A colleague has isolated several mutants that behave normally at low temperature, but at high temperatures exhibit anomalous behavior before or during S phase. You suggest that she characterize the mutant phenotypes at high temperatures in more detail by determining if the pre-replicative complex (pre-RC) forms normally and investigating if the replication origins can fire to initiate DNA synthesis. Once she has finished the suggested experiments, she returns for your help in interpreting the data. Match the phenotype in the left column with a proposed defect in protein function from the right column that may explain each phenotype.


A. Cells arrest in G1with normal pre-RC
B. Cells arrest in G1with no pre-RC
C. Origins fire more than once during a single S phase
D. Cells replicate DNA normally but cannot proceed to M phase


1. S cyclins cannot be degraded
2. Cdc6 cannot bind ORC
3. S cyclins cannot bind Cdks
4. Cdc6 cannot be ubiquitinated