Cell Cycle

• Multi-cellular organism

  •  multicellular organism needs to coordinate cell division across different tissues & organs
  • critical for normal growth, development & maintenance
  • not all cells can have the same cell cycle
• Frequency of cell division

  • Frequency of cell division varies by cell type
  • embryo
  • cell cycle < 20 minute
  • skin cells
  • divide frequently throughout life
  • 12-24 hours cycle
  • liver cells
  • retain ability to divide, but keep it in reserve
  • divide once every year or two
  • mature nerve cells & muscle cells
  • do not divide at all after maturity
  • permanently in G0
• Cell Cycle


  is a repeated pattern of growth and division that occurs in eukaryotic cells.

  
  

  Cells have distinct phases of growth, reproduction, and normal functions.
• Interphase


  A cell spends most of its time in what is called interphase, and during this time it grows, replicates its chromosomes, and prepares for cell division. The cell then leaves interphase, undergoes mitosis, and completes its division.

  Subdivided into three stages:

  G1, S, G2
• Mitosis


  Mitosis is the process by which a cell replicates its chromosomes and then segregates them, producing two identical nuclei in preparation for cell division. Mitosis is generally followed by equal division of the cell's content into two daughter cells that have identical genomes.
• • The cell cycle is a regular pattern of growth, DNA replication, and cell division.
• The main stages of the cell cycle are gap 1, synthesis, gap 2, and mitosis.

  • Gap 1 (G1): cell growth and  normal functions
  • DNA synthesis (S): copies DNA
  • Gap 2 (G2): additional growth (chromatids become replicated chromosomes) 
  • Mitosis (M): includes  division of the cell nucleus (mitosis) and division of the cell cytoplasm (cytokinesis
  • Mitosis occurs only if the cell is large enough and the DNA undamaged.
• Checkpoint control system

  • G1/S
  • can DNA synthesis begin?
  • G2/M
  • has DNA synthesis been completed correctly?
  • commitment to mitosis
  • spindle checkpoint
  • are all chromosomes attached to spindle?
  • can sister chromatids separate correctly?
• Interphase is a stage within the cell cycle of eukaryotic cells, occurring between cell divisions. It is divided into three main phases: G1 (Gap 1), S (Synthesis), and G2 (Gap 2). During interphase, the cell undergoes growth, DNA replication, and preparation for cell division.
• Gap 1


  During this stage, the cell grows and carries out its normal metabolic activities. It synthesizes RNA, proteins, and other molecules necessary for cellular functions.
• Gap 2


  The cell continues to grow and prepare for division during this phase. It synthesizes additional proteins and organelles, and it also checks its DNA for errors and repairs any damage.
• Gap 0

  Cells can enter Gap 0 from various points in the cell cycle, including from the G1 phase after completing division or from the G2 phase if conditions are not favorable for division. In Gap 0, cells may remain for an extended period, sometimes indefinitely, until they receive signals or stimuli that trigger them to re-enter the cell cycle and proceed with division.
• Mitosis


  the process by which a cell replicates its chromosomes and then segregates them, producing two identical nuclei in preparation for cell division. Mitosis is generally followed by equal division of the cell's content into two daughter cells that have identical genomes.
• Prophase

  1. Chromosomes condense and are more visible.
  2. The nuclear membrane (envelope) disappears.
  3. Centrioles have separated and taken positions on the opposite poles of the cell.
  4. Spindle fibers form and radiate toward the center of the cell.
• Prometaphase


  Starts abruptly with the breakdown of the nuclear envelope. Chromosomes can now attach to spindle microtubules via their kinetochores and undergo active movement.
• Metaphase


  The chromosomes are aligned at the equator of the spindle, midway between the spindle poles. The kinetochore microtubules attach sister chromatids to opposite poles of the spindle.
• • Mitosis divides the cell’s nucleus in four phases


  During metaphase, chromosomes line up in the middle of the cell.
• Anaphase

  1. Centromeres that join the sister chromatids split.
  2. Sister chromatids separate becoming individual chromosomes.
  3. Separated chromatids move to opposite poles of the cell.
• Telophase

  1. Chromosomes (each consisting of a single chromatid) uncoil.
  2. A nuclear envelope forms around the chromosomes at each pole of the cell.
  3. Spindle fibers break down and dissolve.
  4. Cytokinesis begins.
• Cytokinesis


  Cytokinesis is the final step in the division of a cell, following mitosis or meiosis. During cytokinesis, the cytoplasm of the parent cell is divided into two daughter cells. In animal cells, this typically involves the formation of a cleavage furrow, which pinches the cell in two, while in plant cells, a cell plate forms between the two nuclei, eventually developing into a new cell wall. This process ensures that each daughter cell receives a complete set of organelles and genetic material.