Cell Cycle

Cells and the Cell Cycle

  • All living organisms are made of cells
    • Humans have 100 trillion
    • 200 cell types at birth
  • Organisms may be single-celled or complex multicellular
    • Single-celled: Bacteria (Amoeba)
    • Multicellular: human

Cells have 4 features

  1. Structure
  2. Function
  3. Reproduce
  4. Have a cell cycle

Know structure of cell and organelles

Purpose of Cell Division

  1. Growth of organisms and development
  2. Repair of damage to tissues
    1. Skin cells
  3. Reproduction to give more rise to more cells that are identical

Division must generate genetically identical daughter cells

  • Nuclei divide (mitosis)
  • Cytoplasm divide (cytokinesis)

Which cells undergo cell division

Cells that divide frequently

  • Skin, cheek and intestinal cells
  • Millions are created daily

Cells that divide infrequently (once a year)

  • Liver

Cells that do not divide

  • Nerve cells

Cell Cycle must be regulated

  • Cannot divide randomly
    • If it does it leads to cancer
  • Cell divisions is under control of the "cell cycle"

Cell Cycle

  • Series of coordinated steps that results in cell division

Components of Cell Cycle

G1 = first gap phase, cell growth occurs

Cell is metabolically active and continually grows but does not replicate DNA

S = Synthesis of DNA takes place

G2 = Second gap phase, cell are preparing for cell divisions

Organelles and proteins necessary for cell division are produced

M-phase = mitosis

Interphase = G1 +S +G2

Gap phases: cell is preparing for process to take place, preparation step

What is the Cell Cycle?

  • Include the definition and the components of the cell cycle

Cell Cycling Times

1 cycle is around 24 hours

Depends on the needs of the organism

  • If quick repair is needed if damage has occurred

G1 is the longest

M-phase is the shortest

G0 = cell resting phase, no activity

How is the cell cycle regulated

  • Must be controlled
  • Molecules in cell act as signals that can start and stop the cell cycle
  • Signals found in the cytoplasm

When fusing 2 cells in different phases (S -G1/ M -G1), signals in the Higher phase will tell the G1 to reach that phase

  • e.x The S cell makes G1 cell to enter synthesis of DNA phases

Signals that regulate the cell cycle

  • Mitosis-promoting Factor (MPF)
    • Levels are maximum as cell enters into mitosis
    • Closely related to levels of cyclin
      • Also max when cell enters mitosis

How does MPF Work (The reason why both concentration are high at the same time)

  • A protein Kinase (type of enzyme that is involved in signaling)
    • Must combine with cyclin molecule to be activated
  • Cyclin is a regulatory protein > activates the kinase by phosphorylating it (adds a phosphate group which provides energy)
  • Therefore MPF is a cyclin-dependent kinase molecule (CDK) which when combined with cyclin form MPF

  1. Starts as CDK From G1 > S > G2
  2. Some point in G2 Checkpoint, Cyclin comes in
  3. Cyclin and CDK combine to create MPF and signals cell to enter mitosis
  4. Cyclin is than degraded and leaves
  5. Returns to CDK in G1

Checkpoints in the cell cycle

  • Location within cycle that can stop/start the cell cycle

3 checkpoints in the human cells

  1. G1
    1. The most important
    2. If cells don't pass G1, they go into G0 phase (non-dividing)
    3. Cell must have adequate size, nutrients are sufficient, DNA is undamaged
  2. G2
    1. Occurs after the S-phase
    2. MPF complex interacts here
    3. Cell must have chromosomes have replicated successfully, DNA undamaged, activated MPF is present
  3. M-phase
    1. Stops the cells from dividing during mitosis
    2. Cell must have all chromosomes are attached to spindle apparatus

Why have checkpoints

  • To control traffic flow
  • Determine how much cell division is needed
    • To repair damage tissue, once enough cells have divides there is no need for more
  • Determine when cells need to divide and when there is a need

No check-points or check-points fair?

CANCER

Another signal that can activate the cell

Platelet derived growth factor

  • Repairs muscle
  • Derived from blood cells
  • Stimulate cell division to results in production of connective tissue (fibroblasts) to repair the damage
  • Fibroblasts have a receptor to bind tyrosine kinases present in PDGF

Mitosis - Process of Cell Division

  • Results in the production of genetically identical daughter cells

  1. DNA in the nucleus is replicated (S-Phase) and condense with histone proteins to form chromosomes
    1. Consist of 2 sister chromatids joined at the centromere

Stages of Mitosis, PPMAT (Look up)

Prophase

  • Chromosomes appear with sister chromatids
  • The spindle apparatus forms (contains microtubules)

Prometaphases

  • Nuclear envelope breaks down
  • The microtubules attach to chromosomes at the centromere (Kinetochores)

Metaphase (the middle)

  • Sister chromatids migrate to the middle of the cells
  • Spindle apparatus is complete
  • Chromosomes line up

Anaphase

  • Chromatids (chromosomes) are pulled along by kinetochore microtubules

Telophase

  • Spindle apparatus breaks down
  • Nucleus envelope forms around chromosomes
  • Chromosomes decondenses

Cytokinesis

  • Division of cytoplasm and formation of cell membrane around two new cells

Kinetochore microtubules

  • Protein that acts like fishing wire

Stems cells have the capability to divide to form new cells

  • They can give rise to cells in the body that has different structure and function, it can turn into any cell
  • Commonly used stem cells are from embryonic tissues, recovered from early stage ( (4-5 days old) in embryo development > blastocysts
  • Can also be derived from the umbilical cord or amniotic fluid surround the fetus
    • Adult stem cells from blood cells or bone marrow cells can give rise to new cells
  • Pluripotent stem cells
    • Can be re-programmed to become cell lines of a different tissue type
    • Skin cells become heart cells

Benefits of Stem Cells Research

  • Potential to regenerate tissues that normally would not grow again
    • Heart tissue, nerve cells (regenerative medicine)
  • Technical challenges have not provided many examples of success
    • Those with spinal cord injury or heat damager

Success

  • Stem cell transplant (bone marrow transplant)
  • Stem cells replace cells damaged by chemo or disease or to fight some types of cancer and blood related disease
  • Transplants use adult stem cells or umbilical cord blood
  • Testing adult stem cells to treat other conditions, including a number of degenerative disease like heart failure

Potential Problems

  • Embryonic stem cells can grow irregularly or grow in different cell types spontaneously, researchers are studying how to control the growth and differentiation
  • Patient's immune system might reject and attack the stem cells as foreign invaders
  • Stem cell might fail to function as expected with unknown consequences