The Question is from neet-botany , and it was created by the author NISHANTH

Question

A researcher isolates chromatin fibers and treats them with a nuclease that specifically cleaves linker DNA.  Following this treatment, what would be the approximate size distribution of the DNA fragments observed?

NISHANTH · Accepted Answer

## CORE INFORMATION
✓ **Answer**: A
## CONCEPT FRAMEWORK
	**Primary Concept**: Chromatin Structure and Nucleosome Organization
	**Related Topics**: DNA packaging, nucleases, linker DNA, histone proteins
	**Prerequisites**: Understanding of DNA and its organization in eukaryotes.
## SOLUTION PATHWAY
**Given Information**:
	Chromatin fibers are treated with a nuclease specific to linker DNA.
**Method & Steps**:
	1. **Understand Chromatin Structure**: Chromatin is composed of DNA wrapped around histone proteins forming nucleosomes. These nucleosomes are connected by stretches of linker DNA.
		Reasoning:  The 'beads-on-a-string' model describes this structure, where nucleosomes are the beads and linker DNA is the string.
	2. **Nuclease Action**: The nuclease specifically targets and cleaves the linker DNA between nucleosomes.
		Working: This digestion process separates individual nucleosomes from each other.
		Key Point: The DNA wrapped around each nucleosome core remains protected from the nuclease.
	3. **Resulting Fragments**:  The DNA associated with each nucleosome core particle is approximately 147 base pairs.  Along with the small amount of linker DNA that is protected by the histone core, the total DNA fragment size after digestion is roughly 200 bp.
		Result: Fragments of approximately 200 bp in length.
		Verification: This size corresponds to the DNA wrapped around a single nucleosome plus the small protected linker segments, confirming the action of the linker-specific nuclease.
 ## OPTION ANALYSIS
[A] ✓: Approximately 200 bp: This is the correct answer, as explained above.  The nuclease cleaves the linker DNA, leaving DNA fragments of approximately 200 base pairs that represent the DNA wrapped around each nucleosome.
[B] :x: Approximately 1000 bp: This is incorrect. 1000 bp would represent multiple nucleosomes still linked together, indicating incomplete digestion of the linker DNA.
[C] :x: Approximately 50 bp: This is incorrect. 50 bp is too small to represent the DNA associated with a nucleosome.
[D] :x: A wide range of sizes from 200 bp to several thousand bp: This is incorrect. While incomplete digestion could theoretically produce a range of sizes, a nuclease specific for linker DNA should produce a relatively uniform distribution around 200 bp.
 ## LEARNING AIDS
	**Common Mistakes**:
		1. Confusing nucleosome size with linker DNA size: Remember that the ~200 bp fragment includes both the DNA wrapped around the histone core and a small portion of protected linker DNA. 
		2.  Not understanding the specificity of the nuclease: If the nuclease wasn't specific to linker DNA, it would cleave at various points along the DNA, resulting in a wide range of fragment sizes.
	**Quick Tips**:
		1. Visualize the 'beads-on-a-string' model: Imagine the nuclease cutting the string between the beads.
		2. Remember the approximate size of DNA wrapped around a nucleosome: ~147 bp, which with the short protected linker regions adds up to approximately 200 bp.

NISHANTH · Answer

The correct Option is "Approximately 200 bp"

NISHANTH · Answer

Approximately 1000 bp

NISHANTH · Answer

Approximately 50 bp

NISHANTH · Answer

A wide range of sizes from 200 bp to several thousand bp

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