Figure 3.

 Diagram of the chromosome methylation changes occurring during preimplantation development of mouse embryos. (A) Chromosome patterns obtained after 5-MeC antibody binding. (B) Summary of DNA methylation alterations that are compatible with the observed chromosome patterns. (A) At the one-cell stage, maternal and paternal chromosomes display heterogeneous but symmetrical labeling of chromatids. Maternal chromosomes are labeled more intensely than paternal chromosomes and display an R-like banding pattern. At the two-cell stage, maternal and paternal chromosomes become asymmetrically labeled, maternal chromosomes always being stained more strongly than paternal chromosomes. From the four-cell stage to blastocyst the number of asymmetrical chromosomes halves after each S phase. At morula (16/32 cells) and blastocyst stages, two cell populations differing by their chromosome condensation can be distinguished: in the first, chromosomes are normally condensed (A, left); in the second, chromosomes are thick and their condensation seems relaxed (A, right). (B) The presence of asymmetrical chromosomes after two S-phases following fertilization (two-cell stage) indicates that after the first S phase (one-cell stage, first metaphase), DNA is hemimethylated in both chromatids, and after the second S phase (two-cell stage, second metaphase), DNA is hemimethylated in one chromatid and demethylated in the other one. As the number of asymmetrical chromosomes halve as cleavage progresses, a failure in maintenance activity on the new DNA strand of each S phase can explain the progression of demethylation. (S) Sphase; (○) unmethylated CpGs; (•) methylated CpGs.