Cutting of DNA

If a reaction tube floats in a water bath, all parts of it may not receive the required temperature uniformly. This can reduce or make restriction-enzyme activity uneven, leading to incomplete DNA digestion. The enzyme does not become immediately inactive; the main issue is improper incubation temperature. Exam tip: Link efficient restriction digestion with the correct, uniform incubation temperature.

This level focuses on practical accuracy. Enzyme storage cleanup timing and labels make results reliable.

Restriction cutting is not just adding enzyme. Storage buffer cleanup quantification and documentation are all important.

Restriction enzymes cut the vector and the desired DNA insert at specific recognition sites. Correct digestion produces fragments of the expected size and compatible ends, increasing the chance of successful ligation; incorrect or incomplete cutting can cause cloning to fail. Option B describes integration, while C and D describe translation and cell multiplication, not digestion. Exam tip: digestion is commonly checked by comparing the expected band sizes on gel electrophoresis.

The governing concept is substrate quality control before enzymatic digestion. Restriction enzymes work best when the DNA is sufficiently pure, intact, and present at a known concentration. Contaminants such as salts, phenol, proteins, or residual detergents can inhibit the enzyme, while sheared or degraded DNA can produce an unclear fragment pattern. A purity measurement, concentration estimate, and gel-based integrity check therefore provide useful evidence that the sample is suitable. Option A is correct. Cell colour is unrelated to the purified DNA reaction, agar must never be evaluated by taste, and enzyme smell is neither a valid safety practice nor a measure of enzyme activity. These checks cannot replace proper molecular quality control.

Protein contamination can affect enzyme accessibility and activity. Clean DNA sample gives better digestion.

The direct answer is A: pipetting can become inaccurate and mixing can be poor. Very viscous DNA resists flowing through a pipette tip, so the delivered volume may not be reliable. It may also remain as a concentrated string or lump instead of mixing evenly with buffer and restriction enzyme. Poorly mixed DNA can expose some molecules to the enzyme while leaving others less accessible, producing incomplete or variable digestion. Option A is correct because viscosity affects both measurement and uniform reaction contact. Option B is wrong because enzyme does not become protein as a result of viscous DNA; enzymes are already proteins. Option C is wrong because a DNA sample does not automatically melt a gel. Option D is wrong because DNA retains its negatively charged phosphate backbone; viscosity does not remove that charge. Gentle mixing, suitable dilution, and careful pipetting or cleanup can help, while avoiding harsh shearing. Memory cue: thick DNA means inaccurate volume and uneven mixing.

The governing concept is mechanical shearing of long DNA molecules. Genomic DNA consists of very long polymeric molecules that can be broken by vigorous vortexing, forceful pipetting, repeated transfer, or narrow tips. Such accidental fragmentation changes the starting material before the restriction enzyme acts and can produce a smear or an altered fragment distribution, making the intended digestion pattern difficult to interpret. Option A is correct. Genomic DNA is a nucleic acid, not a protein, and it carries a negatively charged phosphate backbone. It can migrate through an appropriate agarose gel, although very large molecules may require suitable gel conditions. Gentle mixing and wide-bore tips help preserve its integrity.

Uncut supercoiled plasmid migrates differently from linear plasmid. It is relevant for understanding digest success.

Genomic DNA is much larger than plasmid DNA. If it remains in a plasmid preparation, it may appear as a high-molecular-weight smear or an extra band near the well on an agarose gel. A restriction enzyme can cut genomic DNA at many sites, producing an additional diffuse DNA signal. Disappearance of the DNA ladder is not caused by genomic DNA contamination. Exam tip: extra slow-moving or smeared DNA close to the well commonly suggests genomic DNA contamination.

Contaminants can inhibit restriction enzyme and ligase reactions. Clean preparation is important.

Restriction sites can be added in PCR primers to make product cloning-ready. Extra bases can help enzyme cutting efficiency.

A primer with restriction site can introduce a cut site into amplified DNA. This is a common cloning strategy.

The direct answer is A: extra bases help a restriction enzyme cut efficiently near the end of DNA. A restriction site added to a primer is often placed at the 5′ end. If the recognition sequence lies exactly at the physical end of a PCR product, some enzymes cannot bind or cut efficiently because they need a short stretch of DNA outside the recognition site. Extra, non-target bases provide this needed flanking region. Thus, after amplification and digestion, the desired sticky or blunt end can be produced more reliably. Option A is correct. B is wrong because primers do not become proteins; proteins are made during translation. C is wrong because extra bases do not stop a gel. D is wrong because they are not added to methylate DNA; methylation is a separate chemical modification. The exact number of extra bases depends on the enzyme and protocol, so laboratory instructions must be followed. Exam cue: restriction site plus a short 5′ flank improves digestion at a DNA end.

PCR mix components can affect restriction enzyme activity. Purified product gives better digestion.

A restriction enzyme needs its recognition site. Without the site expected digestion will not occur.

Linear and circular DNA can give different fragment counts with same cut sites. Shape affects pattern.

Some enzymes do not cut terminal sites efficiently. Adding extra bases in primer design is helpful.

Adapters linkers or engineered primers can provide desired ends. Strategy depends on cloning goal.

Adapter dimers can form small ligation products and confuse cloning. Size selection cleanup is helpful.

The direct answer is A: size selection is used to select the desired-size DNA fragment. After restriction digestion, several fragments of different lengths may be present. In gel-based size selection, the fragments are separated by electrophoresis, the band at the required size is identified with a ladder and expected map, and that band is excised for purification. This removes unwanted fragments and improves the chance that the intended insert or vector is used in cloning. Option A is correct because it describes the purpose directly. Option B is wrong because size selection does not primarily destroy the enzyme; enzyme removal is a separate cleanup concern. Option C is wrong because methylation changes DNA chemistry and is not the aim of choosing a size band. Option D is wrong and unsafe because gel is never tasted and tasting has no scientific purpose. Select the correct size, but also check band identity and DNA quality. Memory cue: size selection keeps the right length and removes the wrong pieces.

The direct answer is A: magnetic bead cleanup can recover DNA fragments by purification or size selection. Magnetic beads bind DNA under particular salt and polyethylene-glycol conditions. A magnet then holds the beads while unwanted liquid is removed, followed by washing and elution of purified DNA. With an appropriate bead-to-sample ratio, larger DNA fragments may be retained while smaller fragments and impurities remain in the liquid, although the exact selection range depends on the kit and protocol. Option A is correct because beads are widely used after restriction digestion to remove salts, enzymes, and other contaminants or to select a desired fragment size. Option B is wrong because beads do not heat or activate an enzyme. Option C is wrong because making an agarose gel requires buffer, agarose, heating, and casting equipment, not magnetic beads as the main method. Option D is wrong because cleanup does not directly perform protein expression; it only prepares DNA. Remember: beads bind, a magnet separates, washing cleans, and elution recovers DNA.

In bead-based cleanup, DNA remains associated with the bead surface and is released after adding an elution buffer. If the beads are over-dried, they can become difficult to rehydrate, so DNA elution may be difficult or inefficient. Option B is the opposite of the expected effect: over-drying does not improve elution. Exam tip: remove residual ethanol after the wash, but do not let the beads dry completely until they crack.

A small amount of ethanol may remain on the DNA pellet after the wash. Residual ethanol can inhibit ligation, restriction digestion, and other downstream enzyme-based reactions, so the pellet must be dried. Drying does not cut DNA; restriction enzymes perform that function. Exam tip: dry the pellet adequately, but avoid over-drying because an over-dried pellet can be difficult to dissolve.

The governing concept is purification and buffer compatibility before a downstream enzymatic reaction. A desalting column can retain or separate unwanted small solutes, such as excess salts, while allowing the DNA-containing fraction to be recovered in a more suitable buffer. Removing inhibitory salt or exchanging into a compatible composition can improve the performance of a subsequent restriction-digestion or related enzyme reaction. Thus option A is correct. The column does not translate DNA into protein, methylate the enzyme as a routine function, or run an electrophoresis gel. The precise retention mechanism depends on the column type, so the manufacturer’s instructions and the expected DNA recovery should be considered when selecting and using it.