Overview
Nucleotides are the monomeric building blocks of nucleic acids — DNA and RNA — and play essential roles far beyond gene storage. They act as chemical messengers in hormone signalling, form the backbone of key enzyme cofactors such as NAD, NADP and FAD, and serve as the universal energy currency of the cell in the form of ATP.
This lesson covers the chemistry of nucleosides and nucleotides, the structure of DNA (A, B and Z forms) and the three major RNA types, nucleotide anabolism and catabolism, and clinically important disorders of purine metabolism such as gout — a topic every medical laboratory technologist must understand to interpret uric acid results correctly.
Learning Objectives
After this lesson you will be able to…- Describe the chemistry of nucleotides and how nucleosides are formed
- Describe the structure of nucleic acids, including the Watson–Crick DNA model
- Explain the characteristics that distinguish DNA and RNA
- Describe the pathways of purine and pyrimidine nucleotide metabolism
- Enlist the functions of nucleic acids and relate purine metabolism disorders to gout
Clinical Story
Why This MattersA 52-year-old man presents to the clinic with sudden, severe pain and swelling in his big toe overnight. The physician orders a serum uric acid test, which returns markedly elevated at 9.8 mg/dL. Understanding how purine nucleotides are synthesized, salvaged, and broken down to uric acid helps the laboratory technologist appreciate why this patient's result points toward gout — and why accurate, well-controlled uric acid testing is critical for diagnosis and monitoring.
Core Concepts
A nucleoside is formed when a nitrogenous base attaches to a pentose sugar (ribose or deoxyribose) through a hemiacetal bond — purines attach via the N-9 nitrogen and pyrimidines via the N-1 nitrogen. A nucleotide is a phosphoric acid ester of a nucleoside, formed by esterifying a phosphate group onto the sugar (commonly the 5' position). Nucleotides may exist as mono-, di-, or triphosphates (e.g. AMP, ADP, ATP).
Nitrogenous bases fall into two structural classes. Purines (adenine and guanine) have a fused double-ring structure. Pyrimidines (cytosine, thymine, and uracil) have a single-ring structure. Uracil is found only in RNA, while thymine is found only in DNA — a key distinguishing feature between the two nucleic acids.
Watson and Crick (1953) proposed that DNA is a double helix of two antiparallel polynucleotide chains held together by hydrogen bonds between complementary bases — adenine pairs with thymine (2 hydrogen bonds) and guanine pairs with cytosine (3 hydrogen bonds). DNA exists in three structural forms:
| Character | A form | B form | Z form |
|---|---|---|---|
| Helix sense | Right handed | Right handed | Left handed |
| Base pairs / turn | 10–11 | 10–11 | 12 |
| Vertical rise / bp | 2.6–3.4 Å | 2.56–3.4 Å | 3.7 Å |
Unlike DNA, RNA is typically single stranded, though it can fold back on itself to form secondary and tertiary structures. Three major types of RNA exist: mRNA (carries genetic information from nucleus to ribosome; eukaryotic mRNA is monocistronic, prokaryotic mRNA is polycistronic), tRNA (65–110 nucleotides, folds into an L-shape and transports amino acids using its anticodon), and rRNA (forms the structural framework of ribosomes, ~74–80% of total cellular RNA).
Purine nucleotides are synthesized de novo (mainly in the liver) via a 10-step pathway building the ring directly onto ribose-5-phosphate, using PRPP, glutamine, glycine, aspartate, and CO2 as building blocks. In the salvage pathway, free bases are recycled by APRTase and HGPRTase — important in tissues like RBCs and brain that lack de novo synthesis. Pyrimidine synthesis builds the ring first (starting with carbamoyl phosphate synthetase II), then attaches it to PRPP to form the ribonucleotide.
Elevated uric acid (hyperuricemia: >7 mg/dL in males, >6 mg/dL in females) results from either overproduction or underexcretion of urate. In gout, urate crystals deposit in the synovial fluid and cooler peripheral tissues (tophi), triggering acute inflammatory arthritis. Primary gout arises from enzyme defects (e.g. PRPP synthetase overactivity, HGPRTase deficiency); secondary gout follows increased nucleic acid turnover (malignancy, chemotherapy) or reduced renal excretion (renal failure, thiazide diuretics).
Laboratory Principle
Nucleic acids and nucleotides show characteristic maximum ultraviolet light absorption near 260 nm because of the conjugated ring structure of their purine and pyrimidine bases. This property is exploited in the laboratory to quantify DNA/RNA concentration and purity by spectrophotometry, and underlies techniques such as melting-temperature (Tm) determination, where the loss of base stacking during DNA denaturation causes a sharp rise in absorbance (the hyperchromic effect).
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Uricase reagent | Enzymatic, working strength | Oxidizes uric acid to allantoin for colorimetric uric acid assay | 2–8°C |
| Phosphotungstic acid reagent | Standard reagent grade | Chemical (non-enzymatic) uric acid method | Room temperature, tightly sealed |
| Tris-EDTA (TE) buffer | pH 8.0 | Nucleic acid extraction and stabilisation | Room temperature |
| Ethanol | 70% and absolute | DNA precipitation during extraction | Room temperature, flammable storage |
Step-by-Step Procedure
Collect a fasting venous blood sample into a plain (serum) tube; avoid haemolysis, which falsely elevates uric acid readings.
Centrifuge at 3,000 rpm for 10 minutes to separate serum from cells promptly to prevent glycolytic/metabolic changes.
Add serum to the uricase (or phosphotungstic acid) working reagent according to the analyzer protocol.
Incubate at 37°C for the manufacturer-specified time to allow the enzymatic reaction (uricase converts uric acid to allantoin + H2O2) to go to completion.
Read absorbance at the specified wavelength (typically 520–550 nm for coupled peroxidase methods) and calculate the uric acid concentration against a calibrator; report in mg/dL.
Flow Diagram
Quality Control
Run normal and abnormal (elevated) uric acid controls with every batch of patient samples. Plot results on a Levey-Jennings chart and apply Westgard rules; investigate any control falling outside ±2SD before releasing patient results.
Participate in an External Quality Assessment Scheme (EQAS) for clinical chemistry, submitting uric acid results for inter-laboratory comparison to verify accuracy and identify systematic bias against peer laboratories.
Reference Values
Normal Ranges⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Uric acid > 7 mg/dL (M) or > 6 mg/dL (F) | Hyperuricemia — risk of gout, urate nephropathy, or crystal deposition | Correlate clinically; consider joint fluid analysis if symptomatic |
| Uric acid within reference range but recurrent joint pain | Gout can occur with normal levels during an acute attack | Repeat testing after the acute episode resolves |
| Very high uric acid with rapid tumour lysis history | Secondary gout from increased nucleic acid turnover (e.g. chemotherapy) | Monitor renal function; consider urgent allopurinol/rasburicase therapy |
Common Errors & How to Avoid Them
Cause: Rupture of red cells releases intracellular purines and interferes with the colorimetric reaction
Prevention: Reject haemolysed samples; redraw using correct venipuncture technique.
Cause: Prolonged contact with cells alters analyte concentration through ongoing metabolism
Prevention: Centrifuge and separate serum within 30–60 minutes of collection.
Cause: A recent high-purine meal (organ meats, seafood) transiently raises uric acid
Prevention: Note dietary history; recommend a fasting or standardised repeat sample.
Laboratory Tips from the Bench
Always check the A260/A280 ratio when quantifying extracted nucleic acids — a ratio well below 1.8 (DNA) or 2.0 (RNA) usually signals protein contamination requiring re-purification.
Uric acid solubility drops sharply below 37°C, which is why urate crystals classically deposit in cooler peripheral joints such as the big toe (podagra) rather than in core body tissues.
Remember base pairing with 'PURe As Gold': PURines (Adenine, Guanine) pair with pyrimidines — A–T (2 bonds, think 'A-Team, 2 members') and G–C (3 bonds, 'Great Couple, 3's a crowd').
Important Notes
Never confuse the two: Uracil replaces thymine only in RNA. A stray thymine in RNA or uracil in DNA is a hallmark exam trap.
A complete deficiency of HGPRTase causes Lesch–Nyhan syndrome, characterised by severe hyperuricemia, gout, and self-mutilating behaviour — a classic board examination correlation.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgePresents to the emergency department with sudden onset of severe pain, redness, and swelling of the right first metatarsophalangeal joint that began overnight. He reports a family history of similar episodes and a recent celebratory meal rich in red meat and seafood.
The markedly elevated serum uric acid, combined with acute monoarticular joint inflammation and negatively birefringent needle-shaped crystals on synovial fluid microscopy, is diagnostic of acute gouty arthritis secondary to hyperuricemia.
- →Uric acid is the end product of purine catabolism; overproduction or underexcretion causes hyperuricemia.
- →Diagnosis of gout is confirmed by identifying monosodium urate crystals in synovial fluid, not by serum uric acid alone.
- →Dietary purine load (organ meats, seafood) and alcohol can precipitate acute attacks in predisposed individuals.
Frequently Asked Questions
A nucleoside contains only a nitrogenous base linked to a pentose sugar. A nucleotide additionally has one or more phosphate groups esterified to the sugar, making it the true monomer of nucleic acids.
Thymine is essentially a methylated form of uracil. This methyl group makes DNA repair more efficient (spontaneous deamination of cytosine produces uracil, which is easily recognised as 'foreign' in DNA and excised), contributing to DNA's greater long-term genetic stability.
Yes. During an acute gouty attack, uric acid can transiently normalise as it precipitates into tissues. Diagnosis should not rely on a single uric acid value; synovial fluid crystal analysis remains the gold standard.
Quick Revision
10-Minute ReviewKey Takeaways
- Nucleotides are the monomeric units of DNA and RNA and also serve as energy carriers (ATP) and enzyme cofactors (NAD, FAD).
- DNA is a double-stranded, antiparallel helix stabilised by complementary base pairing (A–T, G–C); RNA is typically single stranded.
- Purine synthesis is de novo (built on ribose-5-phosphate) or via the salvage pathway; pyrimidine synthesis builds the ring before attaching to PRPP.
- Disorders of purine metabolism, especially overproduction or underexcretion of uric acid, lead to hyperuricemia and gout.
- Accurate uric acid testing requires careful pre-analytical handling to avoid haemolysis and delayed separation errors.
Competency Checklist
Track Your MasteryReferences
- National Institute of Open Schooling. Biochemistry — Module: Nucleotides (Lesson 6).
- Murray RK, et al. Harper's Illustrated Biochemistry. 30th ed. McGraw-Hill.
- Burtis CA, Ashwood ER, Bruns DE. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 5th ed. Elsevier.