📌Definition Table

Term	Definition
Nucleic Acid	A polymer of nucleotides that stores and transmits genetic information.
Nucleotide	Monomer unit of nucleic acids composed of a phosphate group, pentose sugar, and nitrogenous base.
Nucleoside	A nitrogenous base covalently bonded to a sugar, without a phosphate group.
Phosphodiester Bond	Covalent linkage between the 3′ hydroxyl group of one sugar and the 5′ phosphate group of another nucleotide.
Antiparallel	Orientation of the two DNA strands in opposite 5′ → 3′ directions.
Base Pairing	Specific hydrogen bonding between complementary nitrogenous bases (A–T or A–U, G–C).

📌Introduction

Nucleic acids are the essential molecules that store, transmit, and express genetic information in all living organisms. DNA serves as the long-term storage molecule, while RNA plays diverse roles in decoding, regulation, and catalysis, with their structures directly determining their biological functions.

📌 Components of Nucleotides

• Phosphate Group:
  • Negatively charged at physiological pH.
  • Imparts acidic nature to nucleic acids.
  • Links sugars via phosphodiester bonds.
  • Part of the sugar–phosphate backbone.
  • Can be mono-, di-, or triphosphate in energy molecules like ATP.
• Pentose Sugar:
  • Ribose in RNA (–OH at 2′ and 3′ carbons → more reactive).
  • Deoxyribose in DNA (–H at 2′ carbon → more stable).
  • Numbered carbons guide bond locations.
  • Influences chemical stability and function.
  • Determines type of nucleic acid.
• Nitrogenous Bases:
  • Purines (A, G) – double ring.
  • Pyrimidines (C, T, U) – single ring.
  • Uracil replaces thymine in RNA.
  • Sequence encodes genetic information.
  • Hydrogen bonding specificity ensures replication accuracy.

🧠 Examiner Tip: Always label the sugar type and number carbons correctly when drawing nucleotides.

📌 DNA Structure

• Two polynucleotide strands form a right-handed double helix.
• Strands are antiparallel (5′ → 3′ and 3′ → 5′).
• Complementary base pairing: A–T (2 H bonds), G–C (3 H bonds).
• GC-rich DNA is more thermally stable.
• Sugar–phosphate backbone is hydrophilic and faces outward.
• Bases are hydrophobic and stacked inward for stability.
• Major and minor grooves allow protein binding.

🧬 IA Tips & Guidance: Investigate DNA melting temperature with different GC contents using spectrophotometry at 260 nm.

📌 RNA Structure

• Usually single-stranded but folds into hairpins and loops.
• Contains ribose sugar (more reactive than deoxyribose).
• Bases: A, G, C, U (uracil instead of thymine).
• Types: mRNA, tRNA, rRNA, miRNA, siRNA.
• mRNA carries genetic code to ribosomes.
• tRNA links codons to amino acids.
• rRNA forms ribosomal structure and catalyzes peptide bonds.

🌐 EE Focus: Study RNA degradation under different pH or temperatures to link structure to stability.

📌 Phosphodiester Bonds and Backbone Formation

• Covalent bonds between 3′ OH of one sugar and 5′ phosphate of the next.
• Backbone is highly stable in DNA.
• RNA is more prone to hydrolysis due to 2′-OH group.
• Requires polymerase enzymes for synthesis.
• Directionality dictates replication and transcription.
• Protects genetic information from random cleavage.

❤️ CAS Link: Organize a DNA/RNA model-building activity to teach base pairing and backbone structure.

📌 DNA Packaging in Cells

• Prokaryotes: Circular DNA, supercoiled in nucleoid region.
• Eukaryotes: Linear DNA wrapped around histones → nucleosomes → chromatin → chromosomes.
• Euchromatin is loosely packed and transcriptionally active.
• Heterochromatin is tightly packed and inactive.
• Packaging controls accessibility for gene expression.
• Protects DNA from damage.

📌 Central Dogma Connection

• Information flows: DNA → RNA → Protein.
• Transcription uses RNA polymerase to make RNA from DNA.
• Translation uses ribosomes to build proteins from mRNA.
• Reverse transcription (RNA → DNA) occurs in retroviruses.
• RNA editing and alternative splicing modify genetic output.

🔍 TOK Perspective: The Central Dogma is a guiding principle, but exceptions remind us that scientific knowledge evolves.

📝 Paper 2: Data Response Tips: Be ready for questions on DNA melting curves, GC content, and RNA vs. DNA stability.