#DNA digestion

Nucleosome Packing

Micrococcal nuclease preferentially digests:

Micrococcal nuclease from Staphylococcus aureus, calcium dependent endo-exonuclease, preferentially cleaves accessible linker DNA between nucleosomes due to steric exposure, showing mild sequence bias toward AT-rich regions. Limited digestion time course generates characteristic ladder pattern on agarose gel with fragments in multiples of nucleosome repeat length, enabling mapping of positioning and repeat determination. Core DNA tightly wrapped around octamer protected early, centromeric heterochromatin more resistant due to compaction, GC-rich naked DNA not preferentially digested. Assay remains gold standard for chromatin accessibility and nucleosome mapping in epigenetics studies.

Ref: Noll M 1974 Nature; Lodish et al., Molecular Cell Biology, Chapter 8: Micrococcal Nuclease Digests Linker DNA

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Transcript analysis

Which enzyme digests methylated parental DNA in inverse PCR?

Parental plasmids propagated in most Escherichia coli strains are methylated by Dam methyltransferase at N6 position of adenine within sequence GATC. After inverse PCR mutagenesis, mixture contains both methylated wild-type template and unmethylated mutated amplicon. Selective removal of parental background exploits methylation sensitivity. DpnI specifically recognizes GATC only when adenine is methylated, introducing double-strand breaks and fragmenting template into non-functional pieces. Newly amplified mutated DNA lacking methylation resists digestion and remains intact for transformation. This enzymatic selection dramatically improves efficiency, explaining why DpnI treatment is indispensable for site-directed mutagenesis protocols.

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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DNA-Protein Interaction analysis

The PCR mix must be treated with DpnI to:

In inverse PCR mutagenesis, reaction mixture contains parental methylated plasmid propagated in Dam positive bacteria and newly synthesized unmethylated mutated copies produced by high-fidelity polymerase in vitro. To enrich mutants, mixture is incubated with DpnI restriction enzyme which recognizes methylated GATC sites present only in parental template, introducing double-strand breaks and fragmenting it. This selective digestion leaves unmethylated mutated amplicons intact, drastically lowering wild-type background upon transformation. Treatment therefore digests template DNA while retaining mutated copies, rather than cutting all DNA or degrading RNA.

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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Hybridization

In inverse PCR, DpnI is used to:

In inverse PCR-based site-directed mutagenesis, DpnI endonuclease plays crucial role in selection of newly synthesized mutant plasmids. DpnI specifically recognizes methylated GATC sequences, which occur frequently in parental plasmid DNA propagated in dam+ E. coli strains. Unmethylated PCR product carrying mutation is resistant to DpnI cleavage, whereas methylated parental strands are digested into fragments. This selective removal enriches mutant clones and reduces background wild-type colonies after transformation. DpnI does not cleave mutant DNA, synthesize RNA, or ligate fragments; its function is methylation-dependent restriction for background elimination.

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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Restriction Digestion & Mapping

A circular DNA digested with 2 asymmetric sites gives:

Circular DNA cleaved at two distinct positions yields as many fragments as cuts. Asymmetric sites divide circle into two arcs of different contour lengths, producing two double-stranded fragments of unequal size. Complete digestion separates circle into those two linear pieces. Agarose gel resolves them as two distinct bands with different migration. Symmetric sites would give one band due to co-migration. One band would indicate single site, three bands would need three sites. Therefore two asymmetric sites logically generate two bands.

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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Restriction Digestion & Mapping

Linear DNA digested with asymmetric sites shows how many bands?

Asymmetric restriction site placement on linear DNA creates fragments of three distinct lengths upon complete digestion. With two sites, cuts produce left terminal segment, central inter-site segment, and right terminal segment. If distances from ends and inter-site distance differ, all three fragments differ in molecular weight and resolve as three separate bands on agarose gel. Symmetric placement would cause two bands to co-migrate. Therefore asymmetric distribution guarantees maximum band separation, aiding accurate measurement of each inter-site distance during physical mapping.

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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Restriction Digestion & Mapping

What defines complete digestion?

Complete digestion represents reaction endpoint where every copy of the DNA molecule in the sample has been hydrolyzed at all cognate recognition sequences. No uncut or partially cut intermediates remain. On gel this yields crisp bands whose number follows n for circular and n+1 for linear DNA, with fragment sizes summing to parent length. Ladder pattern indicates partial or multiple sites, absence of cut shows failed reaction, single band may reflect linearization or co-migration. The defining criterion is saturation of all target sites by active enzyme under optimal buffer conditions.

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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Restriction Digestion & Mapping

DNA showing only 7kb and 3kb after double digestion indicates:

Double digestion with two enzymes should release an insert if both sites flank it, producing two bands typically insert and vector backbone. Observing only two bands of 7 kb and 3 kb summing to 10 kb total indicates that both enzymes cut efficiently at their unique sites, releasing fragments of expected sizes. Absence of additional intermediate bands or uncut supercoiled forms signifies complete reaction rather than partial cleavage. Circular DNA alone would show one linearization band, symmetric sites would show one size. The pattern confirms successful full digestion.

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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Restriction Digestion & Mapping

A linear DNA with 2 equidistant restriction sites produces how many bands?

If two restriction sites are placed at equal intervals on a linear DNA molecule, the terminal fragments flanking the central segment share identical length. Complete digestion yields three physical fragments, but two possess same molecular weight. In agarose gel, size determines migration, so fragments of equal length co-migrate and appear as one band with double intensity. Hence only two distinct bands are visible, and when intensity differences are overlooked or if both terminal fragments are exactly equal, gel interpretation may report as single band region. Equidistant symmetry causes band merging.

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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Restriction Digestion & Mapping

What’s a cause of smears in partial digestion gel?

When digestion is partial, the DNA population becomes heterogeneous with molecules cleaved at different combinations of sites, generating many overlapping fragment lengths. Instead of crisp discrete bands, closely spaced intermediates may appear blurred, especially if site spacing is small or agarose resolution is limited, producing smear-like zones. Gel drying, overloading, or absence of buffer cause general distortion, not site-specific heterogeneity. The fundamental cause of variable digestion states is inconsistent cleavage efficiency across molecules, creating multiple subpopulations migrating at different positions.

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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Restriction Digestion & Mapping

What is the correct fragment outcome for linear DNA with 3 RS under complete digestion?

Complete digestion means every recognition sequence is cleaved in every molecule. For linear DNA the general rule fragments = number of restriction sites + 1 applies because each cut subdivides one existing molecule. With zero sites, fragment count remains one uncut band. One site yields two, two sites yield three, thus three sites necessarily yield four fragments whose sizes correspond to distances between neighboring sites plus two terminal segments. Summation of fragment lengths must equal original molecule length, a key check during gel analysis and map assembly.

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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Restriction Digestion & Mapping

Partial digestion occurs due to:

Partial digestion is defined by incomplete hydrolysis across the population of DNA molecules, usually caused by conditions that lower enzyme efficiency rather than increase it. Sub-optimal temperature, improper pH, suboptimal Mg2+ or salt concentration, presence of inhibitors like EDTA, SDS, residual organic solvents, short incubation time, or insufficient enzyme quantity prevent every site from being cut. Fully saturated enzymes and random cleaving suggest excessive or nonspecific activity. Blunt versus sticky nature of enzyme does not cause partiality, only reaction environment does.

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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