In chromatin these are seen as beads-on-string

Options

A. Genes
B. Nucleosomes
C. Centromeres
D. Centrioles

Answer

**Step-by-Step Solution:**

1. **Understanding Chromatin Structure**: Chromatin is a complex of DNA and proteins found in eukaryotic cells. It plays a crucial role in packaging DNA into a more compact, dense shape.

2. **Identifying the Beads-on-String Structure**: When chromatin is viewed under an electron microscope, it appears as a "beads-on-string" structure. The "beads" represent nucleosomes, while the "string" represents the DNA that connects them.

3. **Defining Nucleosomes**: Nucleosomes are the fundamental units of chromatin. Each nucleosome consists of a segment of DNA wound around a core of histone proteins. Specifically, a nucleosome is made up of approximately 200 base pairs of DNA wrapped around a histone octamer, which consists of eight histone proteins.

4. **Analyzing the Options**:
- **Genes**: These are sequences of nucleotides in DNA and do not appear as beads on a string.
- **Centromeres**: These are specific regions of chromosomes that play a role during cell division but do not exhibit the beads-on-string appearance.
- **Centrioles**: These are cylindrical structures involved in cell division but are not related to the chromatin structure.

5. **Conclusion**: The correct answer is **nucleosomes**, as they are the structures that appear as beads on a string in chromatin.

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