Cutting of DNA

DNA ligase joins cut DNA by forming a phosphodiester bond between the 3′-OH end of one fragment and the 5′-phosphate end of another fragment. Thus, a 5′-phosphate at a restriction-cut end is required for ligation. DNA in gels is stained by dyes such as ethidium bromide, not by phosphate groups. Exam tip: remember the ligase-end pair—3′-OH and 5′-phosphate.

Blunt ends lack complementary overhang. Therefore concentration and ligation condition become more important.

Sticky ends have short single-stranded overhangs. If the overhangs of the vector and insert are complementary, their bases form hydrogen bonds, allowing the fragments to anneal temporarily. DNA ligase later makes the joining permanent by forming phosphodiester bonds in the sugar-phosphate backbone; it is not the initial reason for annealing. Exam tip: associate annealing with complementary base pairing and ligation with phosphodiester-bond formation.

Besides compatible ends clean DNA and correct ratio are necessary. Salt ethanol or enzyme carryover can block ligation.

Active restriction enzyme can cut newly joined DNA in ligation mix. Cleanup or inactivation is useful.

Heat inactivation can stop enzyme but salts and proteins may remain. Cleanup can improve next reaction.

A cleanup kit separates DNA from reaction components. It is useful before ligation or analysis.

The direct answer is A: use gentle handling and low UV exposure. Large DNA fragments are long molecules and can be broken by strong physical forces. Harsh vortexing, repeated forceful pipetting, and rough transfers may shear them into smaller pieces. UV light used to see DNA bands can also damage DNA, so exposure should be kept as short and low as practical. Option A is correct because careful handling and reduced UV help preserve fragment length and recovery. Option B is wrong because harsh vortexing increases mechanical shearing. Option C is wrong because long UV exposure increases DNA damage rather than protecting the sample. Option D is wrong because unlabeled tubes do not protect DNA and can cause sample mix-ups, although correct labeling is also good laboratory practice. Work gently, use suitable tools, and minimize unnecessary illumination. Memory cue: big DNA needs “soft hands and short UV.”

The correct precaution is to avoid cutting a nearby unwanted DNA band along with the vector band. If another DNA fragment is excised with the vector, the purified sample may be mixed, causing incorrect results in later ligation or transformation steps. Use the DNA ladder and expected fragment size to identify and cut only the target band with a clean scalpel. Cutting an overly large gel piece or including the ladder lane reduces purity; UV exposure should also be kept minimal because prolonged UV can damage DNA. Exam tip: in gel excision, check both the expected band size and the purity of the cut band.

The direct answer is A: compare the band size with the ladder and the expected restriction map. A DNA ladder provides size references, while the expected map predicts which fragments should appear after digestion. First note the expected fragment size, then compare the sample band’s position with the ladder, and finally check whether the pattern agrees with the map. This helps identify the intended band before cutting it from the gel. Option A is correct because it uses objective size and mapping information. Option B is wrong because tube smell is not a reliable way to identify DNA bands. Option C is wrong because enzyme color does not tell the size or identity of a fragment. Option D is wrong and unsafe: buffer must never be tasted, and its taste gives no useful information. A correct band choice prevents purification of the wrong DNA. Memory cue: before cutting, check “ladder, expected size, map.”

A post-purification gel confirms that desired fragment is recovered. It is useful before ligation.

Salt left in a purified DNA fragment can alter the ionic conditions of the ligation mixture and interfere with DNA ligase activity. As a result, joining of the vector and insert ends may become less efficient. Unlike option B, extra salt does not improve ligation. Exam tip: ensure proper washing and drying after DNA purification to avoid salt carryover.

Residual ethanol in purified DNA can reduce the activity of DNA ligase and other enzymes. Therefore, DNA containing ethanol carryover may give poor results in ligation, PCR, or other downstream enzyme-based reactions. Ethanol carryover does not make DNA more visible or cause a gel to run on its own. Exam tip: Avoid over-drying the DNA pellet as well, because an excessively dry pellet can be difficult to dissolve.

The direct answer is A: the ligation ratio can be wrong. Successful ligation depends on the relative amounts of insert DNA and vector DNA, often considered by mass and, more accurately, by molar amount. If the concentration is estimated too high, too much DNA may be used; if estimated too low, too little may be added. The intended insert-to-vector ratio then changes, which can reduce ligation and cloning efficiency. Option A is correct because concentration measurement directly affects reaction setup. Option B is wrong because an incorrect estimate does not change the name of the restriction enzyme. Option C is wrong because concentration error does not automatically change gel color. Option D is wrong because DNA does not turn into protein merely because its concentration was measured incorrectly; protein synthesis is a separate biological process. Measure carefully, account for dilution, and calculate the planned amounts before ligation. Memory cue: wrong concentration means wrong DNA ratio.

Long and short fragments have different molecule numbers at the same mass. Molar ratio is important in ligation.

When the amount of insert is much higher than that of the vector, more than one insert fragment may ligate to a single vector molecule. This can produce concatemers or multiple-insert recombinant products and reduce recovery of the desired single-insert clone. Option B is incorrect because excess insert does not ensure one insert per vector. Exam tip: in ligation, an appropriate insert-to-vector molar ratio is more important than simply adding more DNA.

Low insert availability can increase chance of vector self-ligation or empty colonies. Optimize ratio.

Two different ends can reduce vector self-closing. They help join insert in correct orientation.

Diagnostic digest or PCR can give orientation-specific pattern. Do final confirmation by sequencing.

After cutting and ligation false clones can appear. Screening identifies the correct recombinant.

The direct answer is A: ligation and transformation results can fail. Restriction enzymes determine the ends produced on the vector and insert. If the wrong enzyme is used, the ends may not be compatible, the desired site may be cut incorrectly, or an important sequence may be damaged. DNA ligase may then fail to join the fragments efficiently, and transformation may produce few or no useful recombinant colonies. Option A is correct because an early cutting error can affect every later cloning step. Option B is wrong because PCR does not automatically improve merely because a wrong restriction enzyme was used. Option C is wrong because restriction digestion is not protein digestion. Option D is wrong because using the wrong enzyme does not purify the sample. Memory cue: wrong cut means wrong ends, and wrong ends mean poor joining.

Uncut vector can transform and give background. Complete digestion and gel purification reduce it.

Complete digestion linearizes vector and gel purification removes uncut plasmid. This reduces background.

After restriction digestion, uncut plasmid and the cut linear plasmid can have different mobilities on a gel because they have different conformations. Excising and eluting only the desired linear plasmid band leaves the uncut-plasmid band behind. Cutting out all bands together would defeat the purpose of separation. Exam tip: in gel purification, select the band that matches the expected size and form of the desired DNA product.

Unwanted fragments can interfere in ligation reaction. Purification keeps specific desired fragments.