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Chapter 7. CHAPTER 7: UNDERSTANDING CHARGAFF’S RULES

Unpacking the Problem
true
true
You must read each slide, and complete any questions on the slide, in sequence.

If the GC content of a DNA sample is 48%, what are the percentages of the nucleotides in this sample?

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-3): First, it is necessary to understand the basic structure and nomenclature of DNA.
  • Part B (steps 4 and 5): Second, the mechanism and rules of base pairing in DNA are explored.
  • Part C (steps 6-8): Finally, calculations are performed in order to determine the proportion of each nucleotide in this particular sample of DNA.
1

Describe the structure of DNA by correctly filling in the blanks in the following sentences. To fill in a blank, select the best choice of word or phrase from the pulldown menu.

The three-dimensional structure of DNA is composed of strands of that each other to form . The strands are held to each other by between the of each strand.
1
Correct. DNA is a double helix made of two polynucleotide strands that are oriented such that the nitrogenous bases of each strand face inward and toward each other. Hydrogen bonds between the bases on opposite strands hold the two strands together.
Incorrect. DNA is a double helix made of two polynucleotide strands that are oriented such that the nitrogenous bases of each strand face inward and toward each other. Hydrogen bonds between the bases on opposite strands hold the two strands together.

Study Figure 7-8.

Unpacking the Problem
true
true
You must read each slide, and complete any questions on the slide, in sequence.

If the GC content of a DNA sample is 48%, what are the percentages of the nucleotides in this sample?

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-3): First, it is necessary to understand the basic structure and nomenclature of DNA.
  • Part B (steps 4 and 5): Second, the mechanism and rules of base pairing in DNA are explored.
  • Part C (steps 6-8): Finally, calculations are performed in order to determine the proportion of each nucleotide in this particular sample of DNA.
2

What does G represent when referring to DNA?

A.
B.
C.
D.
E.
1

HINT DNA is a polymer. What is the name of the class of monomers from which DNA is built?

Deoxyriboses
Nucleosides
Nucleotides
Unpacking the Problem
true
true
You must read each slide, and complete any questions on the slide, in sequence.

If the GC content of a DNA sample is 48%, what are the percentages of the nucleotides in this sample?

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-3): First, it is necessary to understand the basic structure and nomenclature of DNA.
  • Part B (steps 4 and 5): Second, the mechanism and rules of base pairing in DNA are explored.
  • Part C (steps 6-8): Finally, calculations are performed in order to determine the proportion of each nucleotide in this particular sample of DNA.
3

What monomers are used to create molecules of DNA?

A.
B.
C.
D.
1

DNA is a polymer. What is the name of the class of monomers from which DNA is built?

Deoxyriboses
Nucleosides
Nucleotides
Pyrimidines

These letters are abbreviations for various molecules or for moieties within molecules.

Unpacking the Problem
true
true
You must read each slide, and complete any questions on the slide, in sequence.

If the GC content of a DNA sample is 48%, what are the percentages of the nucleotides in this sample?

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-3): First, it is necessary to understand the basic structure and nomenclature of DNA.
  • Part B (steps 4 and 5): Second, the mechanism and rules of base pairing in DNA are explored.
  • Part C (steps 6-8): Finally, calculations are performed in order to determine the proportion of each nucleotide in this particular sample of DNA.
4

Which bases pair together in double-stranded DNA?

A.
B.
C.
D.
1

In DNA we normally find purines base paired with pyrimidines. True or false?

True
False

In DNA we normally find purines base paired with purines. True or false?

True
False

In DNA we normally find pyrimidines base paired with pyrimidines. True or false?

True
False

In what molecules is uracil (i.e., U) found?

DNA
RNA
DNA and RNA
Proteins
Unpacking the Problem
true
true
You must read each slide, and complete any questions on the slide, in sequence.

If the GC content of a DNA sample is 48%, what are the percentages of the nucleotides in this sample?

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-3): First, it is necessary to understand the basic structure and nomenclature of DNA.
  • Part B (steps 4 and 5): Second, the mechanism and rules of base pairing in DNA are explored.
  • Part C (steps 6-8): Finally, calculations are performed in order to determine the proportion of each nucleotide in this particular sample of DNA.
5

Within DNA, what holds the two bases in a base pair together?

A.
B.
C.
D.
1

Examine Figures 7-5 and 7-8.

Unpacking the Problem
true
true
You must read each slide, and complete any questions on the slide, in sequence.

If the GC content of a DNA sample is 48%, what are the percentages of the nucleotides in this sample?

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-3): First, it is necessary to understand the basic structure and nomenclature of DNA.
  • Part B (steps 4 and 5): Second, the mechanism and rules of base pairing in DNA are explored.
  • Part C (steps 6-8): Finally, calculations are performed in order to determine the proportion of each nucleotide in this particular sample of DNA.
6

If the GC content in a DNA sample is 48%, what is the percentage of G in the sample?

A.
B.
C.
D.
E.
1

Is G found anywhere else in DNA except base paired with C?

Yes, G can bind to any nucleotide.
No, G binds only with C.

What is the ratio of G to C in any sample of DNA?

3:1
1:1
Unknown because it varies for every DNA sample

If the GC content in a DNA sample is X%, then the percentage of G in that sample is

X%/2
X%/4
The DNA that has entered the cell is a single-stranded plasmid unlike the double-stranded genome of the recipient cell.
Unpacking the Problem
true
true
You must read each slide, and complete any questions on the slide, in sequence.

If the GC content of a DNA sample is 48%, what are the percentages of the nucleotides in this sample?

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-3): First, it is necessary to understand the basic structure and nomenclature of DNA.
  • Part B (steps 4 and 5): Second, the mechanism and rules of base pairing in DNA are explored.
  • Part C (steps 6-8): Finally, calculations are performed in order to determine the proportion of each nucleotide in this particular sample of DNA.
7

If the GC content is 48%, what is the percentage of AT in the sample?
1
Correct. The percentage of AT is found by subtracting the percentage of GC from 100. Thus 100% - 48% = 52%.
Incorrect. The percentage of AT is found by subtracting the percentage of GC from 100. Thus 100% - 48% = 52%.

Are there any nucleotides in DNA other than G, C, T, and A?

Yes, uracil (U) is also found in DNA.
Yes, inosine (I) is sometimes found in DNA.
Yes, serine (S) is found in DNA.
No, there are no other nucleotides found in DNA.

If the amount of GC base pairs in a DNA sample is Y%, then what is the amount of AT base pairs in the same DNA sample?

100% - Y%
Y% - 100%
(1 - Y%)/100%
(100% - Y%)/100%
Unpacking the Problem
true
true
You must read each slide, and complete any questions on the slide, in sequence.

If the GC content of a DNA sample is 48%, what are the percentages of the nucleotides in this sample?

Unpack the Problem: Break this problem into several parts and arrive at a solution using this guided, step-by-step approach.

  • Part A (steps 1-3): First, it is necessary to understand the basic structure and nomenclature of DNA.
  • Part B (steps 4 and 5): Second, the mechanism and rules of base pairing in DNA are explored.
  • Part C (steps 6-8): Finally, calculations are performed in order to determine the proportion of each nucleotide in this particular sample of DNA.
8

At this point you know the percentage of GC in the DNA sample (it was given), and you should have calculated the percentage of AT in the sample as well as the percentage of G alone. To complete this problem, fill in the table below with the correct percentages of each nucleotide in this sample.
Nucleotide % in sample of DNA
G
C
A
T
1
Correct. Congratulations!! You exhibit a clear understand of Chargaff’s rules.
Incorrect. In Step 6, you determined the percentage of G in the sample to be half of the GC content, or G = 24%. Since C base pairs only with G, then C must also be 24% of the total nucleotide content in the DNA sample. In Step 7 the amount of AT content was determined by subtracting the amount of GC content from 100%. Thus the AT content = 52%, which is composed of equal parts A and T.

Is G found anywhere else in DNA except base paired with C? Similarly, is A found anywhere else in DNA except base paired with T?

Yes, each nucleotide can bind to the other three.
Yes, A is capable of binding to G.
No, G binds only with C, and A binds only with T.

What is the ratio of G to C in any sample of DNA?

3:1
1:1
Unknown because it varies for every DNA sample

What is the ratio of A to T in any sample of DNA?

3:1
1:1
Unknown because it varies for every DNA sample

If the amount of AT base pairs in a DNA sample is X%, then what is the amount of GC base pairs in the same DNA sample?

100% - X%:1
X% - 100%
(1-X%)/100%

Conclusion

Within the sequence of DNA resides the information to manufacture specific proteins inside a cell. The cellular machinery uses a DNA sequence as a template, and RNA polymerase reads it and synthesizes an mRNA transcript using the rules of complementary base pairing. The mRNA transcript interacts with ribosomes and the adapter molecules, tRNAs, to build a polypeptide chain that folds into a protein. Again it is complementary base pairing rules that determine which tRNA binds to a codon of the mRNA. The genetic code reveals the relationships between each mRNA codon and its corresponding amino acid. It is the tRNA molecules that functionally perform the code conversion: translating a nucleotide sequence into a sequence of amino acids. In this problem you have used base-pairing rules and the genetic code to reinforce how information encoded in DNA is retained and transferred through RNA molecules to ultimately build a protein.