Rucete ✏ Lehninger Principles of Biochemistry In a Nutshell
8.3 Nucleic Acid Chemistry
This chapter explores the chemical properties, stability, and experimental manipulation of nucleic acids. It covers denaturation and renaturation of DNA, nonenzymatic chemical alterations, DNA damage and repair, enzymatic methylation, synthetic and amplification techniques such as PCR, and sequencing methods from Sanger to next-generation sequencing technologies.
Stability and Denaturation of DNA
• DNA’s long-term information storage depends on its chemical stability; even slow spontaneous reactions can have biological consequences related to aging and carcinogenesis.
• Double-helical DNA denatures (melts) when exposed to heat or extreme pH, disrupting hydrogen bonds and base stacking without breaking covalent bonds.
• Upon cooling or neutralization, separated strands can renature (anneal) spontaneously, first by random collision of complementary regions, then by rapid “zipping” along the rest of the sequence.
• Denaturation leads to an increase in UV absorbance at 260 nm (the hyperchromic effect), while base stacking in native DNA reduces absorbance (the hypochromic effect).
• Each DNA species has a characteristic melting temperature (Tm), the point where half of the molecule is denatured; higher G≡C content results in a higher Tm.
• DNA melting occurs first in G≡C-poor regions, forming “bubbles.” RNA and RNA–DNA hybrids show similar behavior but with higher thermal stability.
Nonenzymatic Transformations of Nucleotides
• DNA bases undergo spontaneous covalent changes at low rates, but because the genome tolerates almost no alterations, these reactions are physiologically significant.
• Deamination removes exocyclic amino groups from bases (e.g., cytosine → uracil), occurring ~100 times per day in a mammalian cell.
• Adenine and guanine deaminate at ~1/100 the rate of cytosine. This slow reaction likely explains why DNA uses thymine rather than uracil—to distinguish deaminated cytosine products and allow repair.
• Depurination (hydrolysis of the N-β-glycosyl bond) removes purine bases, leaving apurinic/apyrimidinic (AP) sites—about 10,000 lost purines per mammalian cell daily.
Radiation- and Chemically Induced Damage
• UV light causes pyrimidine dimer formation, especially between adjacent thymines, introducing kinks into DNA.
• Ionizing radiation (x-rays, γ-rays, cosmic rays) causes ring fragmentation, backbone breaks, and oxidative damage.
• Nitrous acid and related agents accelerate base deamination; alkylating agents (e.g., dimethyl sulfate) add alkyl groups that can alter pairing behavior.
• Oxidative damage from reactive oxygen species (ROS)—hydrogen peroxide, superoxide, hydroxyl radicals—causes strand breaks and base oxidation, contributing to aging and cancer.
• Despite constant damage, DNA integrity is preserved through extensive enzymatic repair systems, unlike RNA or proteins.
Enzymatic Methylation of DNA
• Methylation is an enzymatic covalent modification of DNA bases, primarily at cytosine and adenine.
• All DNA methyltransferases use S-adenosylmethionine (SAM) as the methyl donor.
• In E. coli, Dam methylase adds a methyl group to adenine in 5′-GATC-3′ sequences, aiding in replication and mismatch repair recognition.
• Restriction-modification systems protect bacterial DNA by methylation while degrading unmethylated foreign DNA.
• In eukaryotes, about 5% of cytosine residues (mainly in CpG islands) are methylated to form 5-methylcytidine, influencing gene expression and epigenetic regulation.
Chemical Synthesis of DNA
• Automated oligonucleotide synthesis uses the phosphoramidite method developed from Khorana’s and Caruthers’ work.
• Nucleotides are sequentially added on a solid support, with protecting groups controlling reactions at specific hydroxyls.
• Each addition forms a phosphotriester linkage, and protecting groups are removed at the end to release the purified oligonucleotide.
• The method enables routine synthesis of DNA up to 80 bases, revolutionizing molecular biology by providing custom DNA sequences for cloning, PCR, and gene synthesis.
The Polymerase Chain Reaction (PCR)
• Invented by Kary Mullis (1983), PCR amplifies DNA segments exponentially using repeated cycles of denaturation, primer annealing, and extension by DNA polymerase.
• Requires template DNA, two primers flanking the target region, dNTPs, and a thermostable DNA polymerase (e.g., Taq polymerase from Thermus aquaticus).
• Each cycle doubles the target DNA, yielding a billionfold amplification after ~30 cycles.
• PCR enables genetic testing, forensic analysis, diagnostics, and the recovery of ancient DNA from archaeological samples.
DNA Fingerprinting and STR Analysis
• DNA profiling, introduced by Alec Jeffreys (1985), identifies individuals using variations in short tandem repeats (STRs)—short repeated DNA sequences with population variability.
• STR loci are amplified by PCR using fluorescent primers and analyzed by electrophoresis; fragment length determines genotype.
• The Combined DNA Index System (CODIS) in the U.S. uses standardized STR loci to compare genetic profiles, with accuracy exceeding 1 in 1018.
• This technique is used in criminal identification, paternity testing, and historical analyses.
DNA Sequencing Techniques
• The Sanger (dideoxy) method terminates DNA synthesis at specific nucleotides using ddNTP analogs that lack a 3′-OH, producing labeled fragments for electrophoretic analysis.
• Automated Sanger sequencing uses fluorescently labeled ddNTPs, allowing single-lane electrophoresis and computerized sequence reading.
• This method enabled projects such as the Human Genome Project to determine billions of base pairs over a decade.
Next-Generation Sequencing (NGS)
• NGS technologies sequence entire genomes within days using massively parallel approaches.
• Illumina sequencing (reversible terminator method): fluorescently labeled nucleotides with removable blocking groups allow one base addition per cycle, imaging each cluster for base identification.
• Pacific Biosciences’ SMRT sequencing tracks DNA synthesis in real time using single polymerase molecules in nanometer-scale pores, detecting light pulses from incorporated bases.
• SMRT reads reach 30,000–40,000 bp, essential for resolving long repetitive sequences despite higher error rates.
• Sequencing depth (coverage) ensures accuracy by overlapping fragment alignment into contiguous sequences (“contigs”).
In a Nutshell
DNA’s chemical stability underlies its role as genetic material, though it undergoes reversible denaturation and rare spontaneous damage. Environmental and chemical agents introduce mutations, countered by repair mechanisms and protective methylation. Advances in synthetic chemistry, PCR, and sequencing—from Sanger to next-generation technologies—have transformed molecular biology, enabling genetic analysis, medicine, forensics, and evolutionary research.