Topic D2

Question 1

How is the spindle assembly checkpoint mediated?

Answer 1

• The checkpoint is regulated by the protein Mad2 that its at free kinetochores and blocks APC action, so if a kinetochore is not attatched to a microtubule the cell will not move from metaphase to anaphase

Question 2

What are the 3 types of microtubules during mitosis?

Answer 2

1. Astral MTs: radiate out from the poles and contact the cells plasma membrane, help anchor/position the spindle pole
2. Kinetochore MTs: radiate out from the pole and undergo rapid polymerisation/depolymerisation events to contact kinetochores
3. Polar MTs: make links with polar MTs radiating from the other pole

Question 3

How are the dynamically unstable microtubules stabilised during interphase?

Answer 3

• microtubule associate proteins stabilised the microtubules by binding along their sides

Question 4

Explain the process of dynamic instability in microtubules:

Answer 4

• In a cell microtubules are constantly lengthening and shortening (via polymerisation and depolymerisation). When there is an excess of tubulin heterodimers they will attach to a growing (+) end of a microtubule
• As more and more tubulin heterodimers are attaching to the + end of the microtubule the beta subunits start slowly hydrolysing the GTP present within the newly added heterodimers
• This means that the subunits at the + end of the microtubule where they are being added have a higher proportion of GTP attached whilst the subunits more towards the other end of the microtubule (the – end) have a higher proportion of GDP (as the GTP has been hydrolysed by the beta subunits)
• When the heterodimer concentration reduces in the cell, there will be less of these being added to the + end; which means that there will be no more GTP cap on the + end (as the GTPs will be steadily hydrolysed and no more will be added)
• Steadily the polymerisation rate reduces and the unstable GDP cap which is formed in the place of the GTP cap causes the microtubule to dissociate very rapidly from that end.
• The rapid dissociation of the microtubule causes the concentration of tubulin heterodimers in the cell to increase once more, they will begin to bind to the + end, creating a GTP cap and the microtubule will begin to rapidly grow (polymerise) once more.
• This process is known as dynamic instability.