6.  The DNA double helix is held together by two types of bonds: covalent and hydrogen. Covalent bonds are found within each linear strand and strongly bond the bases, sugars, and phosphate groups (both within each component and between components). Hydrogen bonds are found between the two strands; a hydrogen bond forms between a base in one strand and a base in the other strand in complementary pairing. These hydrogen bonds are individually weak but collectively quite strong.

9.  Helicases are enzymes that disrupt the hydrogen bonds that hold the two DNA strands together in a double helix. This breakage is required for both RNA and DNA synthesis. Topoisomerases are enzymes that create and relax supercoiling in the DNA double helix. The supercoiling itself is a result of the twisting of the DNA helix when the two strands separate.

11.  No. The information of DNA depends on a faithful copying mechanism. The strict rules of complementarity ensure that replication and transcription are reproducible.

13.  The chromosome would become hopelessly fragmented.

15.  b. The RNA would be more likely to contain errors.

19.  If the DNA is double stranded, A = T, G = C, and A + T + C + G = 100%. If T = 15%, then C = [100 − 15(2)]/2 = 35%.

20.  If the DNA is double stranded, G = C = 24% and A = T = 26%.

24.  Yes. DNA replication is also semiconservative in diploid eukaryotes.

26.  5′ ….CCTTAAGACTAACTACTTACTGGGATC…. 3′

28.  Without functional telomerase, the telomeres would shorten at each replication cycle, leading to eventual loss of essential coding information and death. In fact, some current observations indicate that decline or loss of telomerase activity plays a role in the mechanism of aging in humans.

30.  Chargaff’s rules are that A = T and G = C. Because these equalities are not observed, the most likely interpretation is that the DNA is single stranded. The phage would first have to synthesize a complementary strand before it could begin to make multiple copies of itself.