The Cell Cycle (College Board AP® Biology): Study Guide

Events of the cell cycle

• In eukaryotes, cells divide and transmit genetic information during the cell cycle

  • The cell cycle is the highly regulated sequence of events that occurs between one cell division and the next

• The cell cycle has three phases:

  • interphase: the cell grows and carries out its normal cellular functions; interphase consists of:

    • G1: the cell is metabolically active, duplicating organelles and cytosolic components

    • S: DNA is in the form of chromatin and replicates to form two sister chromatids connected at a centromere

    • G2: protein synthesis occurs, ATP is produced in large quantities, and centrosomes replicate

  • mitosis: nuclear division

  • cytokinesis: the cytoplasm and organelles are divided between the new daughter cells

Nondividing cells

• Not all cells divide all of the time; sometimes cells will stop dividing, e.g. when they are mature or when conditions for growth are not ideal

• Nondividing cells enter phase G₀, during which cell division does not occur; this exit from the cycle can occur at any stage of the cycle

• Cells in G₀ may re-enter the cell cycle in response to chemical signals

Mitosis

• Mitosis is a form of nuclear division; it ensures the transfer of a complete genome from a parent cell to the new daughter cells

• Mitosis produces two, genetically identical daughter cells

• The production of genetically identical cells by mitosis is essential for:

  • growth

  • tissue repair

  • asexual reproduction

• Mitosis is part of the cell cycle, and occurs after the completion of interphase

• The process of mitosis involves a series of steps:

  • Prophase

    • The nuclear membrane breaks down

    • The sister chromatids condense

    • Spindle fibers begin to form

    • Centrosomes move to opposite poles of the cell

  • Metaphase

    • Spindle fibres attach to the centromere of each chromosome

    • The chromosomes line up along the equator of the cell

  • Anaphase

    • The spindle fibres shorten, pulling the sister chromatids to opposite poles of the cell

    • The sister chromatids are separated and become chromosomes in their own right

  • Telophase

    • The mitotic spindle breaks down

    • New nuclear membranes form around the separated groups of chromosomes

• Mitosis is followed by division of the cytoplasm, or cytokinesis, resulting in two new daughter cells

  • Cytokinesis in animal cells involves formation of a cleavage furrow

  • Rigid cell walls mean that plant cells cannot divide in this way, so instead a cell plate develops to separate the two daughter cells