In classic experiments, scientists fused two animal cells in culture, so that two nuclei lay in one…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
In classic experiments, scientists fused two animal cells in culture, so that two nuclei lay in one cytoplasm.
Experiment 1: a cell in S phase was fused with a cell in G1. The G1 nucleus soon began to copy its DNA.
Experiment 2: a cell in S phase was fused with a cell in G2. The G2 nucleus did not copy its DNA again.
Experiment 3: a cell in M phase was fused with a cell in interphase. The chromosomes of the interphase nucleus condensed.
- What do experiments 1 and 3 suggest about the cytoplasm of the S phase cell and of the M phase cell?
- What does experiment 2 suggest about a G2 nucleus?
- Which kind of protein pair fits the factor of experiment 3?
Need a reminder? The key ideaShow
There are three checkpoints. The G1 checkpoint checks the size of the cell, its food, the growth factors and the DNA damage. The G2 checkpoint checks that the DNA is copied and undamaged. The spindle checkpoint checks that every chromosome is attached to the spindle. A CDK has a steady amount and works only when bound to a cyclin. The amount of a cyclin rises and falls.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) The cytoplasm of an S phase cell holds a factor that starts DNA copying, and it acts on a G1 nucleus. The cytoplasm of an M phase cell holds a factor that starts mitosis, and it acts on an interphase nucleus. The factors are carried in the cytoplasm.
Step 2. (b) A G2 nucleus has already copied its DNA. It does not copy it again, even though the factor is present.
Step 3. (c) The factor of experiment 3 is the M cyclin with its CDK (MPF).
Answer: (a) the cytoplasm carries factors for S phase and for M phase; (b) a G2 nucleus does not copy its DNA again; (c) M cyclin and CDK.
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