Cell Division

CELL DIVISION CELL CYCLE The cell cycle is an ordered set of events which results in the division of eukaryo+c cells into iden+cal daughter cells. It involves two key phases: Interphase is an ac+ve phase of the cycle with many processes occurring in the nucleus and cytoplasm. It is a con+nuum of three dis+nct stages: • • • G1: Cell grows and prepares for division (organelles are duplicated) S: A synthesis stage in which DNA replica+on occurs in the nucleus G2: Acts as a final growth and prepara+on 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 or Meiosis: A process of nuclear division (DNA is separated) Cytokinesis: A process of cytoplasmic division (the cell splits in two) A T C G1 Growth + metabolism M P Prepara3on Replica3on + growth of DNA G2 SS The cell cycle moves in a clockwise direc3on as per the diagram shown above CHROMOSOMES Within eukaryo+c cells, the DNA is organised into discrete units that are called chromosomes. Normally, these chromosomes are packaged in an uncondensed form called chromaCn to allow for the cellular machinery to access the gene+c instruc+ons that determine cell ac+vity. However, when a cell divides, the DNA supercoils and the chroma+n condenses to form discrete chromosomes that will be visible under a light microscope (aZer division, chromosomes decondense and reform into chroma+n). Prokaryo+c cells only have a single circular chromosome and will not undergo the events of the cell cycle (bacteria divide by binary fission). Eukaryo3c Chromosome SISTER CHROMATIDS When DNA is replicated during the S phase, chromosomes will temporarily consist of two iden+cal DNA molecules. These strands are called sister chromaCds and are held together at a site called a centromere. A protein complex (called a kinetochore) a]aches to the centromere and connects to microtubule spindle fibres. Motor proteins (dynein and kinesin) control the lengthening and shortening of these spindle fibres, which causes the chromosomes to be moved around the cell and also allows the chroma+ds to separate. When chroma+ds separate during mitosis, they become two iden+cal chromosomes (one for each cell). DNA replica3on (S phase) produces chromosomes with sister chroma3ds These will separate during mitosis into two gene3cally iden3cal chromosomes MITOSIS Mitosis is the process of nuclear division, whereby the duplicated DNA molecules (i.e. sister chroma+ds) are arranged into two separate nuclei. Mitosis is preceded by interphase and involves four dis+nct 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 chroma+ds, 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 contrac+le 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 The division of cytoplasm is usually equal, as both daughter cells must receive at least one mitochondrion and any other organelle that can only be made by dividing a pre-exis+ng structure. However, some+mes cells will undergo unequal cytokinesis, such as is seen during oogenesis (egg produc+on) and budding. OVERVIEW OF MITOSIS Mitosis involves four phases (prophase, metaphase, anaphase, telophase) and is preceded by interphase. Stage Diagram Interphase (2n) S phase Before Prophase (2n) Key Events A35).