A figure shows the transverse section of a cilium. Nine pairs of microtubules (doublets) are arranged in a ring at the periphery, and two separate microtubules lie at the centre, enclosed by a central sheath. Radial spokes join the peripheral doublets to the central sheath. How many microtubules does this section show, and what is the arrangement called?
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The axoneme of a cilium has nine doublets of peripheral microtubules and a pair of central microtubules, the 9 + 2 array. That gives 9 × 2 + 2 = 20 microtubules in the section.
- A.
Nine doublets make 18 microtubules, but the two central microtubules must be added, and the arrangement is 9 + 2.
- B.
Nine peripheral triplets with no central pair describe a centriole or basal body, not a cilium.
- C.
Each of the nine peripheral units is a doublet of two microtubules, so the total is 9 × 2 + 2 = 20, not 11.
NCERT: Class 11 Biology, Chapter 8