UnitE 1

• Cell Cycle


  The mechanism by which a cell duplicates its contents and divides in two
• Cell Cycle


  1. Replication of DNA and other cellular contents

  2. Partitioning of the duplicated material into two "daughter" cells

  
• Phases of the eukaryotic cell cycle

  • G1 (Gap 1) phase
  • S phase
  • G2 (Gap 2) phase
  • M phase

  
• G1 phase


  Interval between M phase and S phase
• S phase


  DNA replication
• G2 phase


  Interval between S phase and M phase
• M phase


  Includes mitosis (division of the genetic material) & cytokinesis (physical separation of the cell into two cells)

  
• Interphase


  The period between one M phase and the next (i.e. G1/S/G2)
• During interphase, the cell continues to transcribe its genes and translate its proteins, and it also grows and duplicates some of its organelles and grows in mass

  
• Prior to mitosis (M phase)

  1. Centrosomes have duplicated (during S phase) but remain together

  2. The DNA has duplicated (during S phase)

  3. The cell has grown (during interphase)

  4. The nucleus is intact and the DNA is decondensed (i.e. not forming recognizable, mitotic chromosome structures)

  
• Cohesion


  Protein complex that holds the condensed chromosomes together

  
• Centromere


  Point where the sister chromatids remain attached before separation during mitosis

  
• Kinetochore


  Large complex that assembles at the centromere and serves as the attachment point for microtubules

  
• First Phase of Mitosis: Prophase


  1. Two daughter centrosomes separate and move to opposite poles of the cell

  2. Some of the microtubules growing from opposite centrosomes overlap and are stabilized by microtubule-associated proteins

  3. The interacting sets of microtubules form the mitotic spindle

  4. The chromatin starts to condense

  
• Second Phase of Mitosis: Prometaphase


  1. Nuclear envelope breaks down, facilitated by the phosphorylation of the lamin proteins

  2. Microtubules now have access to the chromosomes and attach to the chromosomes via the kinetochore

  3. Chromosomes are now fully condensed

  
• Third Phase of Mitosis: Metaphase


  1. Chromosomes align at the equator of the spindle, forming the metaphase plate

  2. The kinetochore microtubules from one spindle pole attach to one sister chromatid, and the second sister chromatid attaches to a kinetochore microtubule from the second spindle

  
• Types of microtubules during mitosis

  • Kinetochore microtubules
  • Interpolar microtubules
  • Astral microtubules

  
• Fourth Phase of Mitosis: Anaphase


  1. Cohesin is cleaved by cysteine protease called separase, allowing paired chromatids to separate

  2. Each daughter chromosome is gradually pulled toward the spindle pole to which it is attached

  
• Dynamics of mitotic microtubules

  • Kinetochore microtubules disassemble but remain attached to the chromosomes, pulling the chromatids to opposite spindle poles
  • Spindle kinesins generate a sliding force to push the interpolar microtubules apart, forcing the spindle poles further apart
  • Aster microtubules are pulled towards the cell periphery by dynein, also forcing the spindle poles further apart

  
• Fifth Phase of Mitosis: Telophase


  1. The separated chromosomes arrive at the spindle poles

  2. The nuclear envelope begins to reassemble around the chromosomes

  3. A contractile ring composed largely of actin microfilaments starts to form at the equator between the two spindle poles

  
• Cytokinesis


  1. The cytoplasm is divided in two by the contractile ring

  2. Other cytosolic components are equally distributed between the newly-formed daughter cells

  3. Chromosomes are fully decondensed and the nuclear envelope is reformed

  4. Microtubules emerge from the single centrosome in each daughter cell

  5. The remaining microtubules from the mitotic spindle that are no longer attached are referred to as the midbody

  
• Myosin II can slide actin filaments along each other to cause muscle contraction, and the same principle happens during cytokinesis to create the cleavage furrow and ultimately separate the daughter cells
• Improper segregation of the genome leads to a condition called aneuploidy where there is an abnormal number of chromosomes in a cell

  
• Aneuploidy can be caused by defects in chromosome segregation, such as improper attachment of microtubules to kinetochores or improper spindle pole formation

  
• In yeast, mis-segregation occurs every 100,000 cell divisions (highly accurate)
• Down syndrome is caused by trisomy 21, where chromosome 21 is present in 3 copies, resulting in intellectual disability, growth delay, and characteristic facial features
• Cell Cycle

  The mechanism by which a cell duplicates its contents and divides in two
• Cell Cycle

  1. Replication of DNA and other cellular contents

  2. Partitioning of the duplicated material into two "daughter" cells

  
• Phases of the eukaryotic cell cycle

  • G1 (Gap 1) phase
  • S phase
  • G2 (Gap 2) phase
  • M phase

  
• G1 phase

  Interval between M phase and S phase
• S phase


  DNA replication
• G2 phase

  Interval between S phase and M phase
• M phase

  Includes mitosis (division of the genetic material) & cytokinesis (physical separation of the cell into two cells)

  
• Interphase


  The period between one M phase and the next (i.e. G1/S/G2)
• During interphase, the cell continues to transcribe its genes and translate its proteins, and it also grows and duplicates some of its organelles and grows in mass

  
• Prior to mitosis (M phase)

  1. Centrosomes have duplicated (during S phase) but remain together

  2. The DNA has duplicated (during S phase)

  3. The cell has grown (during interphase)

  4. The nucleus is intact and the DNA is decondensed (i.e. not forming recognizable, mitotic chromosome structures)

  
• Cohesion


  Protein complex that holds the condensed chromosomes together

  
• Centromere

  Point where the sister chromatids remain attached before separation during mitosis

  
• Kinetochore

  Large complex that assembles at the centromere and serves as the attachment point for microtubules

  
• First Phase of Mitosis: Prophase

  1. Two daughter centrosomes separate and move to opposite poles of the cell

  2. Some of the microtubules growing from opposite centrosomes overlap and are stabilized by microtubule-associated proteins

  3. The interacting sets of microtubules form the mitotic spindle

  4. The chromatin starts to condense