In the "beads on a string" structure of unfolded chromatin, the "beads" are __________.

• A. looped domains
• B. heterochromatin
• C. genes
• D. nucleoids
• E. nucleosomes

Answer: (E)

Verified step by step guidance

1. Understand the context: The 'beads on a string' model is a way to describe the structure of chromatin in its unfolded form. Chromatin is composed of DNA and proteins, and this model helps visualize how DNA is organized within the cell nucleus.
2. Identify the components: In this model, the 'string' represents the DNA molecule, while the 'beads' are structures that help package the DNA.
3. Define nucleosomes: Nucleosomes are the fundamental units of chromatin. They consist of a segment of DNA wound around a core of histone proteins. This structure helps compact the DNA and regulate its accessibility for processes like transcription.
4. Differentiate from other terms: Looped domains refer to larger loops of chromatin that are organized within the nucleus. Heterochromatin is a tightly packed form of DNA, which is generally transcriptionally inactive. Genes are segments of DNA that code for proteins. Nucleoids are structures found in prokaryotic cells, not related to eukaryotic chromatin.
5. Conclude with the correct identification: In the 'beads on a string' model, the 'beads' are nucleosomes, as they represent the repeating units of DNA wrapped around histone proteins, giving the appearance of beads on a string.