Cutting of DNA

Quality control helps understand digestion success and errors. Controls and expected bands help in this.

The expected restriction-digestion pattern is predicted from the restriction-enzyme recognition sites in DNA and the fragment sizes between those sites. This information is obtained from a restriction map or DNA sequence analysis, so option A is correct. Enzyme concentration may affect whether digestion is complete, but it does not determine the predicted fragment pattern. Exam tip: use the number and positions of cut sites on a map to predict fragment number and size.

After DNA is cut with a restriction enzyme, the fragments are separated by gel electrophoresis. The number and positions of bands visible on the gel give the observed pattern. The enzyme name may indicate possible cutting sites, but it does not provide the actual band pattern. Exam tip: when a question mentions an observed or band pattern, look for an option related to bands on a gel.

Matching bands support correct digestion. Sequencing can be better for final sequence confirmation.

A restriction enzyme cuts DNA only at its specific recognition sites. After gel electrophoresis, a restriction digest can show a pattern of fragment sizes, but it cannot determine the actual order of A, T, G and C within each fragment. DNA sequencing is required to identify the nucleotide sequence. Option B describes sequencing, not restriction digestion. Exam tip: distinguish fragment-length analysis from DNA sequencing.

Direct answer: Option A, sequencing can confirm the exact insert sequence. A restriction digest mainly answers questions such as whether fragments have expected sizes and whether selected sites were cut. A band of the expected length does not prove that every base is correct; a small mutation, deletion, insertion, or rearrangement may remain hidden. Sequencing reads the nucleotide order and can verify the insert and its junctions more directly. Option A is correct because it provides sequence-level confirmation after the preliminary digest check. Option B is wrong: sequencing is not used to colour an enzyme. Option C is wrong: it does not stop a gel. Option D is wrong: water cannot be changed into DNA by sequencing. The logical chain is digest for approximate structure, then sequencing for exact bases. Memory cue: size check is not sequence check.

Sometimes a wrong construct can also give similar size bands. Therefore informative enzymes and sequencing are useful.

A single enzyme may only show plasmid size or linearization. Additional sites are needed for orientation.

Relative positions of two sites change with orientation. Therefore band pattern can be informative.

Uncut plasmid can show supercoiled and open circular forms. Comparison with cut sample explains digestion.

Supercoiled form has different mobility than linear DNA. Linearized plasmid is better for size estimate.

A nick in only one strand of plasmid DNA relaxes its supercoiling, producing the open circular (relaxed) form. Therefore, a single-strand nick is the main cause of an open circular plasmid band. In contrast, cutting both strands at the same position produces a linear plasmid. Exam tip: remember the forms—nicked = open circular, one cut in both strands = linear, intact = supercoiled.

The direct answer is A: a nicked plasmid can migrate with a different mobility and may look like an extra band. A nick is a break in only one strand of the plasmid DNA. The other strand remains continuous, so the molecule is not completely fragmented, but its topology changes, often relaxing a supercoiled plasmid toward an open circular form. Step by step: one strand receives a nick; supercoiling is relaxed; the molecule moves through the gel differently; its band may be mistaken for another DNA product. A is correct. B is wrong because nicking does not convert DNA into protein. C is wrong because a nicked plasmid does not normally destroy the restriction enzyme. D is wrong because nicked DNA can still enter and migrate through the gel. An uncut plasmid control helps identify common conformations and prevents incorrect interpretation. Memory cue: “one-strand nick changes shape, and shape changes mobility.”

In complete digestion, the restriction enzyme cuts DNA at all of its target sites. Therefore, agarose gel electrophoresis shows bands of the expected DNA fragments, with no band for the original uncut DNA. Seeing only an uncut band indicates that digestion has not occurred or is incomplete. Exam tip: confirm digestion by comparing the observed band pattern with the expected fragment sizes.

Direct answer: Option A, a partial digest may show expected bands together with uncut or larger intermediate bands. In complete digestion, nearly every DNA molecule is cut at all available target sites. In partial digestion, enzyme amount, time, temperature, buffer, or DNA accessibility is insufficient for every molecule to finish. Some molecules remain uncut, while others are cut at only some sites. Therefore the gel may contain the original larger band, intermediate-sized fragments, and some expected final bands. Option A is correct. Option B is wrong because a ladder is a size reference and is normally loaded; its absence is not the defining pattern. Option C is wrong because the result concerns DNA bands, not only protein stain. Option D is wrong because a water lane cannot describe digestion. Exam cue: incomplete cutting produces a mixture, not one clean pattern.

Partial digestion often occurs due to insufficient enzyme time or wrong condition. Follow recommended conditions.

During star activity, a restriction enzyme may cut DNA at sequences similar to, but different from, its normal recognition site. Hence, gel electrophoresis can show additional unexpected DNA fragments or bands besides the expected pattern. Only uncut DNA is more suggestive of incomplete digestion, not star activity. Exam tip: associate star activity with non-specific or relaxed-specificity cleavage.

In star activity, a restriction enzyme may cut DNA sequences that resemble, but are not its normal recognition sites. Using the recommended buffer, correct pH, and a limited amount of enzyme maintains enzyme specificity; therefore, option A is correct. Excess enzyme, prolonged incubation, or an inappropriate pH can increase star activity. Exam tip: in restriction digestion, follow the recommended buffer, enzyme amount, and incubation time.

If a methylation-sensitive enzyme does not cut methylation can be a cause. Alternative enzyme or host source can help.

Dam methylation is linked with adenine modification in bacterial DNA. It can affect cutting by some enzymes.

Dcm methylation is linked with cytosine modification in bacterial DNA. Some restriction enzymes can be affected by it.

A methylation-deficient host can make DNA suitable for some sensitive enzymes. Check enzyme requirements first.

EDTA is a chelating agent that binds \,\(Mg^{2+}\) ions. Most restriction endonucleases require \,\(Mg^{2+}\) as a cofactor to cut DNA. When EDTA is present in excess, free magnesium becomes unavailable, so DNA-cleaving activity may decrease or stop. Option D is incorrect because restriction enzymes generally cannot function efficiently without the required cofactor. Exam tip: Remember the role of \,\(Mg^{2+}\) in restriction-enzyme buffers.

Phenol carryover can inhibit the activity of protein enzymes such as restriction endonucleases. As a result, DNA digestion may be incomplete or may fail entirely. Increased ligation efficiency is incorrect because contaminants such as phenol can also interfere with ligase and other enzymes. Exam tip: Purify DNA thoroughly before a restriction digest to remove phenol and other contaminants.

If a DNA pellet is not dried properly after an ethanol wash, residual ethanol may inhibit restriction enzyme activity. This can cause weak or incomplete digestion. A change in enzyme colour is not a reliable indication of ethanol carryover. Exam tip: For a poor restriction digest, check DNA purity, residual ethanol, and correct buffer conditions.