Chapter 15 The Replicon

15.1 Introduction The initiation of replication occurs at a unique site called the origin, which lies within the replicon.

15.2 An Origin Usually Initiates Bidirectional Replication A replication fork is initiated at the origin and then moves sequentially along DNA. Replication is bidirectional when an origin creates two replication forks that move in opposite directions.

15.2 An Origin Usually Initiates Bidirectional Replication Figure 15.02: Replication eyes can be uni- or bidirectional.

15.3 The Bacterial Genome Is a Single Circular Replicon Bacterial replicons are usually circles that replicate bidirectionally from a single origin. The origin of E. coli, oriC, is 245 bp in length.

15.3 The Bacterial Genome Is a Single Circular Replicon Figure 15.05: Replication requires DNA synthesis and chromosome separation.

15.3 The Bacterial Genome Is a Single Circular Replicon The two replication forks usually meet halfway around the circle. There are ter sites that cause termination if the replication forks go too far.

15.3 The Bacterial Genome Is a Single Circular Replicon Figure 15.06: Forks usually meet before terminating.

15.4 Methylation of the Bacterial Origin Regulates Initiation oriC contains eleven GATC/CTAG repeats that are methylated on adenine on both strands. Replication generates hemimethylated DNA, which cannot initiate replication. There is a 13-minute delay before the GATC/CTAG repeats are remethylated.

15.4 Methylation of the Bacterial Origin Regulates Initiation Figure 15.07: The Dam methylase maintains methylation.

15.5 Each Eukaryotic Chromosome Contains Many Replicons A chromosome is divided into many replicons. The progression into S phase is tightly controlled. Eukaryotic replicons are 40 to 100 kb in length. Individual replicons are activated at characteristic times during S phase. Regional activation patterns suggest that replicons near one another are activated at the same time.

15.6 Replication Origins Bind the ORC Origins in S. cerevisiae are short A-T-rich sequences that have an essential 11-bp sequence. The ORC is a complex of six proteins that binds to an ARS. It remains bound through the cell cycle but is activated only during S phase. Related ORC complexes are found in higher eukaryotes.

15.7 Licensing Factor Controls Eukaryotic Rereplication and Consists of MCM Proteins Licensing factor is necessary for initiation of replication at each origin. Licensing factor is present in the nucleus prior to replication. It is removed, inactivated or destroyed by replication. Initiation of another replication cycle becomes possible only after licensing factor re-enters the nucleus after mitosis. The ORC is a protein complex that is associated with yeast origins throughout the cell cycle.

Figure 15.13: Licensing factor controls replication. 15.7 Licensing Factor Controls Eukaryotic Rereplication and Consists of MCM Proteins Figure 15.13: Licensing factor controls replication.

15.7 Licensing Factor Controls Eukaryotic Rereplication and Consists of MCM Proteins Cdc6 protein is an unstable protein that is synthesized only in G1. Cdc6 binds to ORC and allows MCM proteins to bind. When replication is initiated, Cdc6 and MCM proteins are displaced. The degradation of Cdc6 prevents reinitiation. Some MCM proteins are in the nucleus throughout the cycle, but others may enter only after mitosis.

Figure 15.14: The ORC complex controls the origin. 15.7 Licensing Factor Controls Eukaryotic Rereplication and Consists of MCM Proteins Figure 15.14: The ORC complex controls the origin.