Molecular Genetics

For DNA replication to be well coordinated with cell division, no origin of replication may function more than once each cycle.

Facts

• Recall that a cell's DNA has many origins of replication, many of which are not used in any given cell cycle. Each origin that does function in a cell cycle (fire), must only fire once. Any model for regulation of DNA synthesis must account for this fact.
• The DNA around an origin of replication was footprinted while part of G1, G2 and S chromatin. DNA extracted from chromatin (naked) was used as control. A diagram of results is at right.
• DNA transfected into vertebrate cells is taken up by the nucleus and undergoes one round of DNA replication. No further replication of the added DNA takes place until the cell enters S phase.

Interpretations

• The assembly and initiation complex model suggests that replication preinitiation complexes form on G1 chromatin. These complexes can not initiate DNA synthesis. At the beginning of S phase, preinitiation complex formation ceases, and conditions are now such that formed complexes initiate DNA synthesis. This model is particularly consistent with the footprinting data.
• The licensing factor model stipulates that a cytoplasmic factor can be picked up by replication origins when that chromatin is in contact with the cytoplasm. That factor, called a licensing factor allows the origin to fire once when conditions are right. The model stresses the possible relevance of nuclear envelope breakdown and reformation during each mitosis (in many, but not all, species) (ref).

Further information

• Licensing factor has been purified from Xenopus laevis embryos (ref1, ref2).
• Licensing factor appears to be equivalent to a complex designated MCM (for mini chromosome maintenance). MCM consists of six related polypeptides that form a ring structure. This ring structure appears to be loaded onto DNA only during G1 and encircles the DNA. MCM has the activity of a DNA helicase, needed to open the origin for initiation of DNA synthesis. The MCM complex requires activation by a S-cyclin dependent kinase. These observations are consistent with identity of the MCM complex as the licensing factor.
• The two models are not mutually exclusive.
• Cyclins may play a role in one shot replication.

This is page 1334 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 1999, 2004