Section 8: Cilia and flagella

Question 1

Compare flagella and cilia

Answer 1

• They are 2 versions of the same thing
• Cilia are 2-10 μm, flagella are 10-2000 μm
  
  • Flagella propel cells
  • Cilia sweep material across tissue
  • Cilia and flagella bend due to sliding

Question 2

What is the axoneme?

Answer 2

• The underlying structure of cilia and flagella is called the axoneme
• Consists of over 250 proteins
• A 9+2 (9 doublets, 2 singlets) array of microtubules is typical in humans

Question 3

How is the axoneme stabilised?

Answer 3

• Nexin and radial spoke heads: stabilise the doublets
• Axonemal dynein: bound to the tail end of the A tubule, head reaches toward the B tubule

Question 4

What is the basal body?

Answer 4

• The MTOC for axoneme/cilia and flagella
• Consists of 9 triplets (same # as axoneme doublets) with A, B, C tubulin
  
  • C does not pass through the transition zone at the cell surface; it becomes a doublet
• Basal bodies are triplet shaped barrels that are 90^o to each other
  
  • Related to centrioles

Question 5

How does the axoneme bend?

Answer 5

• The bending of cilia and flagella is generated by the sliding of microtubules against one another
  
  • Powered by axonemal dynein
• The A tubule walks along the neighbour B
  
  • While the axonemal dynein tail is permanently attached to A, the head portion binds to and moves B
• Bending occurs because sliding is prevented by nexin

Question 6

What is intraflagellar transport?

Answer 6

• Intraflagellar transport moves material up and down
  
  • Utilises kinesin 2 motor proteins, which move the molecule up to the (+) end (flagellum tip)
  • Utilises cytoplasmic dynein, which moves the molecule down to the (-) end (flagellum base)

Question 7

Describe the importance and function of non-motile primary cilia

Answer 7

• The main role is cell-cell signalling (releases and captures signals)
  
  • Thus it is crucial for development
• They are found in interphase cells
• They are doublets without an axonemal dynein or nexin, highlighting their signalling function over movement function
• Mutations in cilia structure can have lethal embryonic consequences

Question 8

Briefly summarise the 2 components of mitosis

Answer 8

• Karyokinesis: dividing up the chromosomes
  
  • Centrosome duplication
  • Breakdown interphase microtubules
  • Replacement of mitotic asters
  • Assembly of contractile ring (actin filament)
• Cytokinesis: dividing up the cytoplasm
  
  • Reformation of the interphase microtubule array
  • Contractile ring forms the cleavage furrow

Question 9

What happens to the centrosomes and other interphase microtubules before mitosis?

Answer 9

• Interphase microtubule are broken down
• The centrosome replicates and gives rise to two unique structures, spindle poles
  
  • Interphase microtubule half life is 5 minutes, which drops to 15 seconds during mitosis (therefore centrosomes and not like spindle poles)

Question 10

Why is it important that spindle pole microtubules have a short half life?

Answer 10

The short half life is important because the microtubules must capture the chromosome by guessing and checking for chromosome grabbage (reach, miss, gone)

Question 11

What is the mitotic apparatus?

Answer 11

The mitotic apparatus refers to the collection of:

• Polar and kinetochore microtubules (make “spindle”)
  
  • Polar spindle: microtubule that does not contact the chromosome
    
    • They overlap
  • Kinetochore spindle: microtubule that hit a kinetochore on a chromosome
• Astral microtubules
  
  • These move away from the pole

Question 12

What is the centromere?

Answer 12

• Centromere: attachment site for microtubules
  
  • Kinetochore proteins mediate this attachment
• Note: the (+) end of the microtubule has to be free, because it must polymerize and depolymerize to move the chromosome (this is an error with many diagrams)

Question 13

What proteins are involved in chromosome movement, such as alignment during metaphase?

Answer 13

• Kinesin 13: involved in depolymerisation
• Other kinesins: (+) end directed motors

Question 14

How does the cell ensure the chromosome is captured on both sides before being moved?

Answer 14

Using Ndc80; tension assures bi-orientation (attachement at both spindle poles)

• No tension = phosphorylation of Ndc80 proteins at the kinetochore
  
  • Results in weak microtubule interactions
• Tension = dephosphorylation of Ndc80
  
  • Microtubule interactions will be strong

Question 15

Describe anaphase A and B

Answer 15

• Anaphase requires separation of the chromosomes
• 2 parts:
  
  • Anaphase A: moving chromosomes to the pole (requires kinetochore microtubule shortening)
    
    • Depolymerisation even occurs at the (-) end to speed things up
  • Anaphase B: the poles separate (requires motors)

Question 16

Describe the 2 motor proteins involved in anaphase B

Answer 16

• Kinesin 5 interacts with the overlapping interdigitation polar microtubules
  
  • It causes sliding and undoes the overlap
  • This pushes the poles apart
• Anchored dynein pulls apart astral microtubules
  
  • Dynein motor walks to the (+) end
  • This pulls the astral microtubules, separating the poles further