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Assertion: The arrangement of axonemal microtubules in cilia or flagella is called 9+2 array.
Reason: The axoneme usually has nine pairs or doublets of radially arranged peripheral microtubules and a pair of centrally located microtubules.
A. Both assertion and reason are correct and Reason is the correct explanation for assertion
B. Both assertion and reason are correct but Reason is not the correct explanation for assertion
C. Assertion is correct but the reason is incorrect
D. Both assertion and reason are incorrect

Answer
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Hint: Axoneme, with a diameter of 150 nanometres, is a highly complex and precisely constructed support structure with more than 250 adherent proteins that form the foundation of a cilium or flagellum to provide locomotion.

Complete answer: Axoneme is the key extracellular aspect of cilia and flagella in eukaryotic cells. It comprises a cytoskeletal element microtubule, which usually consists of nine doublets. In motile cilia, dynein ATPase protein molecules develop sliding movement among adjacent microtubules, that are assembled into well-orchestrated pounding or rotary movement. Thus, the arrangement of microtubules in the axonemal of that of cilia and flagella is called a 9+2 array. The reason behind this is that axonemes have nine pairs or doublets arranged radially peripheral to microtubules and pairs of centrally located microtubules.
So, the correct option is (A).
Additional information:
The architecture of the axonemal in non-motile primary cilia indicates some variance of the standard "9x2 + 2" structure. No dynein arms were found on the outer doublet microtubules, and that there are no singlets of central microtubules. This arrangement of the axonemal is referred to as "9x2 + 0." In addition, "9x2 + 1" axonemes, just with one central microtubule, are being found to have an impact. Primary cilia appear to serve sensory functions. The key piece of the axoneme is the microtubule (motile cilia). Each axoneme comprises many parallel spaced microtubules. To be precise, the microtubules are grouped in a particular sequence known as "9x2 + 2. Nine sets of "double" Microtubules form a ring all around the "central pair" of individual microtubules.

Note: The axoneme functions as the "skeleton" of cilia or flagella, both offering security for the structure and, in certain cases, for the power to twist. While the distinction of role and length could be made among cilia and flagella, the inner layer of the axoneme is common to both. In contrast to microtubules, axoneme contains multiple proteins and protein complexes that are required for all of its functioning. The dynein arms, for example, are motor complexes that provide the pressure needed to stretch.

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