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 nitrogen base: Purines (adenine, guanine) and pyrimidines (cytosine, thymine, uracil).

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

• Phosphate group: Provides negative charge and enables linkage between nucleotides.

Example: ATP (adenosine triphosphate) is a nucleotide with three phosphate groups.

Nucleosides vs. Nucleotides

A nucleoside consists of a sugar and a base, while a nucleotide includes a phosphate group. Nucleotides can have one (monophosphate), two (diphosphate), or three (triphosphate) phosphate groups.

• Nucleoside: Sugar + base

• Nucleotide: Nucleoside + phosphate

Nucleic Acids: DNA and RNA

Polynucleotide Formation

Nucleic acids are linear polymers of nucleotides, connected by phosphodiester bonds. The backbone consists of alternating sugar and phosphate groups, with the sequence written from the 5' phosphate end to the 3' hydroxyl end.

• Phosphodiester bond: Links the 3' carbon of one sugar to the 5' phosphate of another.

• Sequence convention: 5' → 3'

DNA vs. RNA: Key Differences

DNA and RNA differ in their sugar, bases, and structure:

• DNA: Deoxyribose sugar, bases A, G, C, T, double-stranded.

• RNA: Ribose sugar, bases A, G, C, U, 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' → 3', the other 3' → 5'.

• Phosphate backbone: Negatively charged, faces outward.

• Base stacking: Planar bases provide stability via dispersion forces.

Base Pairing and Stability

Base pairs are formed by hydrogen bonding between complementary bases:

• A-T: 2 hydrogen bonds

• G-C: 3 hydrogen bonds

Stacking interactions between bases (especially GC pairs) provide additional stability.

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.

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

• Nucleosome: DNA wrapped around histone proteins (rich in Arg and Lys).

• Chromosome: Highly compacted DNA-protein complex visible during cell division.

Protein-DNA Interactions

Proteins interact with DNA primarily through ionic interactions between positively-charged amino acids (Arg, Lys, His) and the negatively-charged phosphate backbone. These interactions are crucial for DNA packaging and regulation.

Complementarity and Base-Pairing

Chargaff’s Rules and DNA Complementarity

Chargaff’s rules state that in double-stranded DNA, the number of adenine (A) equals thymine (T), and the number of guanine (G) equals cytosine (C). This complementarity allows for accurate replication and information transfer.

• Complementary pairs: A-T, G-C

• Reverse complement: The sequence of one strand determines the other, written in the opposite direction.

Mutations in DNA

Causes and Consequences

Mutations are changes in the DNA sequence, often caused by mutagens such as radiation or chemicals. If not repaired, mutations can be propagated during DNA replication, potentially affecting protein synthesis and cellular function.

• Mutagen: Substance causing mutation (e.g., X-rays, UV, ROS).

• Semiconservative replication: Each new DNA molecule contains one old and one new strand.

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information: DNA is replicated, transcribed into RNA, and translated into protein. Each process involves initiation, elongation, and termination phases.

• Replication: DNA → DNA

• Transcription: DNA → RNA

• Translation: RNA → Protein

Summary Table: DNA vs. RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, G, C, T	A, G, C, U
Strands	Double	Single
Function	Genetic storage	Genetic decoding, catalysis