Molecular Genetics

Facts

• Both strands of mammalian mitDNA are synthesized continuously as leading strands.
• Synthesis of the first mitDNA strand proceeds partially, pauses, and is then completed upon receipt of appropriate signals. The result is that most mitDNA has D-loops.
• Synthesis of the second strand is initiated after the fork synthesizing the first strand passes the origin of second strand synthesis.
• During plastid DNA replication RNA primers are incompletely excised and replaced. As a result the plastid genome is a chimeric DNA-RNA molecule. Origins of plastid DNA replication have been identified.
• Two D-loops are found in plastid DNA. They represent initiation on opposite DNA strands.

Interpretations

• Replication of plastid and mammalian mitDNA differs from that of nuclear DNA. Specifically, origins for leftward and rightward replication are separate for mitDNA, but coincident for nuclear DNA.
• As with nuclear DNA replication, mitochondrial DNA replication is regulated.

Further information

• Small segments of RNA in replicated DNA are common to rapidly replicating small circular genomes, being also found in some viral DNAs.
• Plastid replication is not syncronous with nuclear replication. Young leaves have more ctDNA/cell than old leaves.
• Since mitDNA strands are initiated from RNA primers, DNA replication and transcription (rev) are linked (ref).
• Many components for organelle DNA replication and transcription are imported from the cytoplasm.

This is page 1375 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 2003