Rucete ✏ Lehninger Principles of Biochemistry In a Nutshell
8.2 Nucleic Acid Structure
This chapter explains the discovery, molecular architecture, and structural variations of DNA and RNA. It covers the historical experiments that identified DNA as the genetic material, the Watson-Crick double-helix model, structural variants such as A-, B-, and Z-DNA, unusual DNA configurations like triplexes and tetraplexes, and the structural complexity of RNA molecules that enable diverse biological functions.
Discovery of DNA as Genetic Material
• DNA was first isolated by Friedrich Miescher in 1869 and termed “nuclein.”
• The Avery–MacLeod–McCarty experiment (1944) demonstrated that DNA, not protein, carries hereditary information by transforming nonvirulent Streptococcus pneumoniae into virulent forms.
• The Hershey–Chase experiment (1952) confirmed that DNA, not viral protein, transmits genetic information in bacteriophage infection.
Chargaff’s Rules and X-ray Evidence
• Erwin Chargaff found that in all organisms, A = T and G = C, implying purine-pyrimidine pairing.
• Rosalind Franklin and Maurice Wilkins used X-ray diffraction to reveal DNA’s helical structure with two periodicities: 3.4 Å between base pairs and 34 Å per helical turn.
The Watson-Crick Double Helix
• In 1953, James Watson and Francis Crick proposed the DNA double-helix model:
– Two antiparallel polynucleotide chains coil around a common axis forming a right-handed helix.
– The sugar-phosphate backbones are on the exterior; bases are stacked inside, nearly perpendicular to the helical axis.
– Complementary base pairing (A═T, G≡C) explains Chargaff’s rules and ensures accurate replication.
– Each helical turn has 10.5 base pairs and a pitch of 36 Å in solution.
– Major and minor grooves form due to the offset pairing of strands.
Complementarity and Stability
• The two DNA strands are complementary: A pairs with T, G with C.
• Hydrogen bonds ensure specificity, but structural stability mainly arises from base-stacking interactions and metal ion shielding of the phosphate backbone.
• DNA regions with higher G≡C content exhibit greater thermal stability due to stronger stacking interactions.
DNA Replication and Information Transfer
• Complementary strand pairing suggested a mechanism for replication: each parent strand acts as a template for a new complementary strand.
• The sequence of bases encodes genetic information that is faithfully transmitted during cell division.
Conformational Variants of DNA
• DNA is flexible, with rotation possible around several backbone bonds, allowing bending and twisting.
• The most common form, B-DNA, is the standard right-handed double helix in physiological conditions.
• A-DNA: right-handed, 11 bp per turn, shorter and wider than B-DNA, with tilted base pairs; forms under dehydrated conditions.
• Z-DNA: left-handed, 12 bp per turn, zigzag backbone; stabilized in GC-rich sequences and may play a role in gene regulation.
Sequence-Dependent Structural Variations
• Runs of adenine residues induce bends in DNA, aiding in protein-DNA interactions.
• Palindromic sequences form hairpin or cruciform structures; mirror repeats form symmetric sequences but not base-paired hairpins.
• Self-complementary single strands can fold into secondary structures in both DNA and RNA.
Unusual DNA Structures
• Triplex DNA forms when a third strand binds in the major groove via Hoogsteen base pairing, stabilized at low pH and in purine/pyrimidine-rich sequences.
• Tetraplex (G-quadruplex) DNA arises from guanosine-rich regions, forming stable four-stranded helices via Hoogsteen hydrogen bonding.
• Such structures often occur in telomeres and regulatory regions of eukaryotic genomes.
RNA as the Genetic Messenger
• mRNA carries genetic information from DNA to ribosomes for translation into proteins.
• Bacterial mRNAs can be monocistronic (single gene) or polycistronic (multiple genes).
• mRNA includes untranslated regions essential for initiation, termination, and regulation of translation.
Structural Diversity of RNA
• RNA is single-stranded but folds into complex secondary and tertiary structures through base pairing and stacking.
• G–C and A–U base pairs dominate, but G–U wobble pairs also occur, adding flexibility.
• Common structural motifs include hairpins, bulges, and internal loops; hairpin loops with UUCG sequences are particularly stable.
• Double-stranded RNA regions adopt an A-form right-handed helix, while extensive mismatching creates complex tertiary folds.
Functional Classes of RNA
• mRNA — carries coding information.
• tRNA — transfers specific amino acids during protein synthesis; its 3D L-shape arises from base pairing and unusual hydrogen bonding.
• rRNA — structural and catalytic component of ribosomes.
• Ribozymes — catalytic RNAs that perform enzymatic functions, such as self-splicing introns and RNA cleavage.
• Noncoding RNAs — regulatory and structural RNAs involved in gene control and processing.
In a Nutshell
DNA’s double-helix structure explains genetic stability and replication through complementary base pairing. Variations such as A-, B-, and Z-DNA and special configurations like triplex and quadruplex DNA contribute to structural flexibility and regulation. RNA molecules, derived from DNA transcription, fold into intricate 3D forms enabling catalysis, recognition, and information transfer. Together, these nucleic acid structures underpin heredity, gene expression, and the molecular basis of life.