Questions: Cilia and Flagella: Structure and Function

Question 1

A researcher engineers cells in which the dynein arms are fully functional but the nexin links and radial spokes are absent. What would you predict about ciliary movement in these cells?

• A. Cilia would beat faster than normal because the constraining links are removed
• B. The doublet microtubules would slide freely past each other rather than bending into coordinated waves
• C. Cilia would function as sensory organelles only, since bending requires intact links
• D. There would be no movement at all because dynein requires nexin to generate force

Answer: (B)

Nexin links and radial spokes are what convert sliding force into bending. If they are absent, dynein can still walk along adjacent doublets and generate sliding — but without structural constraints, the doublets slide apart rather than bending. This is exactly the mechanistic insight of axoneme biology: the 9+2 structure does not generate motion by direct bending but by constraining sliding into bending. Dynein does not require nexin to generate force — it requires nexin to channel that force productively.

Question 2

Primary (non-motile) cilia differ structurally from motile cilia in which key way, and what is the functional consequence?

• A. Primary cilia have a 9+2 axoneme but lack dynein arms, making them very slow movers
• B. Primary cilia have a 9+0 arrangement (no central microtubule pair) and lack dynein arms, making them sensory organelles rather than motile structures
• C. Primary cilia are shorter than motile cilia, causing them to beat at a lower frequency
• D. Primary cilia have additional dynein arms, which makes them more sensitive to molecular signals

Answer: (B)

The structural difference is the absence of the central pair (9+0 vs. 9+2) and dynein arms. Without dynein, there is no motor force and thus no movement. Without the central pair and its associated signaling scaffold, the structure is repurposed as a cellular antenna: it concentrates signaling receptors on its membrane and responds to extracellular signals (Hedgehog, Wnt) and mechanical stimuli (fluid flow). The same basic microtubule scaffold serves completely different functions depending on which components are present.

Question 3

In motile cilia, the dynein arms would cause doublet microtubules to slide freely past each other if not for nexin links and radial spokes that convert this sliding into bending.

• T. True
• F. False

Answer: True

This is the core mechanistic insight of axoneme biology. Dynein is a minus-end-directed motor that walks along adjacent B-tubules, generating a sliding force between doublets. If the doublets were free (like two hands interleaving), they would slide apart. Nexin links and radial spokes constrain this sliding at specific points, so instead of sliding, the doublets bend. Asymmetric activation of dynein arms on one side produces the characteristic back-and-forth beat.

Question 4

In individuals with Kartagener syndrome (primary ciliary dyskinesia), situs inversus (reversed organ placement) occurs in nearly every affected person, because immotile nodal cilia usually reverse left-right organ determination.

• T. True
• F. False

Answer: False

Situs inversus occurs in only approximately 50% of Kartagener syndrome cases, not all of them. Normally, motile nodal cilia during embryogenesis create a directed fluid flow that establishes left-right asymmetry. When these cilia are immotile, the directional cue is absent — but left-right determination still happens, just randomly. The result is a 50/50 chance of normal vs. reversed organ placement, not a consistent reversal. This probabilistic outcome itself confirms that normal cilia provide a directional signal rather than merely permitting random determination.

Question 5

Explain the mechanistic logic by which the 9+2 axoneme structure generates bending. What role do the constraining structures play, and what would happen if they were removed?

Model answer: Dynein arms generate a sliding force by walking along adjacent doublets. Nexin links and radial spokes constrain this sliding at fixed points, converting it into localized bending. Asymmetric dynein activation on one side produces a bend in that direction; alternating sides creates the rhythmic beat. Without constraining links, doublets would slide apart freely rather than bend.

The key insight is that the force-generating mechanism (dynein sliding) is distinct from the movement-producing mechanism (bending). The conversion between them depends entirely on structural constraints. This principle — force generation converted to useful work by structural architecture — is a recurring theme in molecular motors, from muscle myosin to kinesin on cytoskeletal tracks.