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:

• Angle per base pair:

• DNA diameter:

• Number of hydrogen bonds:

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

Base Pair Hydrogen Bonds

Summary Table: Nucleic Acid Parameters

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.