Question

Biochemistry: Describe the structural basis for the "10 nm fiber" and the "30 nm fiber" that are often observed in preparations of eukaryotic genomic DNA. (typed or neat hand writing please and thank you!)

Answer 1

DNA packaging in eukaryotes

DNA consists of two chains of polydeoxyribonucleotides chains joined together by Hydrogen bonding between Nitrogen bases ( A (Adenine) bond with T(Thymine) and G (guanine) bonds with C(Cytosine)).

This long chain of DNA has a diameter of 2 nano mete (2X10^-9meter) and is about 2 meters long. The diameter of the nucleus of the cell where the DNA is located is only 5-10 micro meters . So the DNA needs to be tightly packed inorder to be inside the nucleus.

The double helical DNA with 2 nm diameter is wound around a core of basic proteins known as histone octamers. The Histones are Basic (positively charged) proteins and eight histones join together to form an octamer around which the negatively charged DNA is wrapped around and this wrapping is kept in place by linker histone ( the DNA turns 1 ¾ turns around the histone octamer) . This DNA bound to histone octamer forms a’ beads on string’ structure where each of these beads ( Histone octamer plus wrapped DNA) are called nucleosomes. This packaging forms DNA into a structure having a diameter of 10 nm. The DNA coiled around the histone octamer is called core DNA and is of 146 bps and the DNA that connects two nucleosomes is called a linker DNA which is about 54 bp.

This 10nm form further undergoes further condensation (further coiling) to form a 30nm structure called solenoid fiber.