Molecular Genetics

Replicon clusters are organized in families.

Facts

In yeast, 80-90% of DNA replication occurs in the first 10-25% of S phase. In mammals and plants 90% or more occurs in mid S phase. Sequences replicating very late in S phase are known: the inactive X chromosome of females, accessory B chromosomes of maize, and satellite DNA. Which genes are replicated during which stage of S phase can be determined by sorting cells according to DNA content, fractionating the DNA according to buoyant density and probing the fractions with specific genes. Active genes appear to replicate early. Replication of some series of adjacent genes appears to occur in order of their location on the DNA. Replicons occur in clusters of 2 to 250 (see Table for pea data). Adjacent bubbles have the same size. In A. thaliana: two replicon families have been identified (1800 A, 600 B). The time window of replication of the two families overlaps.

Yeast genes are replicated at different times during S phase (see graph). Human GC-rich DNA is replicated later than human AT-rich DNA. When cells are pulse labeled during S phase with BrUdr, chromosomes with reproducible banded appearances result.

Pea Replicons ~18 replicons/cluster 4.2 x 103 clusters/genome 7.7 x 104 replicons/genome

Interpretations

• At any one time in S phase, each specific replicon is at the same stage of replication in every cell.
• Specific regions of chromosomes replicate at different times during S phase (ref1, ref2).
• Cluster families replicating at different times could reflect family-specific origins that are triggered at different times or differing steric availability for replication of different regions of the chromosomes at different times.

Further information

• Pea replicon clusters have been used to explore the role of protein synthesis in completion of replicon synthesis and fusion of replicon clusters (ref).
• Replicons may be equivalent to chromosome loops and clusters may represent structural superloops (ref).
• Sister chromatid exchanges may be related to replicons (ref).

This is page 13132 of Molecular Genetics by Ulrich Melcher, © 1998, 1999