Nucleoside and Nucleotide Definitions

A nucleoside is defined as a base combined with a (deoxy)ribose sugar. A nucleotide is a nucleoside with an added phosphate group, linked by a 3′-5′ phosphodiester bond.

The 5′ end of an incoming nucleotide carries the triphosphate, which serves as the energy source for bond formation. The alpha-phosphate is the target for a 3′ hydroxyl attack during phosphodiester bond formation.

Purines, Pyrimidines, and Deamination

Purines, such as Adenine (A) and Guanine (G), are characterized by having two rings. Pyrimidines, including Cytosine (C), Uracil (U), and Thymine (T), have a single ring structure.

Important deamination reactions include: Cytosine converting to Uracil, Adenine to Hypoxanthine, Guanine to Xanthine, and 5-methylcytosine to Thymine. Remember that Uracil is typically found in RNA, while Thymine is found in DNA. The methylation of Uracil is what forms Thymine.

Nucleotide Structure Diagram

#boundingbox

This diagram illustrates the fundamental components of a nucleotide. You can see how a nitrogenous base combines with a deoxyribose sugar to form a nucleoside. The addition of a phosphate group to this nucleoside then completes the structure of a nucleotide.

Purine and Pyrimidine Synthesis

Several amino acids are crucial for purine synthesis. These include Glycine, Aspartate, and Glutamine. A mnemonic to remember these is “cats purr until they GAG.”

#boundingbox

The diagram illustrates the synthesis pathways for both purines and pyrimidines. For purines (A, G), you can see the involvement of Aspartate, Glycine, and Glutamine. For pyrimidines (C, U, T), key precursors include Aspartate and Carbamoyl phosphate, with N10-Formyl-tetrahydrofolate also playing a role in some steps.

DNA/RNA Base Characteristics

Here are some key facts about nitrogenous bases and DNA structure. Remember the mnemonics: “Pure As Gold” for Purines (Adenine, Guanine) and “CUT the pyramid” for Pyrimidines (Cytosine, Uracil, Thymine).

Thymine is distinguished by having a methyl group. The Cytosine-Guanine (C-G) bond is stronger than the Adenine-Thymine (A-T) bond because C-G pairs form three hydrogen bonds, while A-T pairs form only two. Consequently, a higher C-G content in DNA leads to a higher melting temperature, often described as “C-G bonds are like Crazy Glue” due to their strong association.

Topics Covered:

• Nucleoside and Nucleotide Basics
• Base Types and Deamination
• Visualizing Nucleotide Structure
• Nucleotide Synthesis Pathways
• Base Pairing and DNA Stability