Are histone proteins present around DNA in the nucleus during all of interphase (including G1) or do histone proteins only form later on when chromosomes are condensing into chromatids?
Thanks in advance!
Yes, DNA is always wrapped around histones.
DNA condensation using histones is not only meant to form the chromatids for mitosis/meiosis, but also one of the factors that control gene expression. Tightly packed heterochromatin is not being expressed, whereas unpacked euchromatin is transcriptionally active. The process of condensation is controlled by enzymes such as histone actelytransferase (HAT) and deacetylase (HDAC) that change the electrostatic properties of the histone proteins. Positively charged lysine in heterochromatin histones allows condensation with negatively charged DNA. By acetylation of these lysines, they become neutral and the heterochromatin turns into euchromatin, which, provided the necessary activators etc., can now be translated. To silence gene expression, the process is reversed.
Gene expression control is very complex and histone acetylation is only one factor out of many. Nonetheless, you see that histones are a crucial part of the chromosomes and are not only formed during mitosis or meiosis.