4. Cellular signals and apoptosis Part 2

Question 1

What are neurons?

Answer 1

Nerve cells which are specialised to recieve, process, and/or send electrical signals.
-Consist of a cell body, dendrites, and a long axon

Question 2

What is a synapse?

Answer 2

The gap between nerves, where the end branches of an axon terminals of one neuron and the dendrites of another neuron lie close to each other, but do not make direct contact.

Question 3

What is a neurotransmitter?

Answer 3

A chemical messenger released by axon terminals into the synaptic cleft (gap) between two neurons which has an excitatory or inhibitory effect on the target cell/neuron.

Question 4

What are neurohormones?

Answer 4

Hormones that are secreted by special nerve cells called neurosecretory cells in the hypothalamus, and travel in the blood to stimulate the nearby pituitary gland.
-Usually, the pituitary gland is stimulated to release hormones

Question 5

Describe how an action potential triggers neurotransmitters to be released:

Answer 5

• Electrical signal arrives and triggers Ca2+ to enter the cell
• This triggers vesicles full of neurotransmitters to exocytose to the synapse
• Neurotransmitters are released into the synaptic gap
• Neurotransmitters bind to specific receptors on the postsynaptic membrane and ion channels open causing a change in cell potential which passes on the action potential

Question 6

Give an example of how the response can alter the original stimulus:

Answer 6

Negative/positive feedback mechanism

Question 7

Give an example of how a signal transduction pathway can be disrupted:

Answer 7

By quickly clearing up the second messengers or dephosphorylating proteins.

Question 8

What happens to the second messenger system depending on the molecules they interact with?

Answer 8

The same second messenger system can produce a different response in one cell compared to another.

Question 9

What is apoptosis? List its 2 pathways:

Answer 9

A highly controlled natural process which breaks a cell down into fragments that are easily cleared away, without inflammation, which occurs in invertebrates, vertebrates, and plants.

• Extrinsic (death-ligand) pathway
• Intrinsic (mitochondrial) pathway

Question 10

Describe the process of apoptosis:

Answer 10

1. Cell receives a pro-apoptotic signal either internally, such as from leakage of a mitochondrial membrane or another organelle, or internally, such as during an immune response.
2. Signal transduction cascade leads to activation of enzymes called caspases that cleave, or cut, intracellular proteins as well as enzymes called endonucleases, that cut up DNA.
3. The nuclear membrane breaks down and the cell fragments into small blebs which contain DNA and organelles which retain their function.
4. Phagocytes engulf the apoptotic debris, which enters a vacuole called the phagosome, fuse with a lysosome producing which is called a phagolysosome.
5. Destruction and recycling of components then occurs.

Question 11

What is the function of a tumor suppressor gene? Give an example:

Answer 11

Produce proteins that prevent cells from dividing when the DNA of the cell is found to carry mutations.
Eg.p53 gene which can activate apoptosis if DNA repair is not possible. If p53 is inactivated by mutation, then a cell containing mutated DNA can escape apoptosis and continue to proliferate, forming a tumour.

Question 12

What are oncogenes and how are they involved in apoptosis?

Answer 12

Are genes whose protein products provide a strong activating signal to the cell, promoting growth signals of much greater duration or intensity than normal.
-These cells can proliferate and evade apoptosis while DNA mutations accumulate and cancer can develop.

Question 13

What are cardiovascular disorders?

Answer 13

Apoptosis of cardiac muscle cells leading to heart failure.

Question 14

How are viruses and bacteria involved in apoptosis? Give an example:

Answer 14

Have an array of mechanisms to manipulate apoptosis in host cells to change the outcome of an infection.
Eg. Preventing apoptosis of phagocytes that have engulfed bacteria.

Question 15

What is apoptosis initiated by?

Answer 15

Stress or a death ligand

Question 16

Describe the general signal transduction pathway:

Answer 16

1. A signaling molecule (ligand) binds with a protein receptor on the surface of the target cell.
2. This binding causes a conformational change in the receptor protein (it changes shape somehow).
3. The change in shape of the receptor protein, leads to the activation of a second messenger, inside the cell.
4. The second messenger is produced in large numbers, so that the signal provided by the original ligand, is amplified within the cell.

Question 17

When can the term ‘ligand’ be used?

Answer 17

The signaling molecule that binds to a receptor can be referred to as a ‘ligand’.

Question 18

List 2 processes that apoptosis is important in:

Answer 18

• Development changes in growing embryos (Eg. In early development, fingers and toes are webbed; as the foetus develops, the cells in the webbing die, freeing the fingers and toes)
• Ridding tissues of old, infected or damaged cells

Question 19

What is the extrinsic pathway of apoptosis initiated by?

Answer 19

It is initiated by a death ligand such as the FASL released from NK or Tc cell.

Question 20

When is the intrinsic pathway of apoptosis initiated?

Answer 20

It is initiated when the cell is damaged due to radiation or some other event, which causes cytochromes and other molecules in the mitochondria to come out into the cytoplasm where they begin apoptotic cascade.