Nucleosome segregation at a defined mammalian chromosomal site

Area of Science:

• Molecular Biology
• Epigenetics
• Cell Biology

Background:

• Chromatin replication is essential for cell division.
• Nucleosomes, the basic units of chromatin, play a crucial role in DNA packaging and regulation.
• Understanding nucleosome segregation during replication is key to comprehending genome stability and inheritance.

Purpose of the Study:

• To investigate the segregation pattern of preexisting nucleosomes during DNA replication in animal cells.
• To determine the fate of nucleosomes when new histone synthesis is inhibited.
• To analyze the impact of nucleosome segregation on DNA replication origins, specifically the simian virus 40 (SV40) origin.

Main Methods:

• Utilizing a simian virus 40 (SV40)-transformed Chinese hamster cell clone with integrated viral DNA.
• Labeling newly replicated DNA with 5-bromodeoxyuridine (BrdUrd) in the presence and absence of the protein biosynthesis inhibitor emetine.
• Employing nuclease digestion and strand-specific analysis to resolve nucleosome-protected DNA sequences and their parental/progeny origins.

Main Results:

• When new histone biosynthesis is blocked, daughter DNA molecules are produced with half lacking nucleosomes.
• Preexisting nucleosomes preferentially segregate to only one of the two daughter DNA duplexes during replication.
• The viral replication origin (ORIsv) showed altered function in the presence of emetine, suggesting a role for protein synthesis in origin activity.

Conclusions:

• Nucleosome segregation during replication is asymmetric, with existing nucleosomes preferentially associating with one daughter DNA strand.
• This asymmetric segregation can influence the accessibility and function of DNA replication origins.
• Inhibition of protein synthesis, potentially affecting viral large tumor antigen, can disrupt the function of the integrated viral replication origin.