Cell Cycle

The cell cycle

Cell division is the process by which cells reproduce (mitosis). The cell cycle is a series of changes the cell goes through from the time it is first formed until it divides into two daughter cells. It starts at mitosis (M-phase) and ends with mitosis. In between are the G-1, S, and G-2 phases. The duration of S, M and G-2 are relatively constant in different tissues.

Between the M-phase and the S-phase is a gap (G-l) where production of RNA, proteins, and enzymes needed for DNA synthesis occurs. The duration of G-1 varies and determines the length of the cell cycle.

The S-phase is when DNA synthesis occurs.

Between the S-phase and M-phase is a second gap (G-2). Cells are thought to prepare for mitosis in G-2 when specialized proteins and RNA are produced. G-0 is a dormant phase.

The four phases of mitosis are:

  1. Prophase
    1. Centrosomes separate and migrate to opposite poles.
    2. Centrioles separate.
    3. Chromatin is transformed into chromosomes composed of pairs of filaments called chromatids (each is a complete genetic copy of its chromosome).
    4. The nuclear membrane disappears.
  2. Metaphase
    1. Paired chromosomes become lined up between the centrioles.
  3. Anaphase
    1. Chromatids are pulled toward the centrioles. One chromatid from each pair goes to each daughter cell.
  4. Telophase (divided into parts I and II)
    1. Telophase I
      1. Chromosomes become more polarized and transformed into thread-like structures.
      2. A nuclear membrane forms around each set of chromosomes forming a new nucleus with a nucleolus.
      3. The centrioles duplicate.
    2. Telophase II
      1. Actual dividing of the cell occurs (cytokinesis).
      2. Cytoplasm splits and two daughter cells are formed.