In most cells, DNA is wrapped around histones. However, when sperm cells are formed, these histones are replaced by small, positively charged molecules called protamines. Understanding the mechanism of this histone-to-protamine replacement pathway is crucial to understanding the passing of epigenetic traits (traits that are influenced by modifications to DNA other than its sequence) between generations. Previous data point to an active displacement model of replacement in which protamines bind directly to the DNA, causing a conformational change that removes the histones. The data also suggest a possible mechanism of replacement in which there are eight histones to begin, and protamine kicks off histones dimer by dimer. In order to investigate this hypothesis, we use atomic force microscopy (AFM) to visualize the DNA wrapped by histones and extract key variables such as length of the DNA and width and height of the histones that tell us the folded state of the DNA. The first step in this process is to make a nucleosome core particle, which is the starting state of the DNA wrapped around eight histones. Therefore, we started by creating chromatin with nucleosomes and then used a partial digestion assay to see if the chromatin had formed. The next step is to image the chromatin in the presence and absence of histones with the AFM. Here, we will present work toward making full nucleosomes and visualizing the DNA. Visualizing states in the histone-to-protamine replacement pathway is important for understanding how epigenetic information is passed down.
Yan et al. (Sun,) studied this question.