Molecular Genetics

Unlike termination of bacterial replicon replication, eucaryotic replicons may not have specific signals for termination of their replication.

Facts

	Single-stranded DNA of the length of replicons or clusters can be recovered from pea cells arrested in G2 by carbohydrate starvation. A gene for a DNA ligase responsible for the joining of replicon-sized DNA fragments has been identified in S.cerevisiae as the CDC9 gene.
In replication of the rDNA tandem repeats of S. cerevisiae, a bidirectional origin of replication is located near the transcription start site. The replication fork moving opposite to the direction of transcription stops at the 3' end of the adjacent transcription unit. The fork moving in the direction of transcription moves through several (average 5) transcription units and stops at the end of the next replicon. Deletion or inactivation of an origin is not lethal to replication as long as there are other origins on the DNA. Adjacent replicons take over the replication of the replicon whose origin was inactivated. In endoreduplicated polytene chromosomes, the degree of polyploidization is gene-specific. Chorion gene amplification in Drosophila is also limited to the chorion gene region.

Interpretations

Replication forks become annihilated when two oppositely directed forks meet. Whether any additional mechanism for termination of replication forks exists is unclear. On the one hand, the variable ploidy in genes along endoreduplicated DNA suggests a specific termination mechanism. The cessation of the anti-transcriptional rDNA fork also supports this view. On the other hand, that origins can be deleted without impairing replication suggests that there are not mechanisms for terminating fork movement other than annihilation. Replication and transcription can occur together efficiently if they proceed in the same direction.

Linkage of separately synthesized DNA strands is an important late event in the completion of replication. Some DNA ends remain unligated after DNA synthesis ceases. The sealing of the ends could serve as a signal for readiness for mitosis.

Further information

• In some organisms, rDNA is replicated semi-discontinuously, forming single stranded displacement loops.
• Termination of replication is regulated in bacteria.
• Special DNA synthesis may be needed at chromosome ends.

This is page 1342 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 1999