Nucleic Acids: Structure, Base Pairing, and Biochemical Principles

Key Concepts

Main Topics

This section introduces the foundational principles of nucleic acid biochemistry, focusing on DNA and RNA structure, base pairing, and related molecular mechanisms.

• Differences Between DNA and RNA

• Nucleic Acid Building Blocks: Nucleotides, Nucleosides

• Structure of Purines & Pyrimidines

• Bonding & Numbering Conventions

• DNA Double Helix: Watson-Crick Model and Properties

• Chemical Stability and Degradation

• Mutation Mechanisms in Nucleic Acids

• Charge and Chemical Nature of Nucleic Acid Backbone

• Base Pairing and Structure

• Major and Minor Grooves

• Complementary Strands and Directionality

• Nucleosome and Chromatin Organization

• RNA Structure and Base Pairing

• Wobble Base Pairs

• RNA Secondary and Tertiary Structures

• RNA as Enzymes (Ribozymes)

Definitions

• DNA: Deoxyribonucleic Acid; contains deoxyribose sugar and bases A, T, G, C.

• RNA: Ribonucleic Acid; contains ribose sugar and bases A, U, G, C.

• Nucleotide: Composed of ribose/deoxyribose + base + phosphate(s).

• Nucleoside: Ribose/deoxyribose + base (no phosphate).

• Phosphate Groups: Designated alpha (closest to sugar), beta, gamma (farthest).

• Purines: Guanine (G), Adenine (A); double-ring structure, 9 atoms.

• Pyrimidines: Cytosine (C), Thymine (T, in DNA), Uracil (U, in RNA); single-ring, 6 atoms.

• Base Pairing: Specific hydrogen bonding between DNA bases (A-T, G-C) and RNA (A-U, G-C, G-U "wobble").

• Glycosidic Bond: A bond between nitrogenous base and sugar (at Carbon 1').

• Nucleosome: DNA wrapped (~146 bp) around histone protein octamer.

• Chromatin: Complex of DNA and proteins, existing as euchromatin (loosely packed) or heterochromatin (tightly packed).

• Wobble Base Pair: Non-standard pair (G-U) seen in RNA.

• Stem-Loop/ Hairpin: Secondary structure in RNA formed by base pairing within a single strand.

• Ribozyme: Enzyme made of RNA instead of protein.

Core Principles

Understanding nucleic acid structure and function is essential for biochemistry. Key principles include:

• Nucleic acids (DNA/RNA) are linear polymers made of nucleotides.

• Backbone: Sugar-phosphate, bases extend from sugar.

• 5' end (prime) and 3' end (three prime) nomenclature critical for directionality.

• DNA and RNA sequences are always written/read as 5' → 3'.

• Base pairing: A-T/U (2 H-bonds), G-C (3 H-bonds)

• Nucleic acids exhibit anti-parallel strand arrangement.

• Key in DNA: 2' deoxyribose (missing an O at 2' carbon); Sugar in RNA: ribose (OH group at 2' carbon)

• RNA is more chemically labile at high pH due to 2'-OH.

• Hydrogen Bonding: A-T (DNA) and A-U (RNA): 2 hydrogen bonds; G-C: 3 hydrogen bonds (stronger stability/melting point)

• Base Stacking: Stabilizes DNA via van der Waals interactions.

• Nucleosome Packing: DNA compaction is achieved via nucleosome assembly forming higher-order chromatin structures.

Practice Problems & Sample Solutions

Practice Problems

• Label a nucleotide, indicating sugar carbons and correct 'prime' notation.

• Determine directionality (5' and 3' ends) in a DNA fragment.

• Identify and name nucleoside vs. nucleotide for given diagrams.

• State number of H-bonds for A-T and G-C base pairs.

• Anticipate outcome if RNA is dissolved in pH 10.5 buffer.

Sample Solutions

• For 5':

  • 5 carbons in ribose labeled as 1', 2', 3', 4', 5'.

  • Phosphate attaches at 5'.

  • Base attaches at 1'.

• For H-bonds:

  • A-T: 2 Hydrogen bonds

  • G-C: 3 Hydrogen bonds

Important Details

Dates/Formulas

• DNA Structure Solved: Watson & Crick (1953), with X-ray data from Rosalind Franklin

• DNA Diameter: ~2 nm (20 Ångstroms)

• Helical Pitch (Full Turn Distance): 10.5 base pairs (one turn); 50 bp used for top exam

• Angle per Base Pair: 36° (360/10)

• Tm (Melting Temperature): Temperature at which 50% of dsDNA is separated

Key Equations:

• Base pairs per turn: $10.5$

• Angle per base pair: $36^\circ$

• DNA diameter: $2\ \text{nm}$

• Number of hydrogen bonds: $\text{A-T: 2},\ \text{G-C: 3}$

Critical Points

• DNA can be stored in slightly alkaline solutions; RNA must be kept at neutral/slightly acidic pH to prevent degradation.

• RNA isolation: sensitivity to basic conditions.

• PCR and Primer Design: Melting temperature (Tm) influenced by base composition (GC content increases Tm).

Study Questions

Quiz Items

• What is the structural difference between ribose and deoxyribose?

• What functional group allows RNA, but not DNA, to be degraded in basic solution?

• Name the bases found in DNA and RNA, highlighting the unique base in each.

• Define: nucleotide, nucleoside, glycosidic bond.

• What are the consequences of spontaneous deamination of cytosine in DNA and RNA?

• What is the typical helical pitch of B-form DNA? Its diameter?

• How many base pairs per turn in B-form DNA?

• Why is GC content important for DNA stability?

Discussion Points

• Discuss why RNA viruses mutate more rapidly than DNA genomes.

• Explain why base pairing is essential for information transfer in DNA/RNA.

• Debate the significance of the 2'-OH group in RNA both for function and stability.

Practice Problems

• Draw the chemical structure of a nucleotide, label all relevant atoms.

• Predict outcome of mixing RNA with 0.1 M NaOH.

• Classify the given structures: purine or pyrimidine, nucleoside or nucleotide.

Review Summary

Quick Reference

• DNA: Double helix, diameter ~2 nm, 10.5 bp/turn, A-T (2 H-bonds), G-C (3 H-bonds)

• RNA: Single-stranded, ribose sugar, uracil replaces thymine, G-U wobble base pair

• Nucleosome: DNA wraps ~146 bp around histone core

• Directionality: 5' → 3' for synthesis and reading

• Wobble G-U: for RNA base pairing

Connection Points

• Replication: All synthesis is 5' → 3'

• Transcription: RNA produced 5' → 3', directionality always maintained

• Lab Applications: PCR primers, gel electrophoresis (use negative backbone charge), RNA stability issues

• Physiological Relevance: DNA's stability over generations vs. RNA's lability for regulatory roles

Flags for Common Test Topics

• Prime notation for sugar carbons

• Base pairing rules and H-bonding

• Mechanistic details of acid/base degradation

• Naming nucleosides/nucleotides (AMP, ADP, ATP, etc.)

• DNA vs RNA differences (sugar, base, chemical stability)

• Meaning of melting temperature (Tm)

• Anti-parallel orientation of DNA strands

• Structural features of purines vs. pyrimidines

• Glycosidic linkage definition

Action Items

• Review sugar structures, practice prime labeling

• Distinguish and draw nucleoside vs. nucleotide

• Memorize base pairing patterns and number of H-bonds

• Understand and explain alkaline degradation of RNA

• Practice sequence notation 5' → 3'

• Learn and use DNA helical parameters (diameter, pitch, bp/turn)

• Prepare to define/recognize glycosidic bond

• Familiarize with chemical reasons for DNA/RNA backbone charges

Tables

Comparison of DNA and RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, G, C	A, U, G, C
Strandedness	Double-stranded (ds)	Single-stranded (ss)
Stability	Stable, less reactive	Labile, more reactive
Base Pairing	A-T (2 H-bonds), G-C (3 H-bonds)	A-U (2 H-bonds), G-C (3 H-bonds), G-U (wobble)
Function	Genetic storage	Genetic transfer, catalysis (ribozymes)

Base Pair Hydrogen Bonds

Base Pair	Number of H-Bonds
A-T (DNA)	2
G-C (DNA/RNA)	3
A-U (RNA)	2
G-U (RNA, wobble)	2

Summary Table: Nucleic Acid Parameters

Parameter	Value
DNA Helix Diameter	~2 nm
Base Pairs per Turn	10.5
Nucleosome Diameter	~10 nm
DNA wraps per nucleosome	~146 bp
Angle per Base Pair	36°

Additional info:

• RNA's 2'-OH group makes it susceptible to alkaline hydrolysis, which is a key reason for its instability compared to DNA.

• Wobble base pairing (G-U) in RNA is crucial for the flexibility of the genetic code during translation.

• DNA's double helix structure provides both stability and a mechanism for accurate replication.

• Nucleosome and chromatin organization are essential for DNA packaging and gene regulation.