Mitosis

• Prophase:

  • Replicated chromosomes start to condense
  • Nuclear envelope starts to break down
  • Nucleolus disappears
  • Centrosomes move to opposite ends of the cell
  • The mitotic spindle forms (spindle fibers are made out of microtubules)
  • The nuclear membrane then disappears and the mitotic spindles attach to the sister chromatids of the chromosome.
• Metaphase: Spindle fibers move the replicated chromosomes + line up the chromosomes single file along the metaphase plate (middle of the cell)
• Telephase: nuclear envelope starts to appear; chromosomes are separated
• Cytokinesis: cytoplasm separates; cell divides into two
• M checkpoint:

  • cell needs to have all of the sister chromatids correctly attached to the spindle fibers to proceed through mitosis
• Anaphase:

  • Spindle fibers pull the sister chromatids (copies of each chromosome) apart
  • Spindle fibers shorten and pull the sister chromatids, now referred to as individual chromosomes, to opposite ends of the cell
  • Spindle fibers not attached to chromosomes push apart to elongate the cell
  • Spindle fibers shorten when microtubules depolymerize (break down)
• Telephase:

  • A nuclear envelope reforms around the unreplicated chromosomes at each end of the cell
  • Unreplicated chromosomes decondense back into chromatin
  • Nucleolus reappears
  • Mitotic spindle fibers break down
• Cytokinesis(Animal Cells):

  1. A ring of contractile microfilaments forms in the middle of the cell.
  2. The microfilaments pinch the cell inwards, forming a cleavage furrow
  3. The cell continues to pinch in half to form 2 cells
• Cytokinesis(Plant cells):

  1. Golgi vesicles carrying proteins and carbohydrates fuse together in the middle of the cell to build the cell plate 
  2. The cell plate gets bigger and fuses with the cell wall
  3. Golgi vesicle membranes form the plasma membrane on each side of the cell wall
  4. The cell is split into 2 cells by the cell wall
• G1:

  • The cell grows 
  • The cell accumulates resources like food and monomers for building DNA.
• G1 checkpoint: cell needs to be big enough, have enough resources, and not have any damaged DNA to move on to the next phase.
• S:

  • The cell replicates/duplicates its DNA. The cell now has twices as much DNA as it did to start with.
  • The cell replicates the centrosome (made up of 2 centrioles if it is an animal cell).
• S checkpoint: If there are problems during DNA replication, a checkpoint is triggered - S phase is put on hold until the problem is fixed
• G2:

  • The cell makes proteins that are needed for cell division.
  • The cell grows and duplicates many of its organelles.
• G2 checkpoint: cell needs to be big enough with enough components, have enough resources, and have successfully copied all of the DNA to move on to the next phase.
• G0:

  • Cells enter the G0 phase if they are not going to prepare to divide
  • Usually cells enter G0 if there aren’t enough resources for division or if the cell has differentiated (become a specialized cell) example- nerve cells and heart cells
  • Cells can be signaled to leave G0 and reenter the cell cycle, or cells can stay in G0 for their entire life
• Purines (two ringed structures): Adenine, Guanine
• Pyrimidines (one ringed structures): Thymine, Cytosine, Uracil
• Chromtin: consists of DNA tightly coiled around histones