Core component of nucleosome. Nucleosomes wrap and compact DNA into chromatin, limiting DNA accessibility to the cellular machineries which require DNA as a template. Histones thereby play a central role in transcription regulation, DNA repair, DNA replication and chromosomal stability. DNA accessibility is regulated via a complex set of post-translational modifications of histones, also called histone code, and nucleosome remodeling. The nucleosome is a histone octamer containing two molecules each of H2A, H2B, H3 and H4 assembled in one H3-H4 heterotetramer and two H2A-H2B heterodimers. The octamer wraps approximately 147 bp of DNA. The different late H2A and H2B mRNAs are present in as few as 200 copies in the egg and each accumulate to 3-5 x 100000 molecules in the gastrula embryo. Monoubiquitination of Lys-119 gives a specific tag for epigenetic transcriptional repression. Phosphorylation of Ser-2 directly represses transcription. Belongs to the histone H2A family.