How Many Such Beads Nucleosomes do you Imagine are Present in a Mammalian Cell?

The DNA helix in a typical nucleosome is 200 bp long. Thus if we divide total number of base pairs with number of base pairs per nucleosome we can conclude that there must be approx 30 million nucleosomes in a mammalian cell, while the actual amount is unknown.

Let's discuss this in detail :

A nucleosome is the primary structural element of DNA packaging in eukaryotes.A nucleosome's structure, which resembles thread looped around a spool, is made up of a piece of DNA wound around eight histone proteins. The core component of chromatin is the nucleosome. A histone octamer, which is an eight-protein group called histones, surrounds a little less than two twists of DNA in each nucleosome.

To fit inside the cell nucleus, DNA must be condensed into nucleosomes. Eukaryotic chromatin is further compressed by folding into a succession of increasingly complicated structures in addition to nucleosome wrapping, which results in the formation of a chromosome. The histone octamer, which is made up of two copies of the core histones H2A, H2B, H3, and H4, is wrapped around 146 base pairs (bp) of DNA in 1.67 left-handed superhelical twists to form the nucleosome core particle. Links in the linker DNA can be up to 80 bp long and connect the core particles.

Linker histones, such as H1 and its isoforms, lie at the base of the nucleosome close to the DNA entry and exit and bind to the linker region of the DNA. They are involved in chromatin compaction. Under an electron microscope, non-condensed nucleosomes without the linker histone resemble "beads on a string of DNA.

Protamines are primarily used by mature sperm cells to package their genomic DNA, in contrast to the majority of eukaryotic cells, most likely leading to an even greater packaging ratio. The discovery of histone equivalents and a condensed chromatin structure in Archaea suggests that nucleosome usage is not limited to eukaryotes.

Nucleosome interactions

• Proteins

The "histone fold," a distinctive structural motif found in the core histone protein, is composed of three alpha-helices (1-3) separated by two loops (L1-2). The histones create H2A-H2B heterodimers and H3-H4 heterotetramers in solution. In the case of histones H3 and H4, two of these dimers combine to create a 4-helix bundle that is stabilised by intense H3-H3' contact. Histones dimerize about their long 2 helices in an antiparallel orientation. Due to interactions between H4 and H2B, which entail the creation of a hydrophobic cluster, the H2A/H2B dimer attaches to the H3/H4 tetramer.

• Histone - DNA interactions

In addition to many hundred water-mediated interactions, the nucleosome has around 120 direct protein-DNA interactions. Direct protein-DNA interactions are not equally distributed throughout the surface of the octamer, but rather are concentrated at specific places. These result from the development of two different types of DNA binding sites in the octamer, the L1L2 site and the 11 site, which utilise the 1 helix from two nearby histones.

The majority of interactions with the DNA are formed through salt linkages and hydrogen bonds between side-chain basic and hydroxyl groups, main-chain amides, and the phosphates in the DNA backbone. This is significant because nucleosome distribution throughout genomes necessitates the existence of a non-sequence-specific DNA-binding factor. Although nucleosomes tend to prefer particular DNA sequences over others, they can attach to virtually any sequence, which is thought to be because these water-mediated interactions can arise in a variety of ways.

An arginine side-chain intercalates into the DNA minor groove at all 14 spots where it contacts the octamer surface, and non-polar connections between protein side-chains and the deoxyribose groups are also made. The DNA within the nucleosome core is altered depending on the number and density of DNA-binding sites on the octamer surface. The DNA has twist faults as well as an uneven bend. In solution, free B-form DNA twists at a rate of 10.5 bp per turn. The nucleosomal DNA twist, which ranges from a value of 9.4 to 10.9 bp per turn, is only 10.2 bp overall.