Histone Major Subtypes and Variants

Cited from Wiki "Histone"

"Histones are subdivided into canonical replication-dependent histones that are expressed during the S-phase of cell cycle and replication-independent histone variants, expressed during the whole cell cycle. In animals, genes encoding canonical histones are typically clustered along the chromosome, lack introns and use a stem loop structure at the 3’ end instead of a polyA tail. Genes encoding histone variants are usually not clustered, have introns and their mRNAs are regulated with polyA tails."

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Cited from the Paper "Histone variants: deviants?"

"In most organisms, there are multiple copies of the histone genes encoding for the major histone proteins. These genes are highly similar in sequence, expressed primarily during the S phase of the cell cycle, and code for the bulk of the cellular histones. While histones are among the slowest evolving proteins known, there are nonallelic variants of the major histones that can have significant differences in primary sequence. Some variants have distinct biophysical characteristics that are thought to alter the properties of nucleosomes, while others localize to specific regions of the genome. The variants are usually present as single-copy genes that are not restricted in their expression to the S phase but are expressed throughout the cell cycle. Unlike the major subtypes, the variant genes contain introns and the transcripts are often polyadenylated."