Week 3 - cell polarity

Question 1

What are the 2 routes by which there can be diversity between 2 daughter cells produced?

Answer 1

1) INTRINSIC - Polar mother cells could divide to generate daughters that have inherited different components.

2) EXTRINSIC - Daughters could be equal at ‘birth’ but then become different by exposure to different environmental signals

Question 2

What are the 3 important steps in generating polarity and cell fate decisions?

Answer 2

1. Have an axis of polarity
2. Mitotic spindle is positioned along the axis.
3. Cell fate determinants are distribuuted differentially to daughter cells.

Question 3

What division was seen in C.elegans?

Answer 3

asymmetric cell division - where zygote divided into one larger (AB) and one smaller cell (P1) where these daughters were commited to different fates.

Question 4

In C.elegans, when there are par mutants, what is seen with AB and P1?

Answer 4

the daughter cells AB and P1 are less pronounced and in extreme cases the two are identical.

Question 5

What is the MTOC?

Answer 5

It is the microtuble organising centre.

Question 6

Explain how symmetry is broken on fertilisation when the sperm delivers a MTOC.

Answer 6

This site becomes the posterior pole and defines the axis of polarity.

• The microtubules generated recruit Par1 and Par2 at the posterior cortex and this antagonises the anterior Par proteins, Par3, Par6 and aPkc which localise to the anterior cortex.
• Par 5 maintains the boundary.
• Phosphorylation is key in the feedback loops that allow the poles to be defined.

Interactions between microtubules and the cortex results in pulling forces which act on the mitotic spindle which causes the spindle to be displaced TOWARD the posterior end.

Question 7

How do drosphila neuroblasts divide?

Answer 7

they divide asymmetrically (undergo repeated rounds of cell division).

each division gives a ganglion mother cell and a larger apical daughter cells.

The GMC divides only once more to give rise to a neuron and a glia cell, while the apical daughter continues to divide asymmetrically.

Question 8

Cell migration depends on the …….beneath the plasma membrane.

What are the 3 main activities needed for movement.

Answer 8

actin-rich complex.

1. Protrusion
2. Attatchment
3. Traction
4. Protrusion – the pushing out of the plasma membrane in front of the cell
5. Attachment – the actin cytoskeleton inside the cell is attached via interacting proteins across the plasma membrane to the substratum (eg extracellular matrix)
6. Traction – the bulk of the cell body is drawn forward through a process of contraction

Question 9

What is chemotaxis?

Answer 9

It is the movement of cells towards or away from a signal.

e.g. movement of neutrophil moving towards a site of bacterial infection. Receptors on the surface of the neutrophils detect very low levels of bacterial peptides.

Question 10

What are the key properties of epithelial cells?

Answer 10

• lateral sides can adhhere to each other
• have polarised actin cytoskeleton.
• can orient their mitotic spindle to allow for division
• Epithelial cells can rapidly lose the epithelial phenotype (epithelial mesenchymal transition – EMT) and re-acquire it (Mesenchymal epithelial transition – MET)

Question 11

What is EMT (epithelial mesenchymal transition)?

Answer 11

lose their epithelial phenotype and acquire mesenchymal properties.

It involves conversion of the epithelial apical-basal polarity axis into a migration axis with front-rear polarity.
EMT is triggered by signals that lead to a loss of E-cadherin. There is also asymmetric activation of small Rho GTPases (Cdc42 and Rac1 at the front and RhoA at the rear)