Nucleic acids Knowledge-Based Study

Study notes

1. What Are Nucleic Acids?

   Nucleic acids are essential biological macromolecules that store, transmit, and express genetic information in all living organisms. The two primary types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

   The Building Blocks: Nucleotides

   Nucleic acids are polymers composed of repeating monomer subunits called nucleotides. Each individual nucleotide consists of three covalently bonded components:

   • A Pentose Sugar: A five-carbon ring sugar, which is either deoxyribose (in DNA) or ribose (in RNA). Ribose has a hydroxyl group ($-\text{OH}$) at the 2' carbon position, whereas deoxyribose has a single hydrogen atom ($-\text{H}$), lacking one oxygen atom.
   • A Nitrogenous Base: An organic ring structure containing nitrogen. These bases are grouped into two categories:
     • Purines: Double-ring structures, specifically adenine (A) and guanine (G).
     • Pyrimidines: Single-ring structures, specifically cytosine (C), thymine (T) (unique to DNA), and uracil (U) (unique to RNA).
   • A Phosphate Group: A functional group ($-\text{PO}_4^{3-}$) attached to the 5' carbon of the pentose sugar that imparts a negative charge to the overall molecule.

   Formation of Polynucleotide Chains

   Nucleotides link together through condensation (dehydration synthesis) reactions to form long polymers known as polynucleotide strands.

   Phosphodiester Bonds

   During chain elongation, the phosphate group attached to the 5' carbon of an incoming nucleotide forms a covalent bond with the hydroxyl group ($-\text{OH}$) on the 3' carbon of the existing sugar. This linkage is called a phosphodiester bond.

   Directionality

   Because nucleotides join in a standardized orientation, polynucleotide chains possess distinct chemical polarity or directionality:

   • 5' End: The terminus of the chain with a free or exposed phosphate group attached to the 5' carbon of the terminal sugar.
   • 3' End: The opposite terminus with a free hydroxyl group attached to the 3' carbon of the terminal sugar.

   By convention, nucleic acid sequences are always synthesized and written in the 5' to 3' direction.

   The DNA Double Helix

   DNA functions as the primary hereditary material in cells, encoding the instructions necessary for building proteins and coordinating cellular activities.

   Structural Features

   The molecular structure of DNA, first elucidated by James Watson and Francis Crick, exhibits several key characteristics:

   • Double-Stranded Helix: DNA consists of two polynucleotide strands winding around a central axis to form a right-handed double helix.
   • Antiparallel Alignment: The two strands run in opposite directions; one strand is oriented 5' to 3', while the complementary strand is oriented 3' to 5'.
   • Sugar-Phosphate Backbone: The hydrophilic sugar and phosphate groups form the outer structural rails of the helix.
   • Complementary Base Pairing: Nitrogenous bases face inward toward the center of the helix, pairing specifically through weak hydrogen bonds according to Chargaff's rules:
     • Adenine (A) pairs with Thymine (T) via two hydrogen bonds.
     • Guanine (G) pairs with Cytosine (C) via three hydrogen bonds.

   Properties and Roles of RNA

   While DNA stores genetic information long-term, RNA plays versatile, dynamic roles in gene expression, protein synthesis, and enzymatic catalysis.

   Structural Characteristics

   • Single-Stranded Architecture: RNA is typically synthesized as a single polynucleotide chain, although it can fold upon itself into complex three-dimensional secondary structures via intramolecular base pairing.
   • Uracil Instead of Thymine: RNA incorporates the pyrimidine base uracil (U), which pairs with adenine (A), replacing thymine.
   • Ribose Sugar: The presence of a 2' hydroxyl group makes RNA chemically more reactive and less stable than DNA.

   Major Classes of RNA

   1. Messenger RNA (mRNA): Carries copied genetic instructions from nuclear DNA to ribosomes in the cytoplasm.
   2. Ribosomal RNA (rRNA): Forms the structural and catalytic core of ribosomes, facilitating peptide bond formation.
   3. Transfer RNA (tRNA): Delivers specific amino acids to the ribosome during translation, matching codons through complementary anticodons.

   Key Differences Between DNA and RNA

   A direct comparison highlights the biochemical adaptations that enable DNA and RNA to perform distinct functions within the cell:

   • Pentose Sugar: Deoxyribose in DNA; ribose in RNA.
   • Nitrogenous Bases: A, T, C, G in DNA; A, U, C, G in RNA.
   • Strand Number: Double-stranded (antiparallel helix) in DNA; generally single-stranded in RNA.
   • Stability: Highly stable due to the absence of the 2' $-\text{OH}$ group in DNA; less stable and readily hydrolyzable in RNA.
   • Primary Function: Long-term storage and transmission of genetic instructions in DNA; short-term intermediary in protein synthesis and gene regulation in RNA.