13 public questions tagged with this topic.
Nitrogen-15 was used to label the original DNA strands to demonstrate semiconservative replication.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Human Genome Project and DNA Fingerprinting
In DNA polymerization, α-phosphate of deoxynucleoside triphosphate is incorporated into growing phosphodiester backbone, while β and γ phosphates are released as pyrophosphate driving reaction forward. Therefore α-32P-dNTP results in internally labeled products detectable by autoradiography, and 5' end-labeling uses γ-32P-ATP via polynucleotide kinase to label primer terminus. Gamma-32P-labeled dNTP cannot be internally incorporated because γ-phosphate is not retained in final strand; polymerase requires cleavage between α and β phosphates. Consequently, experiments requiring internal radioactive tracing cannot utilize gamma-labeled precursors, distinguishing end-labeling from incorporation strategies in labeling techniques.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.
Radioisotopic labeling commonly uses phosphorus-32 because of its superior nuclear characteristics for nucleic acid detection. 32P is a high-energy beta emitter with a half-life of approximately 14.3 days, offering excellent sensitivity and short autoradiographic exposure times. It can be incorporated internally using [alpha-32P]dATP or dCTP during polymerization, or terminally as [gamma-32P]ATP via kinase reaction. Lower-energy isotopes like tritium, sulfur-35, and carbon-14 emit weak beta particles, require prolonged exposure, and provide poorer resolution. Hence 32P remains the isotope of choice for high-sensitivity probe labeling.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.
Nick translation creates labeled DNA by mimicking cellular DNA repair synthesis. DNase I at low concentration introduces single-strand breaks by cleaving phosphodiester bonds, leaving 3'-hydroxyl and 5'-phosphate ends. Escherichia coli DNA polymerase I then binds at nicks; its 5' to 3' exonuclease activity removes nucleotides ahead while its polymerase activity incorporates new nucleotides, including labeled deoxynucleotides, behind. The nick thus translates along the duplex. This coordinated excision and synthesis yields uniformly labeled double-stranded probes with high specific activity for in situ hybridization and membrane blotting.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.
5'-end labeling introduces a label specifically at the 5' terminus of DNA. T4 polynucleotide kinase catalyzes transfer of the gamma-phosphate from ATP to a free 5'-hydroxyl group of DNA or RNA. When [gamma-32P]ATP is supplied as substrate, a radioactive phosphate is efficiently added to the 5' end, creating a highly detectable probe. Prior dephosphorylation with alkaline phosphatase increases labeling efficiency. This technique labels restriction fragments and synthetic oligonucleotides for sequencing, primer extension, and electrophoretic mobility shift assays, whereas polymerases and ligases perform distinct functions.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.
Non-isotopic labeling is divided into direct and indirect systems based on detection. Indirect labels such as biotin and digoxigenin are haptens requiring secondary reagents like streptavidin or antibody conjugates for visualization. Direct labeling attaches a reporter immediately detectable without additional steps. Fluorescein, a fluorescent dye, exemplifies direct non-isotopic labeling because its emission is observed directly under excitation. Autoradiography is not a label but a detection method for radioactive isotopes. Therefore fluorescein enables rapid one-step detection, widely used in fluorescent in situ hybridization and direct probe imaging.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.
Random priming generates labeled probes by annealing short random oligonucleotides, usually hexamers, to denatured DNA template to create multiple initiation sites with free 3'-hydroxyl groups. Klenow fragment, the large proteolytic fragment of Escherichia coli DNA polymerase I lacking 5' to 3' exonuclease activity, extends these primers while incorporating labeled deoxynucleotides. Retaining polymerase and 3' to 5' proofreading functions and unable to degrade template, it produces uniformly labeled probes with very high specific activity suitable for Southern blotting, Northern blotting, and colony hybridization.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.
End-labeling strategies label only the terminus where radioactive nucleotide incorporation occurs, creating a single reference point for mapping. After partial digestion, many fragments contain the labeled end plus internal segments of varying lengths, while fragments derived purely from interior regions lacking the original terminus remain unlabeled. Autoradiography detects only species retaining the labeled end, so internal fragments generated by two internal cuts without involving labeled terminus stay invisible on film. Therefore unlabeled class corresponds to internal fragments, which become visible only with universal staining like ethidium bromide.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.
Labeling cohesive ends commonly employs DNA polymerases that can fill in 5' overhangs. T4 DNA polymerase possesses strong 3' to 5' exonuclease and polymerase activity that efficiently incorporates labeled dNTPs opposite the single-stranded overhang using the protruding 5' sequence as template. EcoRI is restriction enzyme that creates ends, not label them. Ligase joins fragments, exonuclease III degrades from 3' terminus. Hence T4 polymerase is standard choice for labeling sticky ends in mapping, footprinting, and terminal labeling protocols using radioactive or fluorescent nucleotides.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.
Restriction mapping tracks fragments originating from a defined terminus to deduce site order, so terminal labeling is essential. Restriction enzymes producing 5' overhangs expose a recessed 3'-OH and a free 5'-phosphate, ideal substrate for T4 polynucleotide kinase to add 32P or for Klenow fragment to fill in with radioactive dNTPs. Internal loci remain phosphodiester-linked without free ends, blunt termini incorporate label poorly, and 3'-only labeling lacks efficiency. Therefore labeling free 5' cohesive ends creates a single radioactive reference point for stepwise fragment analysis.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.
Klenow fragment retains polymerase and 3' to 5' exonuclease but lacks 5' to 3' exonuclease activity, making it ideal for labeling. For end labeling, it fills recessed 3' ends templated by 5' overhangs incorporating alpha-32P labeled nucleotides efficiently. Blunt ends possess recessed 3' terminus after transient breathing and can be labeled by exchange reaction under certain conditions with high deoxynucleotide and enzyme concentrations. While sticky 5' overhangs are preferred substrates for fill-in, blunt-end labeling is classic method to create uniformly labeled probes for restriction mapping and Southern analysis when cohesive ends are unavailable.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.
When linear DNA is cut by two enzymes producing three fragments, each fragment has two ends derived from cleavage events. Ends originating from natural termini or restriction cuts may be blunt or sticky depending on enzyme specificity. If only 2 kb fragment becomes labeled after end-filling with Klenow and radiolabeled dNTPs, it must be the only fragment possessing a 5' overhang with recessed 3' OH that polymerase can extend using template strand. Other fragments bearing blunt ends or 3' overhangs cannot incorporate label efficiently, thus selective labeling marks fragments with appropriate cohesive terminus aiding mapping orientation.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.
