Neil Kad

BI300 - Introduction to Biochemistry

Nucleic Acids

• University of Kent

Lecture Outline

• Reminder: Drawing chemical structures

• The chemical structures of bases and nucleotides

• DNA and RNA: Polymers of bases

• Principles of base pairing

• Biological functions of DNA and RNA plus nucleotides in other contexts

Nucleotides and Nucleic Acids

Biological Function

• Nucleotides and nucleic acids play a critical role in the storage and expression of genetic information.

Structures of Nucleotides

• Common nucleotides consist of:

  • Nitrogenous bases (purines and pyrimidines)

  • Pentose sugars (ribose in RNA, deoxyribose in DNA)

  • Phosphates

Double-Stranded DNA

• Structure consists of complementary strands held together by hydrogen bonds.

• DNA can undergo denaturation (separating strands) and annealing (rejoining strands).

Chemistry of Nucleic Acids

• Process of mutagenesis can alter the structure and function of nucleic acids.

Drawing Chemical Structures

• Importance of accurately depicting chemical structures of nucleotides.

• Techniques explained through Moodle links, focusing on organic molecule representation.

Nitrogenous Bases

Types of Bases

• Pyrimidines: Single six-membered ring (Cytosine, Thymine, Uracil)

• Purines: Double-ring structure (Adenine, Guanine)

UV Absorption

• These bases are nitrogen-containing heteroaromatic molecules capable of absorbing UV light around 250-270 nm.

Nucleoside and Nucleotide Formation

• Nucleotides are composed of:

  • Nitrogenous base + Sugar (Nucleoside)

  • Nucleoside + Phosphate(s) = Nucleotide (e.g. Adenosine triphosphate - ATP)

• The β−N-Glycosidic bond connects the pentose sugar to the base.

• The bond stability and cleavage mechanisms are important in biochemical reactions.

Nomenclature

• Nucleotides and Nucleic Acids:

  • Purines: Adenine (A), Guanine (G)

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

• Understand the nomenclature of bases, nucleosides, nucleotides, and nucleic acids.

Discovery of DNA Structure

• Key players: Watson and Crick's model illustrated the helical structure of DNA.

• Chargaff’s Rule: Specific pairing (A-T and G-C)

• Contributions from Franklin’s X-ray diffraction studies clarified the structure.

Base Pairing

• DNA strands:

  • Two strands are complementary and run antiparallel.

  • Base pairs linked by hydrogen bonds (A:T has 2, G:C has 3).

  • Base stacking interactions contribute to the stability of the DNA.

DNA Structure

• DNA exists typically as double-stranded with distinct grooves (major and minor).

• Distinct forms of DNA include A-form, B-form, and Z-form based on geometric and environmental factors.

Replication of Genetic Code

• DNA replication involves strand separation and synthesis of new strands using DNA polymerases.

• Newly synthesized strands contain one parent and one daughter strand.

Stability and Denaturation

• Hydrogen bonds provide stability, but can be disrupted by heat (denaturation).

• Denaturation is not uniform—AT-rich regions melt at lower temperatures than GC-rich regions.

Nucleic Acid Functions

• DNA: Information carrier (genetic blueprint)

• RNA: Information carrier and catalyst (e.g., ribozymes, mRNA, tRNA, rRNA)

Nucleotides as Energy Carriers

• ATP functions as an energy currency in biological systems, linking energy release (hydrolysis) to cellular processes.

Other Functions of Nucleotides

• Nucleotides serve not only as building blocks but also play roles in biochemical pathways (e.g., coenzymes like NAD+ and FAD).

Summary

• Understanding the chemical structures, functions, and interactions of nucleic acids and nucleotides is critical in biochemistry.