4. Before the structure of DNA was known, researchers analysed the ratios of the DNA bases found in cells of different species. The tetranucleotide hypothesis, formulated in 1910, proposed that DNA was made up of equal amounts of adenine, guanine, cytosine and thymine. This hypothesis was revised following the publication of Chargaff’s paper in 1951, from which data is shown in the table.
| Source |
A:G |
T:C |
A:T |
G:C |
purines:pyrimidines |
| Ox |
1.29 |
1.43 |
1.04 |
1.00 |
1.10 |
| Human |
1.56 |
1.75 |
1.00 |
1.00 |
1.00 |
| Chicken |
1.45 |
1.29 |
1.06 |
0.91 |
0.99 |
| Salmon |
1.43 |
1.43 |
1.02 |
1.02 |
1.02 |
| Wheat |
1.22 |
1.18 |
1.00 |
0.97 |
0.99 |
| Yeast |
1.67 |
1.92 |
1.03 |
1.20 |
1.00 |
(a) State one example of a purine. [1]
(b) Explain how these results falsify the tetranucleotide hypothesis. [2]
(c) Using these data, evaluate the hypothesis that the ratio of purines to pyrimidines in DNA in all organisms is 1.00. [1]
(d) Discuss how these results are consistent with the existence of a last universal common ancestor (LUCA). [2]
(e) Discuss the evidence of the relationship between genome size and the complexity of organisms. [4]
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(a) State one example of a purine. [1]
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Purines are the larger DNA bases with two rings in their structure.
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Adenine / Guanine
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(b) Explain how these results falsify the tetranucleotide hypothesis. [2]
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What did the tetranucleotide hypothesis predict about the relative amounts of the four bases? How does the data in the table (e.g., the A:G ratio) contradict this prediction?
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The tetranucleotide hypothesis proposed equal amounts (1:1:1:1 ratio) of A, G, C, T. The data shows unequal ratios for A:G and T:C (e.g., A:G = 1.56 in humans, not 1.00) in all species tested, thus falsifying the hypothesis.
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(c) Using these data, evaluate the hypothesis that the ratio of purines to pyrimidines in DNA in all organisms is 1.00. [1]
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Look at the last column of the table (‘purines:pyrimidines’). How close are these values to 1.00? Are there any exceptions?
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The ratio is approximately 1.00 (0.99-1.02) in most organisms shown (Human, Chicken, Salmon, Wheat, Yeast), generally supporting the hypothesis. However, Ox shows a slightly higher ratio (1.10), suggesting it might not be exactly 1.00 in *all* organisms.
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(d) Discuss how these results are consistent with the existence of a last universal common ancestor (LUCA). [2]
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Think about what features shared by all these diverse organisms (from Ox to Yeast) suggest a common origin. Consider both the building blocks and the rules governing their arrangement.
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All organisms studied use the same four DNA bases (A, T, C, G). Furthermore, the consistent base pairing rules (A≈T and G≈C, reflected in the ratios near 1.00) across these diverse species suggest a universal, conserved structure and replication mechanism for DNA, inherited from a common ancestor (LUCA).
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(e) Discuss the evidence of the relationship between genome size and the complexity of organisms. [4]
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Is there a simple, direct relationship? Consider the C-value paradox and the role of non-coding DNA.
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There is no clear, direct correlation between genome size and organism complexity, a phenomenon known as the C-value paradox. Some organisms considered less complex (e.g., Amoeba proteus, certain plants) possess much larger genomes than more complex organisms like humans. This lack of correlation is largely attributed to the significant variation in the amount of non-coding DNA (such as introns, transposons, and repetitive sequences) among different species. Therefore, organismal complexity is thought to be more closely related to factors like the number of protein-coding genes, the complexity of gene regulation networks, and processes like alternative splicing, rather than the sheer size of the genome.
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