Restriction Enzymes

Correct answer: A. By staggered cutting

Sticky ends are produced when a restriction endonuclease cuts the two strands of double-stranded DNA at different positions, creating a staggered break. The resulting DNA fragments have short single-stranded overhangs. These overhangs contain bases that can pair with a complementary sequence on another fragment, making it easier to align the fragments before DNA ligase seals the sugar-phosphate backbone. Therefore option A is correct. Direct heating may denature DNA but does not create the specific, enzyme-defined overhangs used in recombinant DNA work. Reading RNA and folding proteins are biological processes unrelated to restriction cutting. The governing concept is staggered restriction-enzyme cleavage followed by complementary base pairing and ligation.

Correct answer: A. Matching ends are produced

The governing concept is restriction-enzyme cleavage and complementary DNA ends. A restriction endonuclease recognises a particular nucleotide sequence and cuts DNA at or near that sequence. If the same enzyme cuts both the desired gene and the plasmid, the resulting fragments usually possess compatible, complementary sticky ends, or compatible blunt ends in some cases. These ends can align through base pairing, allowing DNA ligase to seal the sugar-phosphate backbone and form a recombinant plasmid. Thus option A is correct. Cutting does not make DNA disappear, fill a plasmid with water, or directly change protein colour. The joining step still requires suitable conditions and ligase.

Correct answer: A. Cutting DNA at specific sites

Restriction enzymes cut DNA at specific recognition sites. For exams remember them as molecular scissors of biotechnology.

Correct answer: B. In bacteria

These enzymes occur in bacteria as a defense system. For exams remember bacteria as their source.

Correct answer: C. Phage DNA

Bacteria use these enzymes to cut foreign bacteriophage DNA. Link this with bacterial defense in exams.

Correct answer: D. Specific nucleotide sequence

These enzymes recognize a fixed sequence in DNA. The term recognition site is important for exams.

Correct answer: D. They cut DNA near recognition sites

Type II restriction enzymes are useful because they recognize particular short DNA sequences and cut at defined positions within or close to those sites. Their predictable action produces reproducible DNA fragments. Scientists can therefore cut a vector and a desired DNA insert in a planned way and join compatible ends during cloning. This control is the important advantage; random cutting would make the result difficult to predict.

The correct statement is that these enzymes cut DNA near recognition sites, so option D follows. They do not cut proteins, manufacture RNA, or act without recognition. A small terminology point is that many Type II enzymes cut within their recognition sequence, while some related enzymes cut nearby. Thus “near recognition sites” is the best available choice and expresses their site-specific behavior used in recombinant DNA work.

Correct answer: B. Escherichia coli

EcoRI is a well known restriction enzyme obtained from Escherichia coli. Remember both naming and source for exams.

Correct answer: C. 5′ single-stranded overhangs (sticky ends)

EcoRI is a restriction enzyme that cuts the recognition sequence 5′-GAATTC-3′ in a staggered manner. This produces sticky ends with 5′ single-stranded overhangs, which can base-pair with complementary DNA ends. A 3′ overhang can also be sticky, but EcoRI specifically produces a 5′ overhang. Exam tip: remember EcoRI with the GAATTC recognition site and the 5′-AATT overhang.

Correct answer: D. They can easily pair with complementary ends

Sticky ends have complementary bases that help joining. In exams connect them with DNA ligation.

Correct answer: B. It reads same on both strands in opposite directions

A palindromic sequence reads the same on both strands in a defined orientation. Many restriction enzymes recognize such sequences.

Correct answer: C. DNA ligase

DNA ligase joins cut DNA fragments. For exams remember restriction enzyme cuts and ligase joins.

Correct answer: A. DNA

Restriction enzymes recognize specific DNA sequences and cut DNA at or near those sites. Therefore, DNA is the correct answer. Enzymes that act on RNA are different; typical restriction enzymes do not cut RNA, proteins, or lipids. Exam tip: associate restriction enzymes with specific DNA recognition sequences and cleavage.

Correct answer: D. Their source bacterium

The governing concept is the nomenclature of restriction endonucleases. Their names are usually derived from the organism in which the enzyme was discovered, especially the genus, species, and sometimes the strain or plasmid. For example, EcoRI is associated with Escherichia coli: “E” comes from the genus, “co” from the species designation, and the remaining notation identifies the strain or enzyme order. Therefore option D is correct. Colour, shape, and temperature are not the standard basis for naming these enzymes. Temperature may affect enzyme activity, and shape may describe a molecule, but neither explains names such as EcoRI, HindIII, or BamHI.

Correct answer: A. Recombinant DNA technology

Restriction enzymes are a basic requirement of recombinant DNA technology. Connect them with gene cloning in exams.

Correct answer: C. DNA sequence recognized by the enzyme

A restriction site is a specific recognition sequence in DNA. The enzyme cuts at or near that region.

Correct answer: B. By methylation

The direct answer is option B: methylation. Restriction enzymes recognise particular DNA sequences and cut DNA at or near those recognition sites. Bacteria possess restriction-modification systems. Their own DNA is modified by adding methyl groups to suitable bases at the recognition sequences. This methylation acts like a chemical identity mark, so the restriction enzyme does not cut the bacterium’s protected DNA. DNA entering from outside may lack the correct methylation pattern and can therefore be recognised and cut. Option A, always breaking their own DNA, would destroy essential genetic information and cannot be a protection method. Option B is correct because methylation protects bacterial DNA from its matching restriction enzyme. Option C, oxidation, is not the standard protective modification in this system. Option D, glycolysis, is a metabolic pathway that breaks down glucose to release energy; it has no role in marking DNA for restriction protection. A useful qualification is that protection is carried out by specific modification enzymes and the exact patterns vary, but methylation is the expected school-level answer. Memory cue: restriction enzyme cuts foreign DNA; methyltransferase marks self DNA.

Correct answer: C. They cut after recognizing specific DNA sequences

Restriction enzymes are molecular tools that identify a particular DNA sequence, called a recognition sequence, and cleave the DNA at a defined location associated with that sequence. Their action is therefore specific rather than random. This specificity allows researchers to select where a DNA molecule will be opened and to produce useful fragments for cloning, mapping, or analysis. The enzyme responds to the nucleotide sequence, not to unrelated cell structures or simple nutrients.

Option C is correct because it says that the enzyme recognizes specific DNA sequences before cutting. Option A is wrong because cutting does not occur everywhere randomly. Option B describes a cell membrane, which is not the substrate of these enzymes, and option D concerns glucose production, unrelated to restriction activity. Recognition followed by cleavage is the defining idea.

Correct answer: D. Genetic engineering

Restriction enzymes are essential for cutting DNA in genetic engineering. Connect them with cloning and vector construction.

Correct answer: A. When they have complementary sticky ends

DNA fragments with complementary sticky ends can anneal because their single-stranded overhangs form hydrogen bonds by complementary base pairing. DNA ligase then forms phosphodiester bonds in the sugar-phosphate backbone to make the joining permanent. Non-complementary ends cannot readily undergo this initial base pairing. Exam tip: DNA fragments cut by the same restriction enzyme often have complementary sticky ends.

Correct answer: A. It cuts DNA internally

An endonuclease cuts bonds within a nucleic acid. So restriction endonuclease cuts inside DNA at specific sites.

Correct answer: C. EcoRI

EcoRI is a famous restriction enzyme. Insulin and hemoglobin are proteins but not restriction enzymes.

Correct answer: A. Compatible ends are produced in both

Cutting with the same enzyme gives compatible ends. This helps foreign DNA join with the vector easily.

Correct answer: C. A restriction enzyme

Direct answer: Option C, a restriction enzyme. HindII is a bacterial restriction endonuclease, a biological tool used to cut DNA at a particular recognition sequence. The name “Hind” comes from Haemophilus influenzae, and “II” identifies the enzyme type or form. Its important action is not hormonal, nutritional, or coloring; it recognizes a defined DNA sequence and cuts the phosphodiester backbone. Option A, a hormone, is wrong because hormones are chemical messengers in organisms, whereas HindII acts on DNA. Option B, a vitamin, is wrong because vitamins are nutrients needed in small amounts and do not specifically cut DNA. Option C is correct because restriction enzymes are molecular scissors used in genetic engineering. Option D, a pigment, is wrong because pigments produce or reflect color and do not cut DNA. Memory cue: restriction enzyme means “DNA-cutting enzyme.”

Correct answer: B. Restriction enzyme

Some restriction enzymes produce sticky ends. These ends tend to join by complementary base pairing.