What evidence did Watson and Crick have at their disposal in 1953? What was their approach in arriving at the structure of DNA?
Table of contents
- 1. Introduction to Genetics51m
- 2. Mendel's Laws of Inheritance3h 37m
- 3. Extensions to Mendelian Inheritance2h 41m
- 4. Genetic Mapping and Linkage2h 28m
- 5. Genetics of Bacteria and Viruses1h 21m
- 6. Chromosomal Variation1h 48m
- 7. DNA and Chromosome Structure56m
- 8. DNA Replication1h 10m
- 9. Mitosis and Meiosis1h 34m
- 10. Transcription1h 0m
- 11. Translation58m
- 12. Gene Regulation in Prokaryotes1h 19m
- 13. Gene Regulation in Eukaryotes44m
- 14. Genetic Control of Development44m
- 15. Genomes and Genomics1h 50m
- 16. Transposable Elements47m
- 17. Mutation, Repair, and Recombination1h 6m
- 18. Molecular Genetic Tools19m
- 19. Cancer Genetics29m
- 20. Quantitative Genetics1h 26m
- 21. Population Genetics50m
- 22. Evolutionary Genetics29m
7. DNA and Chromosome Structure
DNA as the Genetic Material
Problem 23
Textbook Question
Why is Tₘ related to base composition?

1
Understand that Tₘ (melting temperature) is the temperature at which half of the DNA double helix becomes single-stranded. It reflects the stability of the DNA molecule.
Recognize that the stability of DNA is influenced by its base composition, specifically the proportion of guanine (G) and cytosine (C) bases compared to adenine (A) and thymine (T) bases.
Recall that G-C base pairs form three hydrogen bonds, while A-T base pairs form only two hydrogen bonds. This means G-C pairs contribute more to the stability of the DNA molecule than A-T pairs.
Understand that a higher G-C content in the DNA sequence increases the Tₘ because more energy (higher temperature) is required to break the additional hydrogen bonds in G-C pairs.
Conclude that Tₘ is directly related to the base composition of DNA, with higher G-C content leading to a higher Tₘ, and lower G-C content leading to a lower Tₘ.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Tₘ (Melting Temperature)
Tₘ, or melting temperature, is the temperature at which half of the DNA strands are in the double-helix state and half are in the 'melted' single-strand state. It is a critical parameter in molecular biology, influencing the stability of DNA and the conditions required for processes like PCR. Tₘ is affected by various factors, including the length of the DNA strands and their base composition.
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Base Composition
Base composition refers to the relative amounts of the four nucleotide bases (adenine, thymine, cytosine, and guanine) in a DNA molecule. The ratio of these bases, particularly the A-T and G-C pairs, significantly influences the stability of the DNA double helix. Higher G-C content generally leads to a higher Tₘ due to the presence of three hydrogen bonds between G and C, compared to the two hydrogen bonds between A and T.
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Hydrogen Bonding
Hydrogen bonding is a type of weak chemical bond that occurs between the nitrogenous bases of DNA. In the context of base pairing, adenine pairs with thymine through two hydrogen bonds, while guanine pairs with cytosine through three hydrogen bonds. This difference in bonding strength contributes to the overall stability of the DNA molecule, affecting Tₘ; more hydrogen bonds result in a higher melting temperature.
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