Which of the following options is correct about structures visible in the cross-section of a centriole ?

Options

A. `{:("Peripheral","Central","Hub","Spokes","Inter"),("microtubules","microtubules",,,"triplet"),("(triplets)","(singlets)",,,"bridge"),(" 9"," 2"," 1"," 9"," 9"):}`
B. `{:("Peripheral","Central","Hub","Spokes","Inter"),("microtubules","microtubules",,,"triplet"),("(triplets)","(singlets)",,,"bridge"),(" 9"," 2"," 9"," 9"," 9"):}`
C. `{:("Peripheral","Central","Hub","Spokes","Inter"),("microtubules","microtubules",,,"triplet"),("(triplets)","(singlets)",,,"bridge"),(" 9"," 2"," 1"," 2"," 2"):}`
D. `{:("Peripheral","Central","Hub","Spokes","Inter"),("microtubules","microtubules",,,"triplet"),("(triplets)","(singlets)",,,"bridge"),(" 9"," 0"," 1"," 9"," 9"):}`

Explanation

To determine the correct option regarding the structures visible in the cross-section of a centriole, we need to analyze the structure of the centriole in detail. Here’s a step-by-step breakdown:

### Step 1: Understanding the Structure of a Centriole
- A centriole is composed of microtubules arranged in a specific pattern. The typical arrangement is known as the "9 + 0" arrangement, which means there are 9 peripheral triplets of microtubules and no central microtubules.

### Step 2: Identifying the Components
- Each of the 9 peripheral microtubules is made up of three subfibers (A, B, and C), which is why they are referred to as triplets.
- At the center of the centriole, there is a rod-shaped proteinaceous mass known as the hub.
- From the hub, radial spokes extend towards the peripheral triplets.

### Step 3: Analyzing the Options
1. **Option A**: Peripheral microtubules = 9, Central microtubules = 2, Hub = 1, Spokes = 9, Intertriplet bridges = 9.
- Incorrect because there are no central microtubules (0).

2. **Option B**: Peripheral microtubules = 9, Central microtubules = 2, Hub = 9, Spokes = 9, Intertriplet bridges = 9.
- Incorrect because there are no central microtubules (0) and the hub count is wrong.

3. **Option C**: Peripheral microtubules = 9, Central microtubules = 2, Hub = 1, Spokes = 2, Intertriplet bridges = 2.
- Incorrect because there are no central microtubules (0) and the spokes count is wrong.

4. **Option D**: Peripheral microtubules = 9, Central microtubules = 0, Hub = 1, Spokes = 9, Intertriplet bridges = 9.
- Correct because it accurately reflects the structure of the centriole: 9 peripheral triplets, 0 central microtubules, 1 hub, 9 spokes, and 9 intertriplet bridges.

### Conclusion
The correct option is **D**: Peripheral microtubules = 9, Central microtubules = 0, Hub = 1, Spokes = 9, Intertriplet bridges = 9.

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