Question

Identify each of the following bases as a pyrimidine or a purine: a. cytosine b. adenine c. uracil d. thymine e. guanine

Short answer

Cytosine, uracil, thymine are pyrimidines; adenine, guanine are purines.

Step-by-step solution

- Understanding Purines and Pyrimidines

Purines and pyrimidines are the two categories of nitrogenous bases in nucleic acids. Purines have a double-ring structure while pyrimidines have a single-ring structure.

- Identify Cytosine

Cytosine is a pyrimidine because it contains a single-ring structure.

- Identify Adenine

Adenine is a purine because it contains a double-ring structure.

- Identify Uracil

Uracil is a pyrimidine because it contains a single-ring structure.

- Identify Thymine

Thymine is a pyrimidine because it contains a single-ring structure.

- Identify Guanine

Guanine is a purine because it contains a double-ring structure.

Key concepts

purines

Purines are one of the two main types of nitrogenous bases found in nucleic acids like DNA and RNA. These bases have a unique structure characterized by two rings, making them larger than pyrimidines. The two most common purines are adenine (A) and guanine (G). Purines play a crucial role in forming the backbone of nucleic acids by pairing with pyrimidines through hydrogen bonds. This bond formation is critical for the structure and function of DNA and RNA. Keep in mind:

• Adenine pairs with Thymine in DNA
• Guanine pairs with Cytosine in both DNA and RNA

pyrimidines

Pyrimidines are the second category of nitrogenous bases, distinguished by having a single-ring structure. This makes them smaller than purines. Common pyrimidines include cytosine (C), thymine (T), and uracil (U). Thymine is found only in DNA, while uracil is found only in RNA. Pyrimidines are essential for creating the rungs of the DNA ladder by forming hydrogen bonds with purines. Key pairings include:

• Thymine (T) with Adenine (A) in DNA
• Cytosine (C) with Guanine (G) in both DNA and RNA
• Uracil (U) with Adenine (A) in RNA

nucleic acids

Nucleic acids are long molecules made of repeating units called nucleotides. They include DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Each nucleotide consists of three components: a nitrogenous base (which can be a purine or pyrimidine), a sugar molecule (deoxyribose in DNA and ribose in RNA), and a phosphate group. Nucleic acids store and transmit genetic information. DNA contains the instructions needed for an organism to grow, develop, and reproduce. RNA helps in reading these instructions and making proteins based on the genetic code. DNA and RNA differ in one key structural element: DNA uses thymine as a base, whereas RNA uses uracil.

single-ring structure

The single-ring structure is a defining feature of pyrimidines. This structure consists of a six-membered ring made up of carbon and nitrogen atoms. Because of their smaller size, pyrimidines are able to pair with purines, which have a larger double-ring structure. This single-ring shape contributes to the stable and predictable pairing patterns in the DNA and RNA molecules. To sum up, bases with a single-ring structure include:

• Cytosine (C)
• Thymine (T) - only in DNA
• Uracil (U) - only in RNA

double-ring structure

The double-ring structure is characteristic of purines. This structure consists of a fused pair of rings, one six-membered and one five-membered, made up of carbon and nitrogen atoms. This larger structure allows purines to generally pair with the smaller, single-ring pyrimidines. This structural complementarity plays a fundamental role in the double-helix formation of DNA and the various shapes RNA can take. The key bases with a double-ring structure include:

• Adenine (A)
• Guanine (G)

These bases are essential in the stable helical structure of DNA and RNA through hydrogen bonding.