Incorrect base pairing leads to dramatically lower rates of nucleotide addition as a result of a catalytically unfavorable alignment of these substrates. This is an example of kinetic proofreading, in which an enzyme favors catalysis using one of several possible substrates by dramatically increasing the rate of bond formation only when the correct substrate is present. 

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#Unit 2. Cellular Organization

Histone H3.1 is replicative histone.

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#Unit 2. Cellular Organization

H3.3 is expressed throughout the cell cycle, in contrast to most histones that are expressed during S phase during replication.

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#Unit 2. Cellular Organization

H3.3 is available for assembly at any time in the cell cycle, and is incorporated at sites of active transcription, where nucleosomes become disrupted.

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#Unit 2. Cellular Organization

H3.3 is often referred to as a “replacement” histone.

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#Unit 2. Cellular Organization

The H2A variants are the largest and most diverse family of core histone variants.

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#Unit 2. Cellular Organization

H2AX is normally present in only 10%–15% of the nucleosomes in multicellular eukaryotes