Formative Test - Biology > Cell Cycle; Death (p2)

What is apoptosis (programmed cell death)?

• Self-destruction for a cell, for the good of the whole organism

• Genetically controlled and highly regulated

• The cell responds to a specific signal

What is apoptosis used for?

• Cells at the end of their natural life e.g. skin cells

• Dysfunctional, damaged or diseased cells e.g. viruses and infections

• Excessive cells - important in many contexts

What is one example of apoptosis in the sense of removal of excessive cells?

• Embryonic development - final shaping of organs on the programmed cell death of excess cells e.g. “webbing” between digits is removed by week 8

Apoptosis: A process of balance

1. Cells in many of our organs and tissues are in a state of constant turnover e.g. skin and gut cells

2. In adults, the rates of cell renewal and cell death are balanced

3. Apoptosis is a normal healthy process

What is the process of apoptosis?

1. The cell shrinks

2. The cell’s plasma membrane forms bubbles or “blebs” (membrane blebbing)

3. The cell breaks up and its contents from into parcels called apoptosis bodies

4. Apoptosis bodies are engulfed by phagocytic cells

What is necrosis (unplanned cell death)?

• Occurs when a cell is damaged by a mechanical or chemical insult (e.g. disease, lack of blood supply, injury)

• Damage to the plasma membrane means that the cell can no longer control the movement of substances into and out of the cell

• The cell can swell and burst, spilling its contents and causing inflammation to surrounding tissues

What is the process of necrosis?

1. Dying cell swells

2. Plasma membrane ruptures

3. Cell contents spills into surround tissue

4. Local swelling and inflammation

5. If extensive, can lead to gangrene

What are the 2 apoptotic pathways?

Intrinsic and Extrinsic

What is intrinsic?

Intrinsic/mitochondrial pathway - initiated from within a cell

What is extrinsic?

The extrinsic (outside the cell)/death receptor pathway - initiated by factors external to the cell

What is the intrinsic apoptotic pathway?

• Triggered from inside the cell by stress or severe damage (e.g. DNA damage, infection, low oxygen, UV)

• Mitochondria break down and release factors

• This starts a chain reaction that breaks down DNA and proteins

• The cell is dismantled into small pieces

Process of the intrinsic pathway:

1. Internal damage signals to the mitochondria to form pores

2. The pores allow the release of cytochrome c from the mitochondria into the cytoplasm

3. Cytochrome C interacts with other molecules in the cytoplasm to activate a cascade of caspase of enzymes

4. Caspases act as protein-cutting enzymes, triggering further caspase activation and amplifying the death signal

5. The cell shrinks and membrane begins to bleb

6. Cleavage of proteins within the cells leads to break down of the cells nucleus and cytoskeleton.

7. Fragmentation of DNA

8. Apoptotic bodies are formed

What is the extrinsic apoptotic pathway?

Activated when molecules from outside a cell bind to death receptors on the plasma membrane and signal the cell: “its time to die”

Process of the extrinsic pathway:

1. Specific signalling molecules (ligands) bind to death receptor on the plasma membrane

2. Binding of the ligand activate these death receptors, sending a death signal into the cell

3. The signal activated a cascade of caspase enzymes inside the cell

4. The cell shrinks and the membrane begins to bleb

5. Cleavage of proteins within the cells leads to break down of the cells nucleus and cytoskeleton

6. Fragmentation of DNA

7. Apoptotic bodies are formed

Results of to much apoptosis:

• Neurodegenerative disorders e.g. parkinson’s disease and alzheimer’s disease - abnormal progressive loss of nerve cells in the brain due to unregulated activation of caspases and apoptosis.

• AIDS excessive unregulated apoptosis leads to the loss of T-helper cells, which are a group of immune cells

Result of to little Apoptosis

Autoimmune diseases Rheumatoid arthritis is an autoimmune disease where excess cell growth in joint tissue causes damage

Cancers Uncontrolled process of cellular division that forms tumors.

1. Tumours are masses of cells growing out of control

2. Normally, damaged cells should undergo apoptosis (self-destruct)

3. Cancer cells ignore these apoptosis signals

4. Mutations allow cancer cells to grow uncontrollably and crowd out healthy cells