What is the second structural level of DNA packaging after nucleosomes?

The second structural level of DNA packaging after the formation of nucleosomes is known as the 30 nm fiber. During this process, the nucleosomes, which consist of DNA wrapped around histone proteins, further fold into a thicker, more compact structure. This 30 nm fiber represents a higher order of chromatin organization where the nucleosomes cohere in a coiled arrangement, allowing for more efficient packaging of DNA within the nucleus.

The formation of the 30 nm fiber is essential for DNA to fit inside the cell nucleus and also plays a critical role in regulating gene expression. This level of packaging is dynamic and can change based on the needs of the cell, such as during replication or transcription.

In contrast, the 10 nm fiber represents the loosely packed structure of nucleosomes, where the DNA is still relatively extended. The chromatin fiber and 300 nm fiber refer to even higher levels of organization that may involve additional folding and looping mechanisms, but the specific second level after nucleosomes is recognized as the 30 nm fiber.