Molecular Genetics

Entry into mitosis requires completion of DNA synthesis

Facts

• Recall that M phase of the cell cycle (chromosome segregation and cell division) does not occur until well after the end of S phase (DNA synthesis). End ligation occurs in G2. Recall also that cyclins and P34 cyclin-dependent protein kinase are key molecules in the activation of mitosis.
• A yeast chromosome (upper line in diagram) containing an HO endonuclease site adjacent to a telomere, URA3, cycloheximide sensitivity and ADE3 (red-color forming marker) was placed in strains containing a galactose-inducible HO gene.
• Addition of galactose resulted, at first, in an inhibition of cell division.
• The inhbition was not observed in rad9 mutant cells.
• Eventually, cells divided. Many cells among the progeny had simultaneously lost both cycloheximide sensitivity and the red color.
• Cells with white color and cycloheximide resistance were stable. Their chromosomes had regained telomere sequences.
  

Interpretations

• Broken DNA prevents cell division.
• Mitosis is not initiated until DNA synthesis is complete.
• Unligated DNA ends remaining after the completion of DNA synthesis may be responsible for prevention of mitosis until those ends are ligated.
• Broken DNA is sensed by the RAD9 gene product.

Further information

•  Entry into S phase (DNA replication) is also controlled.

This is page 1362 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 1999