Figure 8. Illustration depicting proposed mechanism for DNA replication delay in the paternal pronucleus of H3.1- or H3.2-overexpressing embryos.

In the perinucleolar region, K27 methyltransferase is active only in the paternal pronucleus. Ectopically incorporated H3.1/2 is trimethylated at the H3K27 in the paternal pronucleus alone. H3.1/2-H3K27me3 may contribute to the formation of a tight chromatin structure, leading to a delay in DNA replication in the perinucleolar region of the paternal pronucleus. H3.1/2 deposition in the pronuclei of zygotes is limited by reducing mRNA expression and histone incorporation into chromatin.