278 CHAPTER 20 



20.7. When is DNA single-stranded? 



2o. s. A double helix oi DNA lias a base sequence \l I AG< A on one strand. Can 

 you complete an inversion after breaking the backbone at two places on this 

 single strand? Explain. Can you complete an inversion if the backbone of the 

 complementary chain is also broken at exactly the same two levels? Explain. 



20.9. Given two double helices whose backbones are broken at the places indicated 

 by periods: 



ATCG.GCAT AT.TAG 

 TAGC.CGTA TA.ATC 



draw the base sequences which can occur following reciprocal translocation 



between double helices. 



20.10. In what respects is Figure 20-7 incorrect? 



20.11. M. Green and M. Piha (1963) report that the G + C ratio of a number of 

 human and animal carcinogenic viruses is similar and lower than that of com- 

 parable nontumorigenic viruses. Discuss the possible implications of this anomaly 

 for the origin and action of tumor-inducing viruses. 



20.12. Locate on Figure 20-6 the minor and major grooves of the two pairs of nucleo- 

 tides shown. Discuss the accuracy of this figure. 



