Applied Human Biology I - Nucleic Acids 1

Fundamentals of Nucleic Acids

• Nucleic acids are biological macromolecules composed of repeating monomeric units called nucleotides.

• Primary functions of nucleic acids in biological systems include:

  • Storing genetic information required for life processes.
  • Containing precise instructions for essential cellular activities and protein synthesis.
  • Conducting and regulating biochemical reactions.

• Major classes of nucleic acids:

  • Deoxyribonucleic Acid (DNA):
  • Stores hereditary information needed to self-replicate.
  • Specifies the precise linear sequence of amino acids in polypeptide chains during protein synthesis.
  • Packaged within the cell nucleus as chromatin and condensed into chromosomes.

• Ribonucleic Acid (RNA):
  • Exists in diverse molecular types fulfilling multiple biological roles.
  • Serves as essential components of coenzymes that assist and regulate enzymatic activity.
  • Includes specialized nucleotide derivatives such as Adenosine Triphosphate (ATP), which acts as the primary cellular energy store.

Nucleotide Structure and Chemistry

• Every individual nucleotide monomer consists of three distinct chemical components:
  • A phosphate group attached to the pentose sugar.
  • A 5-carbon pentose sugar.
  • A nitrogen-containing (nitrogenous) base.

• Pentose Sugar Carbon Numbering System:

  • The carbon atoms of the pentose sugar ring are numbered with prime symbols ($1'$ through $5'$) to distinguish them from the atoms in the nitrogenous base.
  • The nitrogenous base is covalently linked to the $1'$ carbon.
  • The phosphate group is linked to the $5'$ carbon.
  • The $3'$ and $2'$ carbon positions carry hydroxyl ($-\text{OH}$) or hydrogen ($-\text{H}$) groups essential for structural differentiation between RNA and DNA.

• Comparison of Pentose Sugars:

  • Ribose: Found in RNA; contains a hydroxyl group ($-\text{OH}$) attached to the $2'$ carbon position.
  • $2'$-Deoxyribose: Found in DNA; contains a hydrogen atom ($-\text{H}$) attached to the $2'$ carbon position, having lost an oxygen atom compared to ribose.

• Nitrogenous Base Classification:
  • Nitrogenous bases belong to two distinct chemical families based on ring structure:
  • Purines: Double-ring heterocyclic aromatic structures.
    • Adenine (A)
    • Guanine (G)
  • Pyrimidines: Single-ring heterocyclic aromatic structures.
    • Cytosine (C)
    • Thymine (T) (exclusive to DNA)
    • Uracil (U) (exclusive to RNA)

Structure of Deoxyribonucleic Acid (DNA)

• DNA nucleotides are composed of a phosphate group, a $2'$-deoxyribose sugar, and one of four nitrogenous bases ($\text{A}$, $\text{G}$, $\text{C}$, or $\text{T}$).

• Double-Stranded Antiparallel Architecture:

  • DNA consists of two polynucleotide strands wound around a central axis in a double helix shape.
  • The sugar-phosphate backbones form the outer structural frame through alternating phosphate and sugar residues.
  • The two strands run in opposite directions and are designated as antiparallel:
  • One strand is oriented in the $5' \rightarrow 3'$ direction.
  • The complementary strand is oriented in the $3' \rightarrow 5'$ direction.

• Complementary Base Pairing and Hydrogen Bonding:

  • Nitrogenous bases project inward toward the center of the double helix perpendicular to the backbone.
  • Strands are held together via specific hydrogen bonds ($\text{H-bonds}$) formed between complementary base pairs:
  • Adenine (A) pairs exclusively with Thymine (T) via two hydrogen bonds.
  • Guanine (G) pairs exclusively with Cytosine (C) via three hydrogen bonds.
  • Helical dimensions: DNA contains approximately $10$\,base pairs ($\text{bp}$) per complete turn of the double helix.

• Representative DNA Strand Pairing Sequence:
  • Upper Strand ($5' \rightarrow 3'$): $5'\text{ -- G -- C -- G -- G -- A -- T -- T -- T -- }3'$
  • Lower Strand ($3' \rightarrow 5'$): $3'\text{ -- C -- G -- C -- C -- T -- A -- A -- A -- }5'$

Structure and Hierarchical Folding of Ribonucleic Acid (RNA)

• RNA nucleotides consist of a phosphate group, a ribose sugar, and one of four nitrogenous bases ($\text{A}$, $\text{G}$, $\text{C}$, or $\text{U}$).

• Strand Topology:
  • RNA is typically single-stranded in cellular organisms, though certain RNA viruses can possess double-stranded RNA genomes.
  • Monomeric nucleotides are sequentially linked via covalent $5'\text{--}3'$ phosphodiester bonds established between the ribose sugar of one nucleotide and the phosphate group of the adjacent nucleotide.

• Structural Hierarchy of RNA:
  • Primary Structure: The linear sequence of RNA nucleotides written from the $5'$ end to the $3'$ end.
  • Secondary Structure: Intramolecular base pairing via hydrogen bonds between complementary regions on the same single strand, producing localized stem-loop or cloverleaf secondary configurations.
  • Tertiary Structure: Complex three-dimensional folding of the secondary structural motifs, yielding a compact overall molecular shape composed of specific spatial helices and grooves.

Systematic Structural Comparison: DNA vs. RNA

• Key structural distinctiveness:
  • The polymer backbone of both nucleic acid types consists of alternating phosphate and pentose sugar units ($\text{--phosphate--sugar--phosphate--sugar--}$).
  • The nitrogenous bases project laterally outward from one side of the sugar-phosphate backbone.
  • Nucleotides are designated and identified by their constituent nitrogenous base, as the base is the variable structural component in a nucleic acid chain.

Adenosine Triphosphate (ATP) and Cellular Energetics

• Structure of ATP:

  • ATP is a specialized nucleotide composed of:
  • An adenosine nucleoside core (consisting of an adenine nitrogenous base bonded to a ribose sugar).
  • A chain of three phosphate groups ($\text{triphosphate}$) covalently attached to the $5'$ carbon of the ribose sugar.

• Mechanism of Energy Storage and Release:

  • ATP functions as a high-energy carrier molecule essential for driving endergonic cellular reactions.
  • The bonds linking adjacent phosphate groups (phosphoanhydride bonds) store substantial chemical potential energy.
  • Cleavage of the terminal bond between the second ($\beta$) and third ($\text{γ}$) phosphate groups via hydrolysis releases free energy:

$\text{ATP} + \text{H}_2\text{O} \rightleftharpoons \text{ADP} + \text{P}_i + \text{energy}$

• Biological Applications of ATP Hydrolysis:

  • Macromolecular Synthesis: Drives anabolic reactions such as peptide bond formation and nucleic acid polymerization.
  • Muscle Contraction: Supplies mechanical energy required for myofilament sliding.
  • Nerve Signal Conduction: Powers active transport ion pumps (e.g., $\text{Na}^+/\text{K}^+$ ATPase) to maintain resting membrane potential across neuronal membranes.

• The ATP-ADP Cycle:

  • Hydrolysis of ATP yields Adenosine Diphosphate (ADP) and an inorganic phosphate group ($\text{P}_i$).
  • ADP is continuously recycled back into ATP through exergonic metabolic pathway reactions (e.g., cellular respiration) that re-attach an inorganic phosphate group to ADP.