Molecular Genetics

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Among the important processes required for transmission of genetic information is replication of the chromosomal DNA. Current models of DNA replication in nucleated eukaryotic cells build on the following concepts:

• DNA is replicated semi-conservatively.
• Initiation of DNA replication requires separation of template strands.
• DNA synthesis is discontinuous with time.

The elements of the model are (beginning top left and proceeding clockwise):

• The DNA contains specific sequences that function as origins, producing replication forks.
• There are multiple origins per chromosome, leading to multiple replicons.
• Origins are binding sites for a complex competent to initiate DNA synthesis.
• The complex contains an origin recognition complex (ORC).
• A signal initiates the DNA synthesis process.
• A licensing factor, required for initiation, is released from the ORC upon initiation.
• New licensing factor is only acquired after the completion of synthesis, in preparation for the next round.