Questions: The formation of what structure would be impaired if a cell was lacking CTCF proteins? Multiple Choice Nucleosome 30-nm fiber Loop domains Double helix

The answer is the third one: Loop domains.

Explanation for each option:

1. Nucleosome: Nucleosomes are the basic units of DNA packaging in eukaryotic cells, consisting of a segment of DNA wound around a core of histone proteins. CTCF proteins are not directly involved in the formation of nucleosomes.

2. 30-nm fiber: This is a higher-order structure of chromatin, where nucleosomes are further coiled into a more compact form. The formation of the 30-nm fiber primarily involves histone interactions and is not directly dependent on CTCF proteins.

3. Loop domains: CTCF proteins are known to play a crucial role in the formation of loop domains in the genome. They act as insulators and are involved in the organization of chromatin into loops, which are important for regulating gene expression and maintaining the structure of the genome. Therefore, if a cell was lacking CTCF proteins, the formation of loop domains would be impaired.

4. Double helix: The double helix refers to the structure of DNA itself, consisting of two strands of nucleotides wound around each other. This structure is not dependent on CTCF proteins.

In summary, CTCF proteins are essential for the formation of loop domains, so their absence would impair this structure.