Cutting of DNA

A plasmid is DNA, so it carries charge and can move through an agarose gel under an electric field. However, DNA molecules with the same length may not move at the same speed if their shapes differ. A supercoiled plasmid is tightly compacted, whereas linear DNA is more extended. The compact form can pass through the gel pores differently and therefore travel a different distance.

For this reason, an uncut plasmid may show several forms, such as supercoiled, nicked circular, or linear DNA, with different mobilities. Its position should not be used for a simple size estimate without caution. Option A correctly identifies compact shape as the cause. The plasmid is not a protein or enzyme, and it is not uncharged, so options B, C, and D are incorrect.

Loading excessive DNA overloads the lane, so DNA accumulates in a small region of the gel. The bands can become broad, smeared, distorted, or overlap, making fragment-size interpretation difficult. DNA overloading does not alter the restriction enzyme’s cutting specificity; digestion occurs before electrophoresis. Exam tip: use an appropriate DNA amount to obtain sharp, well-resolved bands.

A DNA stain, such as ethidium bromide or another fluorescent dye, binds to DNA and makes it visible as bands under UV or blue light. It is therefore used to observe the positions of DNA fragments after gel electrophoresis. Cutting DNA is the role of a restriction enzyme, not a stain. Exam tip: remember that stains help visualise DNA bands, whereas restriction enzymes cut DNA.

A gel image gives proof for future comparison and lab record. It is useful in clone verification.

The direct answer is option A: expected and observed bands are recorded so the result can be interpreted and verified. Before running a digest, the DNA sequence, restriction sites, and fragment sizes allow us to predict the expected bands. After electrophoresis, the visible gel gives the observed bands and their approximate sizes. Comparing the two tells us whether digestion probably worked and whether the DNA construct or sample matches the plan. Option A is correct because this comparison supports both interpretation and verification. Option B is wrong: enzyme smell has no role in identifying DNA fragments. Option C is wrong and unsafe as a laboratory idea; agar is a gel material, not something to taste. Option D is wrong because water volume belongs in the reaction setup record, but it is not the reason for writing expected and observed bands. A difference between prediction and observation may arise from incomplete digestion, wrong enzyme, an incorrect sequence assumption, overloaded DNA, or poor size estimation. Memory cue: expected is the prediction; observed is what the gel actually shows.

Even with similar names enzymes can give different recognition or ends. Exact name is needed in enzyme selection.

Wrong labels can make result interpretation incorrect. Clear labelling is basic lab safety and accuracy.

It is useful to record enzyme buffer time temperature and result. This increases reproducibility.

During restriction digestion, enzymes, buffers, DNA stains and samples must be handled according to laboratory safety procedures. Some DNA stains and other reagents may be hazardous, so PPE such as gloves and proper waste disposal are essential. Ethanol is flammable, making its placement near an open flame unsafe. Exam tip: In laboratory-safety questions, options involving PPE, labelling and correct waste disposal are usually appropriate.

In Cutting of DNA the full chain from enzyme choice to gel interpretation should be understood. Controls make results reliable.

Restriction digestion is important in both cloning planning and verification. Accurate maps controls and records are needed.

Sticky ends are short single-stranded overhangs produced when a restriction enzyme cuts DNA in a staggered way. If two DNA fragments have compatible sticky ends, the exposed bases can form temporary complementary base pairs. This pairing brings the fragment ends together in the correct alignment and makes joining more efficient.

Thus option A is correct. The pairing itself is not a permanent chemical bond. DNA ligase is needed afterward to seal the sugar-phosphate backbones by forming the final phosphodiester bonds. Compatible ends do not make protein, destroy the restriction enzyme, or melt an agarose gel. Their usefulness comes from base complementarity and the alignment it provides during recombinant-DNA construction.

Sticky ends are also called cohesive ends because of complementary overhangs. They help in ligation.

The direct answer is option A: a staggered cut can produce a 5-prime overhang. Double-stranded DNA has two opposite strands, and each strand has a 5-prime end and a 3-prime end. If a restriction enzyme cuts the two strands at different positions, the short unpaired single-stranded part is called an overhang or sticky end. Depending on which strand extends beyond the other, the overhang is described as 5-prime or 3-prime. Option A is correct because a staggered cut creates the unequal ends needed for such an overhang. Option B is wrong because a straight, or blunt, cut across both strands at the same position produces blunt ends, not an overhang; the word “always” also makes it false. Option C is wrong because proteins are not the DNA substrate being cut in this context. Option D is wrong because an RNA-only cut does not describe restriction digestion of DNA. Remember that not every staggered cut specifically gives a 5-prime overhang; it can give either type, so the exact enzyme and cut positions matter.

Some restriction enzymes produce 3 prime overhangs. Overhang type affects ligation compatibility.

5 prime and 3 prime overhangs have different compatibility. End polarity is important in cloning design.

If two different restriction enzymes produce the same complementary sticky overhang, a vector cut by one enzyme can base-pair with a DNA fragment cut by the other. DNA ligase can then join these compatible ends to form recombinant DNA. The other options are incorrect because restriction enzymes cut at specific recognition sites rather than breaking all DNA bonds; they do not synthesise proteins or control gel migration. Exam tip: Ends with matching overhangs are called compatible ends, even when produced by different enzymes.

Compatible ends from different enzymes can ligate and change original sites. The site may not cut again.

Informative restriction sites are needed to confirm orientation. Otherwise sequencing or orientation PCR may be needed.

An informative digest distinguishes correct clone from wrong clone. Enzyme selection is based on this.

A restriction map shows the cut sites of restriction enzymes on a DNA molecule and the expected lengths of fragments produced after digestion. Therefore, it can predict DNA fragments and the expected band pattern on gel electrophoresis. It cannot predict traits such as protein taste or cell-wall colour. Exam tip: the distance between two consecutive cut sites gives the size of the DNA fragment between them.

A restriction map shows the locations of restriction-enzyme cut sites. It allows prediction of the DNA fragment sizes and the band pattern expected after digestion on a gel. Without the map, it is difficult to compare the observed bands with an expected result. Enzyme activity, the negative charge of DNA, and agarose-gel preparation do not depend on a restriction map. Exam tip: In a diagnostic digest, a restriction map is mainly used to predict fragment sizes and band patterns.

Virtual digest can show expected pattern before the actual experiment. It is helpful in planning.

The direct answer is option A: check the supplier information to learn the recommended buffer, temperature, methylation sensitivity, and heat-inactivation conditions. Restriction enzymes are not identical. Each enzyme may work best in a particular buffer and temperature, may be blocked by methylation at certain bases, and may or may not be safely stopped by heating. The supplier’s official datasheet provides these practical details, helping the researcher set up a reliable reaction. Option A is correct because these factors directly affect digestion efficiency and specificity. Option B, enzyme color, is not the main factor for choosing reaction conditions. Option C, DNA smell, is not a valid or safe way to assess DNA. Option D, agar taste, is irrelevant and laboratory materials must never be tasted. The exam cue is: for a restriction enzyme, consult the manufacturer’s protocol before use. Conditions such as DNA amount, incubation time, and star-activity warnings may also be listed and should be followed where applicable.

Provided buffer sets pH salt and cofactors according to the enzyme. This increases digestion reliability.