Answers to Concept Checks
- 1. Without knowledge of the structure of DNA, an understanding of how genetic information was encoded or expressed was impossible.
- 2. c
- 3. c
- 4. No, because carbon is found in both protein and nucleic acid.
- 5. d
- 6. b
- 7. d
- 8. Z-DNA has a left-handed helix; B-DNA has a right-handed helix. The sugar–phosphate backbone of Z-DNA zigzags back and forth, whereas the sugar–phosphate backbone of B-DNA forms a smooth continuous ribbon.
- 9. a
294
WORKED PROBLEMS
Problem 1
The percentage of cytosine in a double-stranded DNA molecule is 40%. What is the percentage of thymine?
Solution Strategy
What information is required in your answer to the problem?
The percentage of thymine in the DNA molecule.
What information is provided to solve the problem?
The DNA molecule is double stranded.-
The percentage of cytosine is 40%.
For help with this problem, review:
The Primary Structure of DNA and the Secondary Structures of DNA in Section 10.3.
Solution Steps
Recall: In double-stranded DNA, A pairs with X whereas G pairs with C; so the percentage of A equals the percentage of X and the percentage of G equals the percentage of C.
If C = 40%, then G also must be 40%. The total percentage of C + G is therefore 40% + 40% = 80%. All the remaining bases must be either A or T; so the total percentage of A + T = 100% − 80% = 20%; because the percentage of A equals the percentage of T, the percentage of T is 20%/2 = 10%.
Problem 2
Which of the following relations will be true for the percentage of bases in double-stranded DNA?
- a. C + T = A + G
- b.
Solution Strategy
What information is required in your answer to the problem?
Indicate whether C + T = A + G and
are true.
What information is provided to solve the problem?
-
The DNA is double stranded.
-
Ratios of different groups of bases.
For help with this problem, review:
The Primary Structure of DNA and the Secondary Structures of DNA in Section 10.3.
Solution Steps
An easy way to determine whether the relations are true is to arbitrarily assign percentages to the bases, remembering that, in double-stranded DNA, A = T and G = C. For example, if the percentages of A and T are each 30%, then the percentages of G and C are each 20%. We can substitute these values into the equations to see if the relations are true.
- a. 20 + 30 = 30 + 20 This relation is true.
- b.
≠ 
. This relation is not true.