Nucleolar organizer is a :-

Options

A. Primary constriction
B. Secondary constriction
C. Tertiary constriction
D. Centriole

Answer

To determine what a nucleolar organizer is, we can follow these steps:

### Step 1: Understand the Definition
A nucleolar organizer is a specific chromosomal region that is associated with the formation of the nucleolus.

**Hint:** Look for terms related to the nucleolus and its formation in the context of chromosomes.

### Step 2: Identify the Function
The nucleolar organizer region (NOR) contains genes that encode ribosomal RNA (rRNA), which are essential for ribosome formation and function.

**Hint:** Focus on the role of the nucleolar organizer in ribosome production.

### Step 3: Recognize the Chromosomal Structure
The nucleolar organizer is characterized by secondary constrictions in the chromosomes. These are specific regions where the nucleolus forms during cell division.

**Hint:** Consider the types of constrictions in chromosomes and their relevance to the nucleolus.

### Step 4: Distinguish Between Constrictions
There are different types of constrictions in chromosomes:
- **Primary constriction**: This is the centromere.
- **Secondary constriction**: This is where the nucleolar organizer is located.
- **Tertiary constriction**: This is not typically associated with nucleolar organization.
- **Centriole**: This is a structure involved in cell division but is not related to the nucleolar organizer.

**Hint:** Compare the definitions and functions of primary, secondary, and tertiary constrictions to clarify their roles.

### Step 5: Conclusion
Based on the information gathered, the nucleolar organizer is best defined as a **secondary constriction** of chromosomes, which is crucial for the formation of the nucleolus.

**Final Answer:** The nucleolar organizer is a **secondary constriction**.

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