Section 8.2 Study Guide: Structure of DNA
Vocabulary
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What are nucleotides? Please explain.
Double helix
Base pairing rule
Review Questions
1. DNA is a compound of four types of nucleotides. On the back of this study guide (or someplace you
have space) draw a nucleotide and label its three parts.
2. Where you have space, draw a DNA double helix. Label the sugar-phosphate backbone, the
nitrogen-containing bases, and the hydrogen bonds.
3. How many types of nucleotides are present in DNA?
4
4. What did Rosalind Franklin’s data reveal about the structure of DNA?
That the DNA molecule was a two stranded helix with a constant width.
5.
How did Watson and Crick determine the three dimensional shape of DNA?
Building on the x-ray from Rosalind Franklin, Watson and Crick realized that a base with one ring
would bond with a base with two rings.
6. How does the DNA base pairing result in a molecule that has a uniform width?
Because a singled ringed nitrogenous base (thymine and cytosine) always pairs with a double
ringed nitrogenous base (adenine and guanine) – thus each rung is always the same width.
7. What nucleotide pairs with T? C?
Adenine with Thymine and Cytosine with Guanine
8. How do the nucleotides differ?
Two of the nucleotides have single ringed nitrogenous bases (C and T) while two of the bases are
double ringed (A and G).
9. Which base is most similar in structure to thymine?
Cytosine is the most similar because they are both single ringed bases.
10. What sequence of bases would pair with the sequence TGACTA?
ACTGAT
11. How do the base pairing rules relate to Chargaff’s rules?
Chargaff found that the same four bases are found in all organisms but the proportions of the
bases were unique to each organism.
He found that the amount of adenine equals the amount of thymine (thus they pair); and that the
amount of cytosine equals the amount of guanine (thus they pair).
12. Which part of a DNA molecule carries the genetic instructions that are unique for each individual:
the sugar-phosphate backbone or the nitrogen-containing bases? Please explain.
The nitrogen-containing bases carry the genetic instructions. Each species has its own unique
(found in each chromosome) base sequences. The base sequences make up an organism’s genes.
13. In a sample of yeast DNA, 31.5% of the bases are adenine (A). Predict the approximate percentage
of C, G, and T. Please explain how you arrived at your answers.
31.5% thymine, 18.5% cytosine, and 18.5% guanine.
14. The DNA of all organisms contains the same four bases (adenine, thymine, cytosine, and guanine).
What might this similarity indicate about the origins of life on Earth?
It indicates that the wide diversity of life that we observe on Earth has a common origin since
there is only one genetic code (DNA).