Structure and Properties of Nucleotides and Nucleic Acids

Structure of Nucleotides and Nucleic Acids

Introduction

Nucleotides and nucleic acids are fundamental biomolecules responsible for the storage, transmission, and expression of genetic information in all living organisms. This section explores their chemical structure, classification, and the molecular interactions that stabilize nucleic acid polymers.

Main Types of Nucleic Acids

DNA and RNA

• Deoxyribonucleic Acid (DNA): The primary molecule for long-term storage of genetic information in cells. DNA contains genes and regulatory sequences that control gene expression.

• Ribonucleic Acid (RNA): Involved in the expression of genetic information through processes such as transcription (mRNA) and translation (tRNA, rRNA). Various small RNAs also play regulatory roles.

• Viruses: Some viruses use RNA instead of DNA to store genetic information.

Structure of Nucleotides

Components of Nucleotides

• Nitrogenous Base: Aromatic heterocyclic molecules, either purines or pyrimidines.

• Pentose Sugar: Ribose in RNA, deoxyribose in DNA.

• Phosphate Group(s): One or more phosphate groups attached to the 5' carbon of the sugar.

The repeating unit of RNA is a ribonucleotide, while that of DNA is a deoxyribonucleotide.

Definitions

• Nitrogenous Base: The aromatic base component (adenine, guanine, cytosine, thymine, uracil).

• Nucleoside: A nitrogenous base covalently linked to a pentose sugar.

• Nucleotide: A nucleoside with one or more phosphate groups attached.

Nitrogenous Bases

Classification

• Purines: Double-ring structures (adenine, guanine).

• Pyrimidines: Single-ring structures (cytosine, thymine, uracil).

Both purines and pyrimidines are aromatic and planar, allowing them to participate in stacking interactions and absorb UV light at 260 nm.

Structures and Functional Groups

• Adenine (A): Purine with an amino group.

• Guanine (G): Purine with an amino and a carbonyl group.

• Cytosine (C): Pyrimidine with an amino and a carbonyl group.

• Thymine (T): Pyrimidine with two carbonyls and a methyl group (found in DNA).

• Uracil (U): Pyrimidine with two carbonyls (found in RNA).

Other Biologically Relevant Purines

• Hypoxanthine, Xanthine, Theobromine, Caffeine, Uric Acid, Isoguanine: These purines have roles in metabolism and signaling. For example, theobromine is found in chocolate and is toxic to some animals.

Table: Toxicity of Theobromine in Different Species

Species	Oral Toxicity (LD50, mg/kg)
Cat	200
Dog	300
Human	1000
Rat	1200

Additional info: Theobromine toxicity explains why chocolate is dangerous for dogs and cats.

Biochemical Roles of Adenosine and Caffeine

• Adenosine: Component of ATP, involved in energy transfer, blood vessel dilation, and sleep regulation.

• Caffeine: Structural analogue of adenosine; blocks adenosine receptors, promoting wakefulness.

Nucleosides and Nucleotides

Formation and Linkages

• Nucleoside: Base linked to the 1' carbon of ribose or deoxyribose via a β-N-glycosidic bond.

• Nucleotide: Nucleoside with one or more phosphate groups attached to the 5' carbon.

• Phosphoanhydride Bonds: High-energy bonds between phosphate groups in di- and tri-phosphate nucleotides (e.g., ATP).

Phosphate Group Designations

• Phosphates are labeled as alpha (α), beta (β), and gamma (γ) from the sugar outward.

Acidity of Nucleic Acids

• Nucleic acids are acidic due to the ionization of phosphate groups at physiological pH.

• At physiological pH, the phosphate group is deprotonated, giving nucleic acids a net negative charge.

Hydrogen Bonding and Base Pairing

Hydrogen Bond Donors and Acceptors

• Nitrogenous bases have functional groups that can act as hydrogen bond donors or acceptors.

• These interactions are essential for the formation of double-stranded DNA and for protein-DNA interactions.

• pH can influence the ionization state of these groups, affecting nucleic acid stability.

Base Pairing Rules

• Adenine (A) pairs with Thymine (T) in DNA via two hydrogen bonds.

• Guanine (G) pairs with Cytosine (C) via three hydrogen bonds.

• In RNA, uracil (U) replaces thymine and pairs with adenine.

Structure of Nucleic Acid Polymers

Directionality and Backbone

• Nucleic acids are polymers of nucleoside monophosphates linked by phosphodiester bonds.

• They have directionality: a 5' phosphate end and a 3' hydroxyl end.

• The sugar-phosphate backbone is hydrophilic and surface-exposed, while the bases are hydrophobic and stacked inside.

Double-Stranded DNA (dsDNA)

• Consists of two complementary strands running in antiparallel directions (5' to 3' and 3' to 5').

• Base pairing follows strict rules (A-T, G-C).

• Double helix is stabilized by hydrogen bonding, hydrophobic effect, and base stacking (van der Waals interactions).

Stabilizing Forces in DNA

• Hydrogen Bonding: Between complementary bases on opposite strands.

• Hydrophobic Effect: Bases are shielded from water, favoring helix formation.

• Base Stacking: Van der Waals (London dispersion) forces between adjacent bases provide major stability.

Table: Major Stabilizing Interactions in dsDNA

Interaction Type	Description	Relative Contribution
Hydrogen Bonding	Between base pairs	Moderate
Hydrophobic Effect	Exclusion of bases from water	Significant
Base Stacking	Van der Waals forces between bases	Major

Helical Forms and Topology of DNA

Forms of Double-Stranded DNA

• B-form: Predominant cellular form; right-handed helix; 10-11 nucleotides per turn; width ~2.4 nm; pitch ~3.4 nm.

• A-form: Right-handed, more compact; occurs under dehydrating conditions.

• Z-form: Left-handed helix; occurs in specific sequences or under certain conditions.

Major and Minor Grooves

• The geometry of base pairs creates major and minor grooves in the helix.

• Proteins often bind to the major groove, where more functional groups are accessible for sequence-specific recognition.

Biological Relevance and Applications

Protein-DNA Interactions

• Proteins involved in replication, transcription, and regulation recognize specific DNA sequences by interacting with exposed functional groups in the major groove.

Measurement and Analysis

• DNA concentration is commonly measured by UV absorbance at 260 nm due to the aromaticity of the bases.

Example: DNA Sequence Complementarity

• Given a DNA strand 5'-AGTC-3', the complementary strand is 3'-TCAG-5'.

Summary Table: Key Differences Between DNA and RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, G, C	A, U, G, C
Strandedness	Double-stranded (usually)	Single-stranded (usually)
Stability	More stable	Less stable

Additional info: The initial images of animals mimicking bird droppings (e.g., Theloderma asperum, Celaenia excavata, Eudryas unio) are likely used as an analogy for molecular mimicry, such as caffeine mimicking adenosine in biochemistry.