Question

Compare and contrast the molecular, physical properties as well as the cellular functions of microtubules and actin. Does actin play a role in cell division? Do microtubules play a role in migration or metastatic invasion?

Answer 1

MICROTUBULES:

1. They are hallow, cylindrical filamentous rods found in both plants and animals
2. 25nm in diameter with 6nm thick wall having 13 protofilaments/subunits      
3. Composed of α & β tubulins
4. They are polar structures having plus end/fast growing end and minus end/slow growing end.
5. MAPs : HMW proteins & tau proteins
6. Microtubule organising centres : Basal bodies, Centrioles
7. Polymerisation requires GTP and calcium ions (Calmodulin)

Functions :

• Mechanical function
• Contraction of spindle microtubules and chromosome separation/movement
• Transport function  

Yes microtubules are involved in migration or metastatic invasion as they exhibit:

• Treadmilling (the addition of subunits at one end and their loss at the other end &
• Dynamic instability (the oscillation between growth and shrinkage)

ACTIN FILAMENTS

1. Aka microfilaments made up of double chain of actin proteins with 7nm diameter.
2. Found in the microvilli of the brush border of intestinal epithelium and in cells that exhibit amoeboid movement.
3. They are 7 nm in diameter and composed of two strands.
4. G actin (Globular and Unpolymerised) & F actin (Filamentous and Polymerised).
5. Functions:

• Muscle contraction
• Cytoplasmic streaming in plant cells
• Pseudopodia (Amoeba)
• Cleavage furrow formation
• Maintainance and changes in cell shape   

Yes actin plays a important role in cell divisin as: Cell membrane completes constriction in telophase. Nuclear membrane forms followed by cytokinesis. Actin filaments decides the plane of contractile ring formation.