Proteins that bind to DNA and package it into compact structures to form nucleosomes —integral structural components of chromosomes. The compact packaging of DNA must be relaxed somewhat for DNA copying (replication) and gene expression (‘transcription’) to occur. Modification of histones through the attachment of acetyl (-COCH3) or methyl (-CH3) groups (acetylation or methylation), and of biotin (biotinylation) has been shown to affect the structure of histones, thereby affecting replication and transcription of DNA.