DNA Replication and Mark Propagation

DNA replication is the moment of greatest challenge for epigenetic memory: as the fork passes, nucleosomes are displaced, parental histones are split between two daughter strands, and DNA methylation becomes transiently hemimethylated. How chromatin marks are copied or restored during and after replication determines whether expression states survive into the next cell generation.

Definition

Mark propagation at DNA replication is the set of processes by which DNA methylation and histone modifications are copied to, or re-established on, the newly replicated daughter strands so that the parental chromatin state is inherited rather than lost.

Scope

The topic covers the molecular events at and behind the replication fork that propagate epigenetic information: maintenance DNA methylation of newly copied CpG sites, the recycling and segregation of parental histones, and the restoration of histone modifications on daughter chromatin. It is a reference topic in molecular biology and does not provide clinical guidance.

Core questions

• How is DNA methylation copied to the new strand at hemimethylated sites after replication?
• How are parental histones recycled and distributed between the two daughter strands?
• How are histone modifications, which are not template-copied, restored to full density before the next division?

Key concepts

• Hemimethylated CpG recognition
• Maintenance methyltransferase DNMT1 and UHRF1
• Histone recycling and deposition
• Symmetric segregation of parental histones
• Restoration of modifications behind the fork
• Replication timing and chromatin state

Key theories

Recycled-histone seeding of mark restoration

Parental modified histones are recycled onto both daughter strands at replication, where they serve as seeds; writer enzymes recognize the residual mark and copy it onto adjacent new histones, restoring the modification pattern - a proposed mechanism for inheriting histone-based states that are not directly templated.

Mechanisms

Semiconservative DNA replication leaves each new CpG site hemimethylated; the maintenance methyltransferase DNMT1, recruited via UHRF1 which binds hemimethylated DNA, copies the methylation pattern to the daughter strand. For histones, parental nucleosomes are evicted ahead of the fork and their components are recycled onto both daughter strands, mixed with newly synthesized, largely unmodified histones, which halves the local density of any modification. Replisome-associated machinery helps distribute parental histones roughly symmetrically to the two strands, and writer enzymes then re-instate modifications using the recycled histones as templates. DNA methylation maintenance and replication-coupled histone restoration together allow the parental chromatin state to be reconstituted before the next cell cycle.

Clinical relevance

Errors in maintenance methylation and replication-coupled chromatin assembly are discussed in relation to genome instability and disease, and the topic is part of foundational education on how heritable chromatin states are kept faithful. It describes molecular processes and is not a basis for individual diagnosis or treatment.

History

The idea that DNA methylation could be copied at hemimethylated sites was proposed when methylation patterns were first described, and was substantiated by the identification of maintenance methyltransferase activity and, later, of UHRF1 as the reader that recruits DNMT1 to hemimethylated DNA. In parallel, decades of work on chromatin assembly clarified how parental histones are recycled at the fork, with more recent studies addressing how their distribution to the two daughter strands is controlled.

Debates

How symmetric is parental-histone segregation, and does it matter for memory?

Whether recycled parental histones are distributed equally to both daughter strands, and how strongly asymmetry would bias inheritance of chromatin states, is an active question studied through replisome components that influence histone deposition.

Key figures

• Genevieve Almouzni
• Steven Jacobsen
• Zhiguo Zhang
• Anja Groth

Seminal works

• probst-2009
• bostick-2007
• margueron-reinberg-2011

Frequently asked questions

How is DNA methylation copied when a cell divides?

After replication each site is hemimethylated; UHRF1 recognizes the hemimethylated DNA and recruits the maintenance methyltransferase DNMT1, which adds methylation to the new strand to match the parental pattern.

If histone modifications are not template-copied, how are they inherited?

Parental modified histones are recycled onto the daughter strands and act as seeds; writer enzymes recognize the residual marks and copy them onto neighboring new histones, restoring the pattern before the next division.