regulation is crucial for comprehending how cells divide, how genetic material is faithfully transmitted, and how disruptions in these processes can lead to cancer. Additional info: Academic context was added to clarify the molecular mechanisms and genetic aspects of cancer, as well as to provide a self-contained summary suitable for exam preparation.
