Visual Guide to Topoisomerases, Licensing, MMR, and Telomerase

CSIR NET | SET: Advanced DNA Replication Regulation

Visual Guide to Topoisomerases, Licensing, MMR, and Telomerase

1. Topoisomerase: Relieving Torsional Strain

As the helicase unwinds DNA, it creates intense positive supercoiling ahead of the replication fork. Topoisomerases relieve this stress by creating temporary nicks in the DNA.

Animation: Topo II cutting supercoils and passing strands to relax DNA.

Feature	Type I Topoisomerase	Type II Topoisomerase
Strand Cuts	Single-strand break (nick)	Double-strand break
ATP Requirement	Usually No	Yes (ATP-dependent)

2. Units of Replication: The Replicon Model

A Replicon is any piece of DNA that replicates as a single unit containing an origin.

Animation: Trans-acting Initiator protein finding and binding to the Cis-acting Replicator.

3. Regulation of Bacterial Replication

To prevent chaotic, uncontrolled replication, bacteria strictly regulate initiation using SeqA and DnaA states:

Animation: SeqA binds to hemimethylated DNA to block DnaA from re-initiating.

• DnaA-ATP: Active form; melts the origin.
• SeqA: Binds to hemimethylated GATC sites, physically blocking DnaA.
• HdaA (RIDA): Forces DnaA to hydrolyze ATP into inactive DnaA-ADP.

4. Prokaryotic Mismatch Repair (MMR)

Post-replication MMR distinguishes the correct parent strand from the newly synthesized mutated strand via methylation.

Animation: MutS scanning the DNA and binding to the structural distortion (mismatch).

5. Eukaryotic Replication: Origins and Licensing

Eukaryotes ensure "Once Per Cell Cycle" replication by strictly separating helicase loading (G1) from origin firing (S phase).

Animation: Inactive MCM loaded in G1 is activated by Phosphorylation in S Phase.

6. Termination and The End Replication Problem

Because lagging strand synthesis requires an RNA primer, removing the final primer leaves a 3' overhang that cannot be filled. Telomerase (a reverse transcriptase) extends this end.

Animation: Telomerase RNP extending the 3' parent strand overhang so synthesis can finish.

7. Chromatin Replication: Nucleosome Dynamics

During eukaryotic replication, the fork must pass through nucleosomes. Histone octamers are disassembled ahead of the fork and reassembled immediately behind it.

Animation: Old and new histones being loaded by chaperones behind the replication fork.