The Cell Cycle

Overview of Cell Division

Cell division is a fundamental process that enables organisms to reproduce, grow, and repair tissues. In multicellular organisms, it starts from a single cell that divides to form two genetically identical daughter cells. - Cell division is essential for reproduction, growth, and tissue repair. - Genetic material is accurately distributed to daughter cells during division.

Cellular Organization of Genetic Material

The genetic material in cells is organized as DNA, which is packaged into chromosomes. - Genome: All DNA in a cell; can be a single molecule (prokaryotes) or multiple molecules (eukaryotes). - Chromatin: DNA-protein complex that condenses into chromosomes during cell division. - Somatic cells: Nonreproductive cells with two sets of chromosomes. - Gametes: Reproductive cells with half the chromosome number of somatic cells.

Chromosome Structure and Replication

Chromosomes are duplicated before cell division, forming two sister chromatids joined at the centromere. - Sister chromatids: Identical copies of a chromosome, joined by cohesin proteins. - Centromere: Region where chromatids are most closely attached.

Phases of the Cell Cycle

Interphase

Interphase is the period of cell growth and DNA replication, comprising about 90% of the cell cycle. It is divided into three phases: - G1 phase: Cell grows and carries out normal functions. - S phase: DNA is replicated. - G2 phase: Cell prepares for division; chromosomes condense, centrioles replicate, and tubulin is synthesized.

M Phase: Mitosis and Cytokinesis

The M phase includes mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm). Mitosis is conventionally divided into five stages: 1. Prophase: Chromosomes condense, spindle apparatus forms, nuclear envelope breaks down. 2. Prometaphase: Microtubules attach to kinetochores, chromosomes begin moving to the cell center. 3. Metaphase: Chromosomes align at the metaphase plate. 4. Anaphase: Sister chromatids separate and move to opposite poles. 5. Telophase: Spindle disassembles, nuclear envelope reforms, chromosomes uncoil. Cytokinesis differs in animals (cleavage furrow) and plants (cell plate formation).

Control of the Cell Cycle

Checkpoints and Regulation

The cell cycle is regulated by checkpoints that ensure proper division. - G1 checkpoint: Determines if the cell will proceed to division or enter G0 (non-dividing state). - G2 checkpoint: Ensures DNA is replicated and undamaged. - M checkpoint: Ensures all chromatids are properly attached to spindle microtubules. - Cyclins and Cyclin-dependent kinases (Cdks): Regulatory proteins that control cell cycle progression. - MPF (Maturation-promoting factor): Cyclin-Cdk complex that triggers entry into M phase.

Molecular Biology of Cancer

Genetic Changes Leading to Cancer

Cancer results from mutations that disrupt normal cell cycle control. - Proto-oncogenes: Genes that stimulate normal cell growth; mutations convert them to oncogenes (promote excessive division). - Tumor-suppressor genes: Inhibit cell division; mutations reduce their function, contributing to cancer. - Ras proto-oncogene: Mutations lead to hyperactive Ras protein and increased cell division. - p53 tumor suppressor gene: Mutations prevent suppression of cell cycle, allowing damaged cells to proliferate. - Multiple mutations are typically required for cancer development; inherited mutations (e.g., BRCA1/2) can predispose individuals to certain cancers.

Summary Table: Key Terms in Cell Cycle and Cancer

Concept Map: Cell Cycle and Cancer

Equations and Formulas

DNA Replication and Chromosome Number

- If a cell has n chromosomes, after DNA replication it will have 2n chromatids. - After mitosis, each daughter cell receives n chromosomes.

Additional info:

- The cell cycle is tightly regulated to prevent uncontrolled cell division, which can lead to cancer. - Understanding the molecular basis of cell cycle control is essential for developing targeted cancer therapies.