Question

Discuss the events in the packaging of DNA to the final event, the chromosome. Be sure to mention names and sizes.

Answer 1

1. The simplest level is the double stranded helical structure of DNA. This form is the naked DNA which is without proteins. (2 nm)
2. DNA is first associated with the proteins to form nucleosomes (11 nm). These proteins are the negatively charged proteins called as histones. The core nucleosome particle consists of the histone proteins, H2A, H2B, H3, and H4. The dimer of H2A & H2B and H3 & H4 forms the octamer. This model is known as beads on string model. This results in seven fold compaction of DNA.
3. This nucleosome particle when associated with the linker H1 histone particle results in the chromatosome.
4. Then the nucleosome is further folded up more to result in the thicker and compact 30nm chromatin fiber (30nm). In this structure, the nucleosome folds up into the solenoid form.
5. This 30 nm fiber then further folds to produce more compact loops (300 nm).
6. These loops further condense to form wide fiber (700 nm).
7. During the metaphase stage during the cell cycle, the DNA is present in most compact form. The 700 nm fiber now condenses to form the compact chromosomes (1400 nm). If the comparison is drawn between metaphase chromosome and naked DNA, the packaging ratio will be 10,000:1.

The DNA become so compact that it is now inaccessible to the replication and transcription. So that to make the DNA available, the modification of histones as well as displacement of histones using chromatin remodeling enzyme is done.