Phosphorylation of serines and methylation and acetylation of lysines in histone tails affect the stability of chromatin structure above the nucleosome level and have important consequences for gene expression. Draw the structures for serine and phosphoserine, as well as those for lysine, monomethylated lysine, and acetylated lysine. Which modifications alter the net charge on a histone tail? Would you expect the changes in charge to increase or decrease the ability of the tails to interact with DNA?