Nucleic acids

1. Nucleotides Overview

Nucleic acids are polymers, and nucleotides serve as their monomers. A nucleotide is composed of three subunits:

• One or more phosphate groups

• A pentose sugar: Either deoxyribose or ribose

• A nitrogen-containing base: Adenine ($\text{A}$), cytosine ($\text{C}$), guanine ($\text{G}$), thymine ($\text{T}$), or uracil ($\text{U}$)

Nucleotides assemble into nucleic acids (DNA or RNA) to store and transfer genetic information, or form adenosine triphosphate (ATP) for short-term energy storage.

2. Nucleic Acids (DNA and RNA)

Nucleic acids are constructed when dehydration synthesis forms bonds between the pentose sugar of one nucleotide monomer and the phosphate group of another, creating a sugar-phosphate backbone with nitrogenous bases projecting outward.

2.1 Types of Nucleic Acids

1. Deoxyribonucleic Acid (DNA)

   • Function: Stores genetic information organized into genes across $46$ chromosomes in human cell nuclei (excluding mature red blood cells).

   • Structure: Contains deoxyribose sugar (which has one less oxygen atom than ribose), a phosphate group, and a nitrogenous base ($\text{A}$, $ext{C}$, $ext{G}$, or $ext{T}$).

   • Form: Typically double-stranded, consisting of two backbones twisted into a double helix structure.

2. Ribonucleic Acid (RNA)

   • Function: Transfers genetic information from DNA to assist in protein synthesis.

   • Structure: Contains ribose sugar, a phosphate group, and a nitrogenous base ($\text{A}$, $ext{C}$, $ext{G}$, or $ext{U}$).

   • Form: Typically single-stranded.

2.2 Nitrogenous Base Classification

• Purines: Nitrogen-containing bases with a double-ring structure.

  • Adenine ($\text{A}$)

  • Guanine ($\text{G}$)

• Pyramidines: Nitrogen-containing bases with a single-ring structure.

  • Cytosine ($\text{C}$)

  • Thymine ($\text{T}$ - exclusive to DNA)

  • Uracil ($\text{U}$ - exclusive to RNA)

2.3 Base Pairing Rules

In double-stranded nucleic acids, bases extending from opposite strands pair via hydrogen bonding:

• Adenine ($ext{A}$) pairs with Thymine ($ext{T}$) in DNA (or Uracil ($ext{U}$) in RNA).

• Cytosine ($ext{C}$) pairs with Guanine ($ext{G}$).

3. Adenosine Triphosphate (ATP)

Adenosine triphosphate (ATP) is a specialized nucleotide used for short-term energy storage and transfer within the body.

3.3 Structure of ATP

• Composed of an adenine base, a ribose sugar, and three phosphate groups.

• Classified as a high-energy compound because covalent bonds connecting adjacent phosphate groups store significant energy due to mutual electrostatic repulsion.

3.2 Hydrolysis and Energy Release

When energy is required for cellular activities (e.g., muscle contraction, active transport, anabolic reactions), the bond holding the third phosphate group is broken via hydrolysis:
$\text{ATP} + \text{H}<em>2\text{O} \rightarrow \text{ADP} + \text{P}</em>i + \text{energy}$

• Adenosine Diphosphate (ADP): Formed when one phosphate group is removed from ATP.

• Adenosine Monophosphate (AMP): Formed if a second phosphate group is cleaved.

3.3 Phosphorylation

• Definition: The chemical addition of a phosphate group to an organic compound.

• Reversibility: ADP can undergo phosphorylation to regenerate ATP. This dehydration synthesis process requires an equivalent reinvestment of energy to form the covalent phosphate bond.