Nucleic acid probe and labelling

Nucleic acid probe and labelling

Which labeling involves a hapten and requires secondary detection?

Labeling methodologies are classified as direct or indirect based on visualization requirements. Direct methods link a reporter that is immediately detectable, such as radioisotope or fluorophore covalently attached to the probe. Indirect methods incorporate a hapten, a small immunogenic molecule like biotin or digoxigenin, which itself produces no signal. After hybridization, a secondary detection reagent such as enzyme-conjugated streptavidin or anti-digoxigenin antibody binds the hapten and generates colorimetric, chemiluminescent, or fluorescent output. This two-step approach allows signal amplification and flexibility in reporter choice, enhancing sensitivity for low-copy targets.

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.

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Nucleic acid probe and labelling

What is the size range of nucleic acid probes?

Nucleic acid probes must balance specificity, hybridization kinetics, and tissue penetration. Probes shorter than about 15 nucleotides lack sequence complexity and hybridize non-specifically to multiple sites, while excessively long probes exhibit slow reassociation, poor membrane transfer, and limited cellular penetration. Optimal oligonucleotide probes range approximately 20 to 100 nucleotides, providing sufficient uniqueness to recognize single-copy sequences in complex eukaryotic genomes while maintaining high melting temperature and stable duplex formation. This size range enables rapid hybridization, good signal-to-noise ratio, and compatibility with Southern blotting, Northern blotting, and fluorescence in situ 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.

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Nucleic acid probe and labelling

Which enzyme is used in 3’-end labeling of nucleic acids?

3'-end labeling adds labeled nucleotides specifically to the free 3'-hydroxyl terminus of nucleic acids. Terminal deoxynucleotidyl transferase is a template-independent DNA polymerase that catalyzes sequential addition of deoxynucleotides to 3' ends without requiring duplex formation. Using substrates like [alpha-32P]dideoxyATP, [alpha-32P]cordycepin, or biotin-dUTP, it creates singly or multiply tailed labeled probes. This enzyme efficiently labels single-stranded DNA, double-stranded DNA, and RNA primers. Klenow fragment requires a template and primer, Taq polymerase needs thermal cycling and template, and ligase joins ends rather than incorporating labels.

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.

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Nucleic acid probe and labelling

Which probe type must be synthesized chemically and end-labeled?

Synthetic oligonucleotides are short single-stranded DNA sequences assembled chemically by solid-phase phosphoramidite chemistry rather than by enzymatic template-directed synthesis. Because they are chemically produced, they emerge unlabeled and require subsequent end-labeling to function as hybridization probes. Common methods include 5' phosphorylation with T4 polynucleotide kinase using [gamma-32P]ATP or 3' tailing with terminal transferase incorporating labeled nucleotides. In contrast, longer probes such as cloned DNA, cDNA, and RNA can be labeled internally during enzymatic synthesis via nick translation, random priming, or in vitro transcription, producing higher specific activities.

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.

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Nucleic acid probe and labelling

Which protein has high affinity for biotin in labeling systems?

Biotin-labeled nucleic acid probes are visualized through exceptionally strong non-covalent protein interactions. Avidin, a tetrameric glycoprotein from egg white, and streptavidin from Streptomyces avidinii bind biotin with a dissociation constant near 10^-15 M, among the strongest known biological affinities. This stability allows efficient capture of biotinylated probes followed by detection using enzyme-conjugated or fluorophore-conjugated avidins. Amplification steps improve sensitivity in blots and histochemistry. Antibodies, alkaline phosphatase, and T4 ligase do not exhibit specific high-affinity biotin binding, making avidin-streptavidin essential for indirect non-isotopic detection systems.

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.

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Nucleic acid probe and labelling

What type of radioactive isotope is commonly used in DNA labelling?

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.

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Nucleic acid probe and labelling

Which labeling technique uses nicking of DNA followed by polymerase activity?

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.

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Nucleic acid probe and labelling

Which enzyme is used in 5'-end labeling of DNA?

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.

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Nucleic acid probe and labelling

In direct non-isotopic labeling, which of the following is used?

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.

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Nucleic acid probe and labelling

Which of the following is used in random priming labeling?

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.

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