In biology, histones are highly basic proteins abundant in lysine and arginine residues that are found in eukaryotic cell nuclei. They act as spools around which DNA winds to create structural units called nucleosomes. Nucleosomes in turn are wrapped into 30-nanometer fibers that form tightly packed chromatin. Histones prevent DNA from becoming tangled and protect it from DNA damage. In addition, histones play important roles in gene regulation and DNA replication. Without histones, unwound DNA in chromosomes would be very long. For example, each human cell has about 1.8 meters of DNA if completely stretched out, however when wound about histones, this length is reduced to about 90 micrometers (0.09 mm) of 30 nm diameter chromatin fibers.
A nucleosome is the basic structural unit of DNA packaging in eukaryotes. The structure of a nucleosome consists of a segment of DNA wound around eight histone proteins and resembles thread wrapped around a spool. The nucleosome is the fundamental subunit of chromatin. Each nucleosome is composed of a little less than two turns of DNA wrapped around a set of eight proteins called histones, which are known as a histone octamer. Each histone octamer is composed of two copies each of the histone proteins H2A, H2B, H3, and H4.
Histone acetyltransferases (HATs) are enzymes that acetylate conserved lysine amino acids on histone proteins by transferring an acetyl group from acetyl-CoA to form ε-N-acetyllysine. DNA is wrapped around histones, and, by transferring an acetyl group to the histones, genes can be turned on and off. In general, histone acetylation increases gene expression.
DNA replication licensing factor MCM6 is a protein that in humans is encoded by the MCM6 gene. MCM6 is one of the highly conserved mini-chromosome maintenance proteins (MCM) that are essential for the initiation of eukaryotic genome replication.
Eukaryotic DNA replication is a conserved mechanism that restricts DNA replication to once per cell cycle. Eukaryotic DNA replication of chromosomal DNA is central for the duplication of a cell and is necessary for the maintenance of the eukaryotic genome.
Histone-binding protein RBBP4 is a protein that in humans is encoded by the RBBP4 gene.
DNA replication licensing factor MCM2 is a protein that in humans is encoded by the MCM2 gene.
DNA replication licensing factor MCM3 is a protein that in humans is encoded by the MCM3 gene.
DNA replication licensing factor MCM4 is a protein that in humans is encoded by the MCM4 gene.
Histone H2B type 2-E is a protein that in humans is encoded by the HIST2H2BE gene.
Chromatin assembly factor 1 subunit A is a protein that in humans is encoded by the CHAF1A gene.
Chromatin assembly factor 1 subunit B is a protein that in humans is encoded by the CHAF1B gene.
Histone chaperone ASF1A is a protein that in humans is encoded by the ASF1A gene.
Protein HIRA is a protein that in humans is encoded by the HIRA gene. This gene is mapped to 22q11.21, centromeric to COMT.
Histone H3.1 is a protein that in humans is encoded by the HIST1H3C gene.
Histone H4 is a protein that in humans is encoded by the HIST1H4F gene.
Histone chaperone ASF1B is a protein that in humans is encoded by the ASF1B gene.
Nucleosome assembly protein 1-like 4 is a protein that in humans is encoded by the NAP1L4 gene.
Origin recognition complex subunit 1 is a protein that in humans is encoded by the ORC1 gene. It is closely related to CDC6, and both are the same protein in archaea.
Chromatin assembly factor-1 (CAF-1) is a protein complex — including Chaf1a (p150), Chaf1b (p60), and p50 subunits — that assembles histone tetramers onto replicating DNA. CAF-1 functions as a histone chaperone that mediates the first step in nucleosome formation by tetramerizing and depositing newly synthesized histone H3/H4 onto DNA rapidly behind replication forks. Several studies have shown that the interaction between CAF-1 and PCNA, which stabilizes CAF-1 at replication forks, is important for CAF-1's role in nucleosome assembly
