Restriction Enzymes

Correct answer: A. Unexpected extra DNA cuts

During star activity, a restriction enzyme may cut DNA at sequences similar to, but not exactly, its normal recognition site. This produces unexpected extra DNA fragments or bands on a gel. Complete absence of DNA is not the usual effect of star activity. Exam tip: remember star activity as non-specific or relaxed-specificity DNA cleavage.

Correct answer: A. Giving proper pH and ionic condition for enzyme

Buffer makes the reaction environment suitable for enzyme activity. Wrong buffer can affect digestion.

Correct answer: A. Keep them cold and protected from heat

The direct answer is option A: restriction enzymes should be kept cold and protected from heat. Restriction enzymes are proteins, and their three-dimensional shape is necessary for recognizing and cutting the correct DNA sequence. Heat can disrupt this shape and reduce or destroy enzyme activity. Therefore laboratories store these enzymes under cold conditions, commonly in a freezer, and avoid repeated warming or unnecessary exposure. Option A is correct because cold handling helps preserve activity. Option B is wrong because boiling generally denatures proteins and would inactivate the enzyme. Option C is wrong because dry soil is not a controlled, sterile storage condition and can damage or contaminate the preparation. Option D is wrong because sunlight and heat can harm the enzyme. The exact storage temperature depends on the manufacturer, so instructions must be followed. Memory cue: enzymes are delicate proteins—keep them cold, clean, and brief outside storage.

Correct answer: A. DNA phosphodiester backbone

Restriction enzymes make cuts in the phosphodiester backbone of DNA. Therefore their target is DNA.

Correct answer: A. endonuclease

In restriction endonuclease endonuclease means cutting inside a nucleic acid. This fact is linked with the meaning of the name.

Correct answer: A. Restriction enzyme can make a specific internal cut

Restriction enzyme makes internal cuts like an endonuclease. Exonuclease removes nucleotides from ends.

Correct answer: A. Same reading on both strands in opposite directions

Many restriction sites are palindromic. This is a common feature in restriction enzyme recognition.

Correct answer: A. Its own DNA may be cut by restriction enzyme

Methylation protects self DNA from restriction enzymes. Without it self DNA can become vulnerable.

Correct answer: A. They cut invading phage DNA

Restriction enzymes are part of the defence system of many bacteria. When foreign DNA, such as DNA injected by an invading bacteriophage, enters the cell, a suitable restriction endonuclease recognises a particular nucleotide sequence and cleaves the DNA, often producing defined ends. This cleavage can prevent the phage genome from being replicated and expressed. Therefore option A is correct because it directly describes cutting phage DNA. Option B is biologically unrelated, while options C and D describe impossible transformations and are not functions of restriction enzymes. In genetic engineering, the same cutting ability is used deliberately to prepare DNA fragments for cloning.

Correct answer: A. Cutting DNA at specific site

Restriction enzymes are important for making desired cut sites in recombinant DNA. Without them controlled cutting is difficult.

Correct answer: A. Bacterial defense and lab cloning

The natural role is bacterial defense and artificial lab use is in cloning. This is complete concept revision.

Correct answer: A. Using two different restriction enzymes

The direct answer is option A: using two different restriction enzymes can reduce the insert-orientation problem. If the same enzyme cuts both ends, the insert may sometimes join in either direction because both ends are compatible. Two different enzymes create two different, non-equivalent ends. The insert can then fit into the vector in only one planned orientation, a method called directional cloning, provided the insert and vector are designed with the matching sites. Option A is correct. Option B is wrong because boiling can denature the enzyme and damage the cloning reaction. Option C is wrong because removing DNA prevents construction rather than controlling orientation. Option D is wrong because ligase is needed to seal the joined DNA; avoiding it prevents stable recombinant formation. This method does not guarantee success in every experiment, but it strongly helps when the design is correct. Memory cue: two different cuts give one planned direction.

Correct answer: A. Enzyme can cut plasmid at a selected site

In plasmid cloning a restriction enzyme can cut at a selected site. This creates space for inserting DNA.

Correct answer: A. Joining cut fragments by covalent bonds

DNA ligase joins DNA fragments by forming phosphodiester bonds. Restriction enzyme is for cutting.

Correct answer: A. An uncut DNA band

A restriction enzyme cuts DNA only when its specific recognition sequence, called a restriction site, is present and accessible. If that site is absent from the DNA molecule, the enzyme has no correct target and does not cleave the molecule. During gel electrophoresis, the DNA can therefore migrate as the original uncut molecule, producing an uncut band rather than the smaller fragments expected after digestion. Option A is correct. The gel separates nucleic-acid fragments, not free amino acids or chlorophyll. An absent restriction site also does not remove the sample tube or make the DNA disappear; it simply prevents the expected enzymatic cleavage.

Correct answer: A. DNA digestion may be incomplete

If the amount of restriction enzyme is too low, it may not be sufficient to cut all recognition sites in the DNA. Some DNA molecules may remain uncut or be cut at only a few sites, producing incomplete or partial digestion. Option B is incorrect because a low enzyme amount reduces the chance of complete cutting. Exam tip: Complete digestion requires an appropriate enzyme amount, the correct buffer, and suitable incubation time.

Correct answer: A. Suitable buffer and active enzyme

Active enzyme and suitable buffer are needed for restriction digestion. Check conditions in practical MCQs.

Correct answer: A. enzyme source recognition sequence end type / Enzyme source recognition sequence end type

Restriction enzyme questions may ask source sequence and end type. Revising them in table form is useful.

Correct answer: A. They cut near defined recognition sites

Type II enzymes cut at defined sites. That is why they are considered more useful in biotechnology.

Correct answer: A. They cut DNA molecules

The term molecular scissors is due to their DNA cutting ability. It is a popular name for restriction enzymes.

Correct answer: A. DNA ligase

Compatible sticky ends can pair first. DNA ligase forms phosphodiester bonds for stable joining.

Correct answer: A. Bacterial enzymes that cut specific DNA sequences

This definition gives both source and function. It is the most useful definition for Class 11 exams.

Correct answer: A. Restriction enzyme helps make DNA fragments

Making recombinant DNA needs different DNA fragments. Restriction enzymes prepare them with specific cuts.

Correct answer: A. starch molecule

Restriction enzymes cut DNA at specific recognition sequences. Therefore, a DNA molecule, plasmid DNA, and foreign phage DNA can be their targets. Starch is a polysaccharide, not DNA, so it cannot be a target of a restriction enzyme. Exam tip: associate restriction enzymes with specific DNA sequences, not carbohydrates.

Correct answer: A. Bands show DNA fragments

After restriction digestion gel bands show positions of DNA fragments. Their size can be compared with a ladder.