Textbook Question
Is a DNA molecule neutral, negatively charged, or positively charged? Explain.
Ch.26 Nucleic Acids and Protein Synthesis
McMurry8th EditionFundamentals of General, Organic, and Biological ChemistryISBN: 9780134015187
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Table of contents
- Ch.1 Matter and Measurements27
- Ch.2 Atoms and the Periodic Table20
- Ch.3 Ionic Compounds8
- Ch.4 Molecular Compounds23
- Ch.5 Classification & Balancing of Chemical Reactions12
- Ch.6 Chemical Reactions: Mole and Mass Relationships28
- Ch.7 Chemical Reactions: Energy, Rate and Equilibrium64
- Ch.8 Gases, Liquids and Solids24
- Ch.9 Solutions52
- Ch.10 Acids and Bases25
- Ch.11 Nuclear Chemistry22
- Ch.12 Introduction to Organic Chemistry: Alkanes57
- Ch.13 Alkenes, Alkynes, and Aromatic Compounds47
- Ch.14 Some Compounds with Oxygen, Sulfur, or a Halogen50
- Ch.15 Aldehydes and Ketones45
- Ch.16 Amines42
- Ch.17 Carboxylic Acids and Their Derivatives57
- Ch.18 Amino Acids and Proteins82
- Ch.19 Enzymes and Vitamins63
- Ch.20 Carbohydrates44
- Ch.21 The Generation of Biochemical Energy65
- Ch.22 Carbohydrate Metabolism45
- Ch.23 Lipids42
- Ch.24 Lipid Metabolism33
- Ch.25 Protein and Amino Acid Metabolism24
- Ch.26 Nucleic Acids and Protein Synthesis45
McMurry8th EditionFundamentals of General, Organic, and Biological ChemistryISBN: 9780134015187Not the one you use?Change textbook
Chapter 26, Problem 11a
DNA and RNA, like proteins, can be denatured to produce unfolded or uncoiled strands. Heating DNA to what is referred to as its “melting temperature” denatures it (the two strands of the double helix become separated). Why does a longer strand of DNA have a higher melting temperature than a shorter one?
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Understand the concept of DNA melting temperature: The melting temperature (Tm) is the temperature at which half of the DNA strands in a sample are denatured, meaning the hydrogen bonds between the complementary base pairs are broken, and the double helix separates into single strands.
Recognize that DNA stability is influenced by the number of hydrogen bonds: Each base pair in DNA contributes to the overall stability of the molecule. Guanine (G) and cytosine (C) pairs form three hydrogen bonds, while adenine (A) and thymine (T) pairs form two hydrogen bonds. More base pairs mean more hydrogen bonds, which require more energy (heat) to break.
Consider the length of the DNA strand: A longer DNA strand has more base pairs compared to a shorter strand. This means there are more hydrogen bonds in a longer strand, making it more stable and requiring a higher temperature to denature.
Account for cooperative interactions: In a DNA molecule, the breaking of one hydrogen bond can destabilize neighboring bonds, but this effect is more pronounced in shorter strands. Longer strands have more regions of stability, which collectively resist denaturation, leading to a higher melting temperature.
Conclude that the melting temperature increases with strand length: The greater number of hydrogen bonds and the cooperative stability of longer DNA strands explain why they have a higher melting temperature compared to shorter strands.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Melting Temperature (Tm)
The melting temperature (Tm) of DNA 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. Tm is influenced by the length of the DNA strands and their nucleotide composition, particularly the number of guanine-cytosine (GC) pairs, which form three hydrogen bonds compared to the two formed by adenine-thymine (AT) pairs.
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Temperature (Simplified) Example 2
Hydrogen Bonds in DNA
DNA strands are held together by hydrogen bonds between complementary base pairs. Each GC pair contributes three hydrogen bonds, while each AT pair contributes two. Therefore, longer DNA strands, which typically contain more GC pairs, have more hydrogen bonds, resulting in greater stability and a higher melting temperature compared to shorter strands.
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Strand Length and Stability
The length of a DNA strand directly affects its stability due to the cumulative effect of base pairing. Longer strands have more base pairs, leading to increased hydrogen bonding and van der Waals interactions, which collectively enhance the stability of the double helix. This increased stability translates to a higher melting temperature, as more energy (in the form of heat) is required to separate the strands.
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Related Practice
Textbook Question
What is the difference between DNA polymerase and DNA ligase?
Textbook Question
Draw the structures of adenine and uracil (which replaces thymine in RNA), and show the hydrogen bonding that occurs between them.
2
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Textbook Question
What are Okazaki fragments? What role do they serve in DNA metabolism?
Textbook Question
Write the complementary sequence of bases for each DNA strand shown next.
a. 5′T-A-T-A-C-T-G 3′
Textbook Question
What is the function of the spliceosome in hnRNA?