Nucleic Acid Labeling

Making a probe visible: isotopic versus fluorescent, hapten, and enzyme labels; direct versus indirect detection; and the ways labels get onto nucleic acid - end labeling, nick translation, random priming, PCR, and in vitro transcription - with where each shows up in today's lab.

5 min
6 steps
4 questions
Lesson 17 of 60

In this lesson

Label and method
Labels
Direct versus indirect
Getting the label on
Uses in the lab
What to take from this

Label and method

DNA is chemically the same from one fragment to the next, so to see a specific sequence you attach a reporter to a probe that will find it. Keep two choices separate ¹:

the label - what makes the probe detectable;
the labeling method - how the label gets attached.

Labels

Family	Examples	How it’s read
Isotopic	³²P, ³³P, ³⁵S, ³H	autoradiography; very sensitive but short half-lives and disposal burden - mostly historical in clinical work ²
Fluorophores	FAM, HEX/VIC, Cy3, Cy5, Texas Red	excite with one wavelength, read emission at another; several colors at once allow multiplexing
Haptens	biotin, digoxigenin (DIG)	detected by a partner: streptavidin binds biotin; an anti-DIG antibody binds DIG; the partner carries the signal ¹
Enzymes	alkaline phosphatase, horseradish peroxidase	turn substrate into a colored (colorimetric) or light-emitting (chemiluminescent) product

Biotin has a catch: tissues like liver and kidney contain endogenous biotin, which can cause background in tissue-based detection; DIG avoids this because it’s a plant compound absent from human cells.

Quick check

Biotin on a probe is detected with:

Direct versus indirect

Direct: the signal is on the probe - a fluorophore-labeled FISH probe is simply illuminated. Fast, few steps, signal limited to the attached dyes.
Indirect: the probe carries a hapten; after hybridization, a labeled partner (streptavidin-enzyme or anti-DIG antibody) binds, and an enzyme turns over substrate. More steps, but each binding event becomes many signal molecules.

Direct detection with a fluorophore on the probe versus indirect detection with a hapten bound by an enzyme-labeled partner that makes many signal molecules; and seven labeling methods: 5-prime end with kinase, 3-prime end with terminal transferase, nick translation, random priming, PCR incorporation, in vitro transcription, and chemical synthesis. — Choose a label, and a way to attach it. Credit: StudyCorner diagram after Green and Sambrook and Buckingham · CC BY 4.0 · Source

Quick check

What is the trade-off of indirect detection compared with direct?

Getting the label on

Most methods use the polymerases, kinases, and ligases from the foundations course ²:

Method	How	Result
5’ end labeling	polynucleotide kinase transfers a labeled phosphate to the 5’ end	one label per molecule; for oligos
3’ end labeling	terminal transferase adds labeled nucleotides to the 3’ end	a few labels at one end
Nick translation	DNase nicks dsDNA; a polymerase with 5’→3’ exonuclease replaces nucleotides from each nick with labeled ones	uniformly labeled dsDNA probe ²
Random priming	denature template, anneal random hexamers, extend with labeled dNTPs	uniformly labeled, high-activity probe from little template
PCR incorporation	labeled dNTPs or a labeled primer during PCR	amplifies and labels in one step
In vitro transcription	a phage RNA polymerase (T7, SP6) transcribes from its promoter with labeled NTPs	single-stranded RNA probe (riboprobe) of defined orientation
Chemical synthesis	dye or hapten built into an oligo on the synthesizer	the norm for qPCR probes and primers

Quick check

Which labeling method uses short random-sequence oligos to prime synthesis on a denatured template?

Uses in the lab

Real-time PCR probes: synthetic oligos with a 5’ reporter (FAM, HEX/VIC) and a 3’ quencher (see Probe Chemistries).
FISH: directly labeled fluorescent probes in two or three colors, such as the dual-color dual-fusion BCR::ABL1 probe set.
In situ hybridization in pathology: DIG- or biotin-labeled probes with enzyme detection, such as EBV-encoded RNA (EBER) in tissue.
Microarrays: sample DNA labeled with Cy3/Cy5 and hybridized to the chip.
NGS hybrid capture: biotinylated probes (“baits”) bind target regions, and streptavidin magnetic beads pull them out - labeling used as a capture handle.
Sanger sequencing: four dye-labeled terminators, one color per base.

Quick check

In NGS hybrid-capture library prep, how are target fragments pulled out?

What to take from this

A probe needs a label (isotope, fluorophore, hapten, or enzyme) and a way to attach it. Direct detection reads the label on the probe; indirect detection adds a labeled partner - streptavidin for biotin, antibody for DIG - for amplified signal. End labeling, nick translation, random priming, PCR incorporation, in vitro transcription, and chemical synthesis put labels on. Today’s lab uses dye-labeled oligos for qPCR and Sanger, fluorescent FISH probes, DIG and biotin for tissue ISH, and biotin-streptavidin for NGS capture.

Lesson complete

Nice work.

1day streak

0/1today's goal

–correct

Up next · 4 min

Restriction Enzymes and RFLP

Next lesson

Back to course Today

Sources for this lesson

1
Lela Buckingham. Molecular Diagnostics: Fundamentals, Methods, and Clinical Applications. 3rd ed. F.A. Davis Company. 2019. verifiedThe standard clinical molecular-diagnostics textbook for MLS/MB programs; author holds MB DLM(ASCP). Covers nucleic-acid chemistry, techniques, lab operations, and applications across infectious disease, oncology, genetics, and identity. Primary topic reference for the ASCP MB program.
2
Michael R. Green, Joseph Sambrook. Molecular Cloning: A Laboratory Manual. 4th ed. Cold Spring Harbor Laboratory Press. 2012. verifiedThe classic three-volume molecular-biology methods manual — authoritative for nucleic-acid isolation, electrophoresis, restriction digestion, labeling, and hybridization techniques. Standard-tier topic reference for the techniques courses.