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Capítulo 13
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2. There are several mechanisms that prevent rereplication that might be affected. For example, the protein kinase (Cdk) that prevents rereplication could be mutated, or there might be a defect in the transport of Mcm proteins out of the nucleus once they are displaced from the origin.

3. If replication were conservative, one of the two chromatids would be labeled after one round and one of the four chromatids would be labeled after two rounds. If replication were dispersive, all chromatids would be labeled after both one or two rounds.

4. Would cause a mutation, because U does not have the same base-pairing properties as the C it was derived from. Whereas C base pairs with G, U can also base pair with A. If this were to occur during replication, an adenosine would be inserted where a guanosine should be incorporated. .

5. The molecule would have a free 5' phosphate, but would lack a free 3' OH group.

6. The mutant genes of the first group could encode a subunit of the core holoenzyme for DNA polymerase III, which is required for continued incorporation; those of the second group would not be required immediately, such as a helicase or DNA polymerase I, allowing replication to continue for a brief period; those of the third group would likely be required for initiation of replication at the origin.

7. Because of their smaller genome, the mutation rate in bacteria is less than one per 100 replication cycles. In contrast, human cells contain about 6 X 10 9 nucleotides, which would mean an average of 6 alterations per cell division. Since most of the human genome does not code for meaningful products, the impact is not as great as this, but one would still expect to see a much greater mutation rate in the human cells (assuming a comparable rate of DNA repair).

8. It would appear as a solid black line, i.e., a continuous string of silver grains. No. You are only looking at one small part of one chromosome. Other segments of the DNA might not be replicating at all at this time (which is the case).

9. A-T-rich regions are more readily unwound than other regions of the chromosome because A-T base pairs are held together by only two hydrogen bonds.

10. The matrix provides a mechanism to spatially organize the various factors involved in replication. The foci allow the proteins required for replication to be concentrated, which would be expected to decrease the time required for specific interactions and thus increase the efficiency of replication.

11. Human cells are typically found at a much higher temperature and would be expected to experience greater damage, as a result. In addition, humans have a much greater lifespan and, consequently, the DNA in their cells would be expected to accumulate less damage in a given period of time to allow them to continue to function normally over an extended period.

12. The cell in S phase would have a much higher silver grain density over its nucleus, but the nonreplicating cell would still show incorporated label due to DNA synthesis that accompanies DNA repair.

13. The model would likely propose that many of the key parts of the DNA repair process, such as the helicases, are also part of the transcription machinery, or at least accompany it.







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