Structural Biochemistry/DNA Packaging

DNA Packaging

DNA packaging is an important process in living cells. Without it, a cell is not able to accommodate large amount of DNA that is stored inside. For example, a bacterial cell which ranges from 1 to 2um in length contains amount of DNA that is 400 times as big (Becker et al. 530). Eukaryotic cells face even bigger challenges. A typical human cell has enough “DNA to wrap around the cell more than 15,000 times” (531). Therefore, DNA packaging is crucial because it makes sure that those excessive DNA are able to fit nicely in a cell that is many times smaller.

The DNA in bacterial cells are either circular or linear. To accommodate the size of bacterial cell, supercoiled DNA are folded into loops with each loop resembles shape of bead-like packets containing small basic proteins that is analogous to histone found in Eukaryotes (533).

In eukaryotic cells, DNA packaging is more complicated because they contain amount of DNA that is much larger than that of bacterial cells. More proteins are therefore required for the process with histone being the most important one. This protein is consisted largely of positive amino acids like lysine and arginine which make the overall structure positive. Thus, histone interacts favorably with the negative phosphate groups from DNA. There are five main types of histone, H1, H2A, H2B, H3 and H4 (533). Two of each H2A, H2B, H3 and H4 joins to form an octamer wrapped around by DNA of 146 base pairs like a bead on a string. This bead, consisting of eight histone molecules and 146 DNA base pairs, is known as the nucleosome. Each nucleosome is connected by a DNA linker of 50 base pairs to form a fiber like structure called chromatin. H1 is believed to be found in these DNA linkers. Chromatin fibers can be further compacted to form higher order of structures called heterochromatin or euchromatin depending on the degree of packing. Ultimately, DNA packaging in eukaryotic cells can lead to the formation of chromosome which is only present during cell division or several other situations (533-535). In eukaryotic cells, DNA packaging is not only in the nucleus but is also in mitochondria and chloroplast. The overall shape of their DNA resembles that of bacteria instead that of eukaryotes.

Histone chaperones and the nucleosome assembly processes

Histones are proteins that allow DNA to be tightly packaged into units called nucleosomes. The DNA wraps itself around the histones.

Chromatin is made of DNA and proteins (Histones). Chromatin is used to give structure to a chromosome.

Nucleosome consists of the acidic chromatin and the basic histone proteins.

Histone chaperones Histone chaperone guided folding pathways, assists in the folding and unfolding of the DNA around the histone.

Organization The tight coiling of DNA allows easier access to the DNA which makes sequencing faster.

Need for histone chaperones Nucleosomes can be assembled or disassembled and are done in stepwise function. Histone chaperones guide the pathway process, they control and regulate.

Structural forms of histone chaperones Since histone chaperones participate at each step of the nucleosome assembly processes, there are different chaperones needed for each different step.

Reference

Becker, Wayne M, et al. The World of the Cell. 7th ed. New York: Pearson/Benjamin Cummings, 2009. Print.

Churchill, Das, Tyler The histone shuffle: histone chaperones in an energetic dance