Question

Nucleotide Structure Which positions in the purine ring of a purine nucleotide in DNA have the potential to form hydrogen bonds but are not involved in Watson-Crick base pairing?

Short answer

Position 7 in guanine is the non-Watson-Crick hydrogen bonding site.

Step-by-step solution

Understanding DNA Nucleotide Structure

A DNA nucleotide consists of a nitrogenous base, a sugar (deoxyribose), and a phosphate group. The bases include purines (adenine, guanine) and pyrimidines (cytosine, thymine). Purines have a two-ring structure which includes six-membered and five-membered rings.

Identifying Purine Bases

The purine bases in DNA are adenine and guanine. Each purine has potential hydrogen bonding sites in the form of nitrogen atoms and exocyclic amine groups.

Examining Guanine and Adenine Positions

In guanine, positions 1, 7, and 3 have the potential to form hydrogen bonds. On adenine, hydrogen bond-forming sites are located at positions 1 and 6. However, in Watson-Crick base pairing, guanine pairs with cytosine involving positions 1, 2, and 6, while adenine pairs with thymine involving positions 1 and 6.

Identifying Non-Involved Hydrogen Bond Sites

For guanine, the nitrogen at position 7 is capable of forming hydrogen bonds but is not involved in Watson-Crick pairing with cytosine. Therefore, position 7 in the purine ring of guanine is a potential hydrogen-bonding site not involved in Watson-Crick base pairing.

Key concepts

Purine Ring

Purines are one of the two types of nitrogenous bases found in nucleotides, the building blocks of DNA and RNA. They are characterized by a two-ringed structure, which consists of a six-membered ring fused to a five-membered ring. This dual-ring system is what distinguishes purines from pyrimidines, which have only a single six-membered ring.

Adenine and guanine are the two purines that are found in DNA. The rings in these bases are rich in carbon and nitrogen atoms. Some of these atoms serve as sites for hydrogen bonding, which are critical in the formation of the DNA double helix structure. The numbering of atoms in the purine ring is important because it helps identify which atoms are capable of forming hydrogen bonds.

Hydrogen Bonds

Hydrogen bonds are a type of weak attraction that occurs between a hydrogen atom, which is covalently bonded to an electronegative atom, and another electronegative atom. They are essential in many biological molecules, including the structure of DNA.

In the context of DNA, hydrogen bonds occur between nitrogenous bases, helping hold the two strands of the double helix together. Though individual hydrogen bonds are weak, collectively, they provide significant stability to the DNA structure. Each purine and pyrimidine base in DNA can form hydrogen bonds at specific sites, which are determined by the structure of the base. Thus, knowing where these potential hydrogen bonds are is crucial for understanding DNA interactions.

Watson-Crick Base Pairing

Watson-Crick base pairing refers to the specific pairing of nitrogenous bases in DNA. It is named after James Watson and Francis Crick, who discovered this pattern. In their model, adenine (A) forms two hydrogen bonds with thymine (T), while guanine (G) forms three hydrogen bonds with cytosine (C).

This complementarity is crucial for the replication and stability of the DNA double helix. Each base pair contains a purine paired with a pyrimidine, maintaining a consistent helix diameter. The hydrogen bonds in Watson-Crick base pairs are specific, meaning that only these pairs will normally bond with one another in DNA. This specificity is vital for DNA replication, as each strand serves as a template for a new complementary strand.

Adenine and Guanine

Adenine (A) and guanine (G) are the purine bases in DNA. These bases contain several potential sites for hydrogen bonding due to the presence of nitrogen atoms and functional groups such as amines.

Adenine primarily forms bonds at positions 1 and 6, while guanine can form bonds at positions 1, 2, and 6. However, not all possible sites are engaged in Watson-Crick base pairing. For example, guanine's position 7 is free to form hydrogen bonds, but it is not used in the Watson-Crick scheme. This highlights the complexity in the DNA structure and the significance of unexplored binding sites.

DNA Nucleotide Structure

A DNA nucleotide is composed of three components: a phosphate group, a deoxyribose sugar, and a nitrogenous base. The nitrogenous base can be a purine (adenine or guanine) or a pyrimidine (cytosine or thymine). This combination is crucial for the formation of the nucleic acid chains.

The phosphate and sugar create the backbone of the DNA strand, while the nitrogenous bases participate in forming hydrogen bonds between two complementary DNA strands. Understanding nucleotide structure is essential, as it forms the foundation for other concepts like base pairing and DNA replication.