Question

Describe the transitions that occur as nucleosomes are coiled and folded, ultimately forming a chromatid.

Short answer

Question: Describe the transitions that occur during the formation of a chromatid from nucleosomes, including the structural intermediates involved in this process. Answer: The formation of a chromatid from nucleosomes involves several transitions, including the binding of histone H1 to form chromatosomes, coiling into the 30-nm fiber, folding into the 300-nm fiber, creating radial loops anchored by scaffold proteins, and compaction into the final structure of a chromatid. These structural intermediates play significant roles in DNA compaction and organization.

Step-by-step solution

Starting with Nucleosomes

Nucleosomes are the basic structural units of chromatin, which consist of around 147 base pairs of DNA wrapped around a histone octamer (two copies each of histone H2A, H2B, H3, and H4). The DNA connected to nucleosomes is referred to as "linker DNA." Understanding nucleosomes is the first step in describing the transitions that occur while forming a chromatid.

Formation of Chromatosome

A chromatosome forms when histone H1 binds to the nucleosome. Histone H1 binds to the linker DNA outside the core histone octamer and helps stabilize the DNA wrapped structure of the nucleosome, ultimately compacting the DNA.

30-nm Fiber Formation

The next level of DNA compaction occurs by the formation of the 30-nanometer (nm) fiber. The chromatosomes approach each other by coiling to form a helical structure, which is the 30-nm chromatin fiber. This compact structure is formed by the interaction of H1 histone tails with nearby nucleosomes, folding the nucleosome chain into the helical shape.

300-nm Fiber Formation

However, the DNA is not yet sufficiently compact. The 30-nm fibers are further folded into a larger fiber, known as the 300-nm fiber. This fiber is organized in a highly structured, solenoid-like arrangement with six nucleosomes per turn.

Radial Loop Formation

Following the formation of the 300-nm fiber, the DNA compaction process continues with the formation of radial loops. The loops are further anchored to scaffold proteins, which belong to the nuclear matrix.

Chromatid Formation

Eventually, the radial loop structure undergoes further compaction, and ultimately, the chromatid forms. Two identical sister chromatids are connected by a centromere. By the end of this process, the initial nucleosome structure is condensed into a chromatid, which is roughly 10,000 times shorter. In conclusion, the transitions that occur during the formation of a chromatid from nucleosomes include the binding of histone H1 to form chromatosomes, coiling into the 30-nm fiber, folding into the 300-nm fiber, creating radial loops anchored by scaffold proteins, and compaction into the final structure of a chromatid.