Bead-like thickened portions of leptotene chromosomes are

Options

A. puffs
B. chromomeres
C. centromeres
D. centrosome

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the bead-like thickened portions found in leptotene chromosomes. We need to identify the correct term that describes these structures.

2. **Identifying Leptotene Stage**: Leptotene is the first stage of prophase I in meiosis. During this stage, chromosomes start to condense and become visible.

3. **Recognizing Chromosome Structure**: As chromosomes condense, they do not appear as solid rods but rather have a beaded appearance. This is due to the presence of certain structures along the length of the chromosome.

4. **Defining the Bead-like Structures**: The bead-like structures on the leptotene chromosomes are known as chromomeres. These are localized thickened areas that represent regions of coiled chromatin.

5. **Understanding Chromomeres**: Chromomeres are formed due to the coiling of chromatin fibers. They are also referred to as idiomeres.

6. **Evaluating the Options**: The options given are:
- A) Pups
- B) Chromomeres
- C) Centromeres
- D) Centrosomes

Among these, the correct answer is B) Chromomeres, as they specifically refer to the bead-like structures on leptotene chromosomes.

7. **Conclusion**: Therefore, the answer to the question is that the bead-like thickened portions of leptotene chromosomes are called chromomeres.

**Final Answer**: B) Chromomeres

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