Apply What You’ve Learned

Review

4.1 The structures of the polynucleotides DNA and RNA enable their functions in storing and transfering genetic information.

4.1 Base pairing between nucleotides in the polynucleotides DNA and RNA provides the structure needed for transfer of genetic information.

4.1 Base sequences of nucleotides in DNA provide the chemical diversity needed for storage of genetic information.

4.1 Nucleotides other than those found in DNA and RNA have diverse functions within the cell.

Original Paper: Chargaff, E. 1950. Chemical specificity of nucleic acids and mechanisms for their enzymatic degradation. Experientia. 6: 201–240.

Nucleic acids show structural similarity across organisms, whether you’re looking at DNA from a bacterium, a wheat plant, or a human. If you run chemical analyses on DNA extracted from several organisms, you always find a one-to-one molar ratio of phosphate groups to deoxyribose groups. This one-to-one ratio results from the repeating backbone structure of DNA, which is constructed from many nucleotide monomers polymerized together. All organisms share this structural similarity.

You know, however, that DNA from different organisms is not identical. After all, DNA carries biological information specific to each organism. What can you learn about DNA if you analyze base composition across several species? What can you learn about RNA? The tables below provide data for analyzing these questions.

Questions

Question 1

Calculate the purine-to-pyrimidine ratio for each DNA data set. What pattern do you observe? What does this pattern indicate about DNA structure?

The ratio of purines (A + G) to pyrimidines (C + T) is always one-to-one. This pattern is observed because of the double helix structure and base pairing between the two strands making up the double helix. There is always one purine on one strand and a pyrimidine that pairs with it on the complementary strand.

DNA A G Purines C T Pyrimidines Ratio purines to pyrimidines
Herring sperm 27.8 22.2 50 22.6 27.5 50.1 1.00
Rat bone marrow 28.6 21.4 50 21.5 28.4 49.9 1.00
Human sperm 30.7 19.3 50 18.8 31.2 50 1.00
E. coli 26 24.9 50.9 25.2 23.9 49.1 1.04
Yeast 31.3 18.7 50 17.1 32.9 50 1.00

Question 2

Calculate the purine-to-pyrimidine ratio for each RNA data set. What pattern do you observe? What does this pattern indicate about RNA structure?

The ratio of purines (A + G) to pyrimidines (C + U) ranges from 0.87 to 1.24, with lots of variation in between. Therefore there is no constant pattern in this ratio in RNA across many species. This indicates that the number of purines and pyrimidines varies within an RNA strand, which we know to be single-stranded.

RNA A G Purines C U Pyrimidines Ratio purines to pyrimidines
Rat liver 19.2 28.5 47.7 27.5 24.8 52.3 0.91
Carp muscle 16.4 34.4 50.8 31.1 18.1 49.2 1.03
Yeast 25.1 30.2 55.3 20.1 24.6 44.7 1.24
Rabbit liver 19.7 26.8 46.5 25.8 27.6 53.4 0.87
Cat brain 21.6 31.8 53.4 26.0 20.6 46.6 1.15

Question 3

What is the significance of any difference in patterns you found for DNA and RNA as you answered Questions 1 and 2?

The difference in ratios of purines to pyrimidines in DNA and RNA across species highlights the double-stranded nature of DNA and the single-stranded nature of RNA. Only in the double-stranded structure would you have a constant ratio of purines to pyrimidines because they are paired in a one-to-one ratio. In single-stranded RNA, there is no requirement for pairing purines and pyrimidines, and the variability in their content reflects differences in the genetic sequences of the strands.

Question 4

Calculate the combined AT content and combined GC content in the DNA of each organism listed in the table. How does DNA from different organisms compare with respect to this calculation?

Only E. coli has about equal AT and GC content. Human sperm and yeast have more AT than GC content, and rat bone marrow and herring sperm have more GC than AT content.

DNA A G C T A+T G+C
Herring sperm 27.8 22.2 22.6 27.5 55.3 44.8
Rat bone marrow 28.6 21.4 21.5 28.4 57 42.9
Human sperm 30.7 19.3 18.8 31.2 61.9 38.1
E. coli 26.0 24.9 25.2 23.9 49.9 50.1
Yeast 31.3 18.7 17.1 32.9 64.2 35.8

Question 5

Identify two organisms having similar AT content and GC content in their DNA from your answer to Question 4. Explain how these organisms can share this similarity yet have completely different genetic makeups.

Herring sperm and rat bone marrow cells have similar AT and GC content. Their genetic makeups are determined by the sequences of bases in DNA, so even though they have similar overall base content, they each have unique sequences of all bases—A, T, G, and C—that encode the genes within the DNA.

DNA base composition (%)
Organism and tissue Adenine Guanine Cytosine Thymine
Herring sperm 27.8 22.2 22.6 27.5
Rat bone marrow 28.6 21.4 21.5 28.4
Human sperm 30.7 19.3 18.8 31.2
Escherichia coli 26.0 24.9 25.2 23.9
Yeast 31.3 18.7 17.1 32.9
RNA base composition (%)
Organism and tissue Adenine Guanine Cytosine Uracil
Rat liver 19.2 28.5 27.5 24.8
Carp muscle 16.4 34.4 31.1 18.1
Yeast 25.1 30.2 20.1 24.6
Rabbit liver 19.7 26.8 25.8 27.6
Cat brain 21.6 31.8 26.0 20.6