Cell Cycle

CELL CYCLE Key Knowledge: • • The eukaryotic cell cycle, including the characteristics of each of the sub-phases of mitosis and cytokinesis in plant and animal cells Disruption to the regulation of the cell cycle and malfunctions in apoptosis that may result in deviant cell behaviour: cancer and the characteristics of cancer cells CELL CYCLE The cell cycle is an ordered set of events which results in the division of eukaryotic cells into identical daughter cells. It involves two key phases: Interphase is an active phase of the cycle with many processes occurring in the nucleus and cytoplasm. It is a continuum of three distinct stages: • • • G1: Cell grows and prepares for division (organelles are duplicated) S: A synthesis stage in which DNA replication occurs in the nucleus G2: Acts as a final growth and preparation stage prior to cell division The M phase is the period of the cycle in which the cell and its contents are divided into two separate daughter cells. It involves two key stages: • • Mitosis: A process of nuclear division (duplicated DNA is separated) Cytokinesis: A process of cytoplasmic division (the cell splits in two) A T C G1 Growth + metabolism M P Preparation Replication + growth of DNA G2 SS The cell cycle moves in a clockwise direction as per the diagram shown above CHROMOSOMES Within eukaryotic cells, the DNA is organised into discrete units that are called chromosomes. Normally, these chromosomes are packaged in an uncondensed form called chromatin to allow for the cellular machinery to access the genetic instructions that determine cell activity. However, when a cell divides, the DNA supercoils and the chromatin condenses to form discrete chromosomes that will be visible under a light microscope (after division, chromosomes decondense and reform into chromatin). DNA REPLICATION When DNA is replicated during the S phase, chromosomes will temporarily consist of two identical DNA molecules. These strands are called sister chromatids and are held together at a site called a centromere. When chromatids separate during mitosis, they become two identical chromosomes (one for each cell). S phase Mitosis DNA replication (S phase) produces chromosomes with sister chromatids These will separate during mitosis into two genetically identical chromosomes MITOSIS Mitosis is the process of nuclear division, whereby the duplicated DNA molecules (i.e. sister chromatids) are arranged into two separate nuclei. Mitosis is preceded by interphase and involves four distinct stages: • • • • Prophase: Chromosomes condense, nucleus dissolves and paired centrosomes move to opposite poles Metaphase: Centrosomes connect to centromeres via spindle fibres and move chromosomes to middle Anaphase: Spindle fibres contract and separate sister chromatids, moving them to the opposite poles Telophase: Chromosomes decondense and nuclei reform around the two separated chromosome sets INTERPHASE PROPHASE METAPHASE ANAPHASE TELOPHASE CYTOKINESIS Mitosis only separates the duplicated DNA into separate nuclei. The division of the cell in two (cytokinesis) occurs concurrently with the final stage of mitosis (telophase). Cytokinesis differs in animal and plant cells: • • Animals: Microtubule fibres form an outer contractile ring (cleavage furrow) – division is centripetal Plants: Vesicles form at the centre of the cell and fuse to form an end plate – division is centrifugal CANCER Errors in the cell cycle can lead to uncontrolled cell division (cancer). The abnormal cell growths are called tumours and can occur in any tissue or organ. Some tumours are benign and will remain in their original location, while other tumours are malignant and will spread to invade neighbouring tissue (metastasis). Disruptions to the cell cycle are caused by gene mutations that are either inherited (genetic predisposition) or triggered by mutagens. Mutagens can be either physical (radiation), chemical (tobacco or asbestos) or biological (certain viruses). A mutagen that specifically causes cancer is called a carcinogen, whereas a mutagen that causes abnormal embryonic or foetal development is called a teratogen. There is a strong positive correlation between cigarette smoking and the development of lung and tongue cancer. Risk Factors: Genetics Radiation Pollution Diseases Asbestos Tobacco OVERVIEW OF MITOSIS Mitosis involves four phases (prophase, metaphase, anaphase, telophase) and is preceded by interphase. Before: After: S phase Nuclear membrane dissolves Centrosomes move to poles Spindle fibres M = Middle Chromatids A = Apart Nuclear membranes reform