Question

Which of the following DNA sequences would have the highest melting temperature? A. CGCAACCATGCG B. CGCAATAATACA C. CGTAATAATACA D. CATAACAAATCA

Short answer

Sequence A (CGCAACCATGCG) has the highest melting temperature due to the highest number of C-G pairs.

Step-by-step solution

- Understand the effect of nucleotide pairs on melting temperature

Nucleotides occur in pairs with Adenine (A) pairing with Thymine (T) and Cytosine (C) pairing with Guanine (G). A-T pairs have two hydrogen bonds while C-G pairs have three hydrogen bonds. DNA sequences with more C-G pairs will have a higher melting temperature because three hydrogen bonds are stronger than two.

- Count C-G pairs in each sequence

Count the number of C-G pairs in each sequence: A. CGCAACCATGCG: 7B. CGCAATAATACA: 4C. CGTAATAATACA: 3D. CATAACAAATCA: 2

- Determine which sequence has the highest C-G content

The sequence with the highest number of C-G pairs would have the highest melting temperature. Sequence A has 7 C-G pairs, which is the highest among the given options.

- Conclude the answer

Based on the count, Sequence A (CGCAACCATGCG) has the highest melting temperature due to its higher number of C-G pairs.

Key concepts

Nucleotide Pairs

DNA is made up of building blocks called nucleotides. Each nucleotide consists of a sugar, a phosphate group, and one of four nitrogenous bases: Adenine (A), Thymine (T), Cytosine (C), or Guanine (G). In a DNA double helix, nucleotides pair up in a specific way. Adenine always pairs with Thymine, and Cytosine always pairs with Guanine. This pairing is crucial because it maintains the structure and function of DNA.

For example:

• An Adenine (A) nucleotide on one strand will always pair with a Thymine (T) nucleotide on the opposite strand.
• A Cytosine (C) nucleotide on one strand will always pair with a Guanine (G) nucleotide on the opposite strand.

These pairs are known as complementary pairs. Complementary base pairing is essential for the processes of DNA replication, transcription, and repair, ensuring genetic information is accurately passed down during cell division.

Hydrogen Bonds in DNA

The nucleotides in DNA are held together by hydrogen bonds. Hydrogen bonds are weak interactions compared to other types of bonds, like covalent bonds, but they are strong enough to hold the two strands of the DNA helix together.

• Adenine (A) and Thymine (T) are connected by two hydrogen bonds.
• Cytosine (C) and Guanine (G) are connected by three hydrogen bonds.

Because Cytosine-Guanine pairs have three hydrogen bonds, they are stronger and more stable than Adenine-Thymine pairs, which only have two hydrogen bonds. This makes DNA sequences with more Cytosine-Guanine pairs more resistant to being separated when heated, meaning they have a higher melting temperature.

Cytosine-Guanine Pairs

Cytosine (C) and Guanine (G) pairs, due to their three hydrogen bonds, play a critical role in the stability of DNA. The presence of more C-G pairs in a DNA sequence increases its melting temperature. For instance, in the provided sequences, Sequence A (CGCAACCATGCG) has 7 C-G pairs. This makes it more stable and gives it a higher melting temperature compared to sequences with fewer C-G pairs. This property is used in various molecular biology techniques, such as PCR (Polymerase Chain Reaction), to determine the optimal conditions for DNA denaturation and annealing.

Adenine-Thymine Pairs

Adenine (A) and Thymine (T) pairs are connected by two hydrogen bonds, which makes them less stable compared to Cytosine-Guanine pairs. Sequences with more A-T pairs have lower melting temperatures because the bonds holding the complementary strands together are weaker. For example, in the provided sequences, Sequence D (CATAACAAATCA) has mostly A-T pairs, resulting in a lower melting temperature. Understanding the balance of A-T and C-G pairs in a DNA sequence helps scientists predict the conditions needed for processes like DNA amplification and hybridization.