Chromosome Structure Proteins

 Chromosome Structure Proteins Background

In 1975 Oudet et al. presented evidence that chromatin structure is comprised of a repeating unit that they described as “beads on a string”. Those beads are now known as nucleosomes, which are small protein discs that DNA (the ‘string’) wraps around. These nucleosomes are separated by 10-60 bp of linker DNA and coil to form an array along the DNA that is about 11 nm in diameter. This array is then supercoiled into a 30 nm fiber, which winds upon itself into the microscopic unit known as a chromosome. The nucleosome is comprised of 146 bp of DNA wrapped 1.7 times around a histone octamer, comprised of a histone H3/H4 tetramer flanked by two H2A/H2B dimers. Nucleosomes serve to package the DNA while still allowing access for gene transcription and DNA replication and repair. In addition to the electromicrographs showing beads on a string, nucleosomes were also identified by digestion of chromatin with nucleases resulting in approximately 200-bp ladders as well as by centrifugal isolation of 11.5S nucleoprotein complexes. It has long been thought that the behavior of chromatin is a direct result of the properties of these nucleosomes. Chromatin is now known to consist of DNA and histones, as well as a plethora of other protein complexes that assist in the DNA-related cellular functions. Histones play both structural and functional roles in these processes, which include replication, repair, recombination, transcription, and segregation.