Secondary constriction in chromosome is referred to as

Options

A. centromere
B. nucleolar organiser
C. balbiani puffs
D. centriole

Answer

To solve the question regarding the secondary constriction in chromosomes, we will follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Secondary Constriction**:
- Secondary constriction refers to specific regions on a chromosome that are narrower than the rest of the chromosome. These regions are important for certain cellular functions.

2. **Identify Types of Secondary Constrictions**:
- There are two main types of secondary constrictions:
- **Joints**: These are areas involved in the breaking and fusion of chromosome segments.
- **Nucleolar Organizing Regions (NOR)**: These regions are responsible for the formation of the nucleolus during the interphase of the cell cycle.

3. **Evaluate the Options**:
- Option A: **Centromere** - This is the primary constriction of the chromosome and is not considered a secondary constriction.
- Option B: **Nuclear Organizer (NOR)** - This is indeed a type of secondary constriction that plays a role in nucleolus formation.
- Option C: **Balbiani Puffs** - These are regions of chromatin that are actively transcribing RNA, not secondary constrictions.
- Option D: **Centriole** - This is a structure involved in cell division, not related to chromosome constriction.

4. **Select the Correct Answer**:
- Based on the definitions and evaluations, the correct answer to the question "Secondary constriction in chromosome is referred to as" is **Option B: Nucleolar Organizer**.

### Final Answer:
**The secondary constriction in chromosome is referred to as Nucleolar Organizer (NOR).**

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