While working on Drosophila, you find a new mutant strain with an abnormal histone H3 gene. This novel histone mutant is predicted to cause the nucleosomes to bind an extra 15 bp of DNA compared to wild type. If you digest isolated chromatin with a nuclease to release the core nucleosomes, what size DNA fragments would you expect from wild-type and mutant flies?

Question

Quizplus · Accepted Answer

The Answer is In wild-type Drosophila, the core nucleosome consists of DNA wrapped around histone proteins. The core particle is typically associated with about 147 base pairs (bp) of DNA. This is the length of DNA that is protected from nuclease digestion when the chromatin is treated with enzymes such as micrococcal nuclease, which preferentially digests the linker DNA between nucleosomes.

If you have a mutant histone H3 that causes nucleosomes to bind an extra 15 bp of DNA, then the mutant nucleosomes would be expected to protect 147 bp (the normal length) + 15 bp (the additional length due to the mutation) from nuclease digestion. Therefore, the size of the DNA fragments you would expect to see after nuclease digestion of the mutant chromatin would be 162 bp.

To summarize:

- Wild-type nucleosomes: ~147 bp DNA fragments
- Mutant nucleosomes: ~162 bp DNA fragments

These sizes represent the DNA that is tightly bound to the nucleosome core and protected from nuclease digestion. The actual size of the fragments could vary slightly due to the presence of linker DNA, which connects adjacent nucleosomes and can vary in length. However, the core protected DNA would be expected to show the sizes mentioned above.

While Working on Drosophila, You Find a New Mutant Strain