Final answer:
DNA compaction involves histone proteins and scaffold proteins that organize DNA into various structures, like the 10nm beadlike nucleosomes and the 30nm solenoid fibers, and eventually condensing into higher-level structures for cell division.
Step-by-step explanation:
DNA Packing and Associated Proteins
DNA packaging is a critical process allowing the long DNA molecules to fit within the microscopic cell nucleus. This compaction involves several levels of organization, each associated with specific proteins. In the context of eukaryotic cells, such as human cells, this intricate packing is necessary due to the considerable length of DNA needing to be accommodated inside a relatively small nuclear space.
10nm Fiber: Beads on a String
The compaction process begins with the formation of nucleosomes, which are structures consisting of DNA wrapped around core histone proteins. These proteins include two each of H2A, H2B, H3, and H4, creating an octamer around which approximately 146 base pairs of DNA are wound. This nucleosome structure resembles beads on a string, with each bead being roughly 10nm in diameter. The string between these beads consists of linker DNA, usually associated with another histone type, H1.
30nm Fiber: Solenoid Structure
The 10nm fiber is then coiled into a denser 30nm fiber. This occurs as nucleosomes stack onto each other with the assistance of a fifth type of histone, often H1, which leads to the solenoid structure. The compaction at this stage makes the DNA molecule approximately 50 times shorter than its extended form.
300nm Fiber
The 30nm fiber further folds to form a series of looped structures. Scaffold attachment proteins, including histone H1 and topoisomerase II among others, bind to these loops, creating a structure composed of rosettes. This level of organization contributes to a significant compaction of DNA.
1400nm Fiber
During the metaphase stage of cell division, the chromosomes attain their highest level of compaction, sometimes reaching up to 700nm in width. This occurs due to the association of DNA with scaffold proteins, which help to tightly pack chromatin into chromosomes. This stage of the cell cycle ensures that the DNA is sufficiently compacted for efficient segregation to the daughter cells.