Nucleic Acid Structure: DNA, RNA, and Their Biological Functions

Nucleotides: The Building Blocks of Nucleic Acids

Structural Components of Nucleotides

Nucleotides are the fundamental units of nucleic acids, consisting of three main components: a heterocyclic nitrogenous base, a five-carbon sugar (ribose or deoxyribose), and a phosphate group. These components are joined together through condensation reactions, forming nucleosides (sugar + base) and nucleotides (nucleoside + phosphate).

• Heterocyclic nitrogenous base: Aromatic, planar, and capable of hydrogen bonding. Two types: purines (adenine, guanine) and pyrimidines (cytosine, thymine, uracil).

• Sugar: Ribose (in RNA) or deoxyribose (in DNA).

• Phosphate group: Provides negative charge and links nucleotides via phosphodiester bonds.

• Nucleoside: Sugar + base (no phosphate).

• Nucleotide: Nucleoside + phosphate.

Nitrogenous Bases and Their Properties

The nitrogenous bases are classified as purines (adenine, guanine) and pyrimidines (cytosine, thymine, uracil). They are aromatic, planar, and contain hydrogen bond donors and acceptors, which are essential for base pairing in nucleic acids.

• Purines: Adenine (A), Guanine (G)

• Pyrimidines: Cytosine (C), Thymine (T, DNA only), Uracil (U, RNA only)

• Hydrogen bonding: Key for base pairing specificity

Formation and Structure of Nucleic Acids

Polynucleotide Formation

Nucleotides are linked together by phosphodiester bonds, forming polynucleotides. The linkage occurs between the 3' hydroxyl group of one sugar and the 5' phosphate group of another, creating a sugar-phosphate backbone.

• Phosphodiester bond: Connects nucleotides in a linear chain

• Directionality: Written from 5' to 3'

DNA vs. RNA: Key Differences

DNA and RNA are the two main types of nucleic acids, differing in their sugar, bases, and structure.

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, G, C, T	A, G, C, U
Strands	Double-stranded	Single-stranded

3D Structure of DNA: The Double Helix

Key Structural Features

Double-stranded DNA forms a double helix, with two antiparallel strands. The phosphate backbone is on the outside, and the bases are stacked perpendicular to the backbone, forming the steps of a spiral staircase.

• Antiparallel strands: One strand runs 5' to 3', the other 3' to 5'

• Phosphate backbone: Negatively charged, on the exterior

• Base stacking: Planar bases provide stability via dispersion forces

Base Pairing and Stability

Base pairs are formed by hydrogen bonds between complementary bases: A-T (2 H-bonds) and G-C (3 H-bonds). Stacking interactions between bases contribute significantly to the stability of the double helix.

• A-T pair: 2 hydrogen bonds

• G-C pair: 3 hydrogen bonds (stronger stacking interactions)

DNA Organization in the Cell

Chromatin, Nucleosomes, and Chromosomes

DNA is compacted in the cell nucleus as chromatin, which consists of DNA wrapped around histone proteins to form nucleosomes. Chromatin is further organized into chromosomes during cell division. Histones are rich in basic amino acids (arginine, lysine) to facilitate electrostatic interactions with the negatively charged DNA backbone.

• Chromatin: Orderly tangle of DNA in non-dividing cells

• Nucleosome: DNA wrapped around histone proteins

• Chromosome: Highly compacted DNA-protein complex

• Gene: Segment of DNA encoding a single polypeptide

Complementarity and Base-Pairing

Chargaff’s Rules and DNA Complementarity

Chargaff’s rules state that in double-stranded DNA, the number of adenine bases equals thymine, and the number of guanine equals cytosine. This complementarity allows for accurate replication and information transfer.

• Complementary base pairing: A-T, G-C

• Reverse complement: Sequence of one strand determines the other, written in opposite direction

Mutations in DNA and the Central Dogma

DNA Mutation and Replication

Mutations are changes in the DNA sequence, often caused by mutagens such as radiation or chemicals. If not repaired, mutations are propagated during DNA replication. DNA replication is semiconservative, meaning each new double-stranded DNA contains one old and one new strand.

• Mutagen: Substance causing DNA mutation

• Semiconservative replication: Each ds DNA product has one old and one new strand

• Central Dogma: DNA → RNA → Protein

Protein-DNA Interactions

Sequence-Specific Recognition

Proteins interact with DNA primarily through ionic interactions between positively-charged amino acids (arginine, lysine, histidine) and the negatively-charged phosphate backbone. Sequence-specific recognition occurs when proteins "read" the unique patterns of hydrogen bond donors and acceptors in the major groove of DNA.

• Salt bridges: Ionic interactions between protein side chains and DNA phosphates

• Major groove: Site of sequence-specific protein binding

Summary Table: DNA vs. RNA vs. Nucleotides

Component	DNA	RNA	Nucleotide
Sugar	Deoxyribose	Ribose	Ribose/Deoxyribose
Bases	A, G, C, T	A, G, C, U	Any one base
Phosphate	Yes	Yes	Yes
Structure	Double-stranded	Single-stranded	Monomer