old hi stones: Q H2A ■ H2B ■ H? fl H4 new bistones: H2A Q H2B H3 □ H4

direction of DNA replication parentaj

DNA replication machinery

DNA replication machinery

HZA-H2B dimer

H3-H4 tetramer nUCleoSOme m

HZA-H2B dimer

H3-H4 tetramer

In experiments that differentially labeled old and new bistones, it was found that the old histones are present on both of the daughter chromosomes (Figure 7-41). Mixing is not entirely random, however. H3'H4 tetramers and H2A-H2B dimers are composed of either all new or ail old histones. Thus, as the replication fork passes, nudeosomes are broken down into their component subassemblies, H3-H4 tetramers appear to remain bound to one of the two daughter duplexes at random and are never released from DNA into the free pool. In contrast, the H2A-H2B dimers are released and enter the local pool available for new nurleosnme assembly.

The distributive inheritance of old histones during chromosome duplication provides a mechanism for the accurate propagation of the parental pattern of hislone modification. By this mechanism, old histones, no matter on which daughter chromosome they end up, tend to be found close, in location, to their position on the parental chromosome (Figure 7-42). This localized inheritance of modified histones provides a limited number of modifications in similar positions on each daughter chromosome. The ability of these modifications to recruit enzymes that add similar modifications to adjacent nudeosomes (see the discussion of bromodomains and chromodomams above) provides a simple mechanism to maintain similar states of modification after DNA replication has occurred« Such mechanisms are likely to play a critical role in the inheritance of chromatin states from one generation to another.

Assembly of Nudeosomes Requires Histone "Chaperooes"

The assembly of nudeosomes is not a spontaneous process. Early studies found that the simple addition of purified histones to DNA

old histones: , H2A

■ H2B

■ H3

■ H4

new histones: H2A

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