In classic experiments, scientists fused two animal cells in culture, so that two nuclei lay in one…
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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?
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The worked solution
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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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