Cutting of DNA

Correct answer: A. Straight-cut DNA ends without overhangs

The governing concept is the structure of DNA ends produced when a restriction enzyme cuts both strands at corresponding positions. A blunt end is a double-stranded DNA end with no single-stranded nucleotide overhang. The two strands terminate at the same level, giving the molecule a relatively straight or flush end; therefore option A is correct. This differs from a sticky end, where the cuts are staggered and a short single-stranded overhang remains. Option B is incorrect because blunt ends describe DNA, not unusually long RNA. Option C refers to neither DNA strand structure nor restriction cutting, and option D is too broad because a blunt end is not simply any single-stranded end.

Correct answer: B. At or near specific recognition sequences

The governing concept is sequence-specific DNA cleavage by restriction endonucleases. Each restriction enzyme recognizes a particular short DNA sequence, often a palindromic sequence, and cuts the phosphodiester backbone at a defined position within that sequence or close to it. Thus option B is correct. The cut may produce blunt ends or staggered sticky ends, depending on the enzyme and its cleavage pattern. Option A describes a cellular location rather than the molecular site recognized by the enzyme. Options C and D are unrelated because protein coats and ribosomes are not the DNA recognition sites used by restriction enzymes.

Correct answer: A. Cutting the vector with a suitable restriction enzyme

The governing concept is preparation of a cloning vector for recombinant-DNA construction. Before a desired gene can be inserted, the circular vector must be opened at a suitable recognition site. A selected restriction enzyme cuts the vector DNA and creates compatible ends, such as sticky ends, that can pair with matching ends on the desired DNA fragment. This makes option A correct. After alignment, DNA ligase is normally used to seal the sugar-phosphate backbone and form recombinant DNA. Boiling would damage or destroy the vector, changing it into sugar is biologically meaningless, and converting it into a ribosome is unrelated to gene cloning. Cutting is therefore the necessary preceding step, not the final joining step.

Correct answer: C. Cutting

The governing concept is the catalytic role of a restriction enzyme in recombinant-DNA technology. A restriction enzyme is an endonuclease that recognizes a particular DNA sequence and hydrolyses phosphodiester bonds in the DNA backbone, thereby cutting the molecule at or near that recognition site. Consequently, option C, cutting, is correct. Synthesis means building a new molecule and is not the primary action of these enzymes. Translation is the ribosome-mediated production of a protein from messenger RNA, while evaporation is a physical change of state and has no connection with DNA cleavage. The enzyme may create blunt or sticky ends, but both outcomes result from cutting DNA.

Correct answer: A. When a foreign gene is to be joined with a vector

The governing concept is cohesive-end joining during gene cloning. A restriction enzyme can make a staggered cut, leaving short single-stranded overhangs called sticky ends. If a foreign gene and a vector are cut with the same suitable enzyme, their overhangs can have complementary sequences. The ends then anneal through hydrogen bonding, bringing the insert and vector into the correct alignment; DNA ligase can subsequently seal the backbone. Therefore option A is correct. Heating water, changing leaf colour, and filtering soil are unrelated processes and do not explain the molecular usefulness of sticky ends. Sticky ends increase the efficiency and specificity of recombinant-DNA assembly.

Correct answer: A. It can be cut with a suitable restriction enzyme

The governing concept is preparation of an insert for recombinant-DNA cloning. Before a foreign gene is inserted into a vector, the gene and the vector can be treated with the same suitable restriction enzyme. This produces compatible sticky ends, or suitable blunt ends when applicable, so that the insert can align with the opened vector. DNA ligase then joins the aligned molecules by forming phosphodiester bonds. Hence option A is correct. The gene is not converted into water, a leaf, or a metal; those choices describe impossible transformations and do not represent molecular cloning steps. The exact enzyme must be selected so the desired gene is not damaged and the ends are compatible.

Correct answer: A. To compare fragment sizes

The governing concept is separation of charged DNA molecules by gel electrophoresis. DNA has a negatively charged phosphate backbone, so it migrates through an agarose gel toward the positive electrode when an electric field is applied. Smaller DNA fragments generally move through the gel matrix more easily and travel farther than larger fragments. After staining, the resulting bands reveal the number and approximate sizes of fragments produced by restriction digestion. Therefore option A is correct: gel electrophoresis allows fragment sizes to be compared, commonly with a DNA ladder as a reference. It does not measure water temperature, determine leaf age, or assess the taste of fat.

Correct answer: A. Joining will be difficult or inefficient

In molecular cloning, the ends of a vector and an insert must usually be compatible so that their bases can pair or their chemical termini can be joined efficiently. If the ends are incompatible, they will not align or base-pair correctly, and DNA ligase cannot readily seal the discontinuity. As a result, the ligation efficiency decreases and few or no recombinant molecules may be formed. Option A is therefore correct. Option B is the opposite of the expected result. DNA does not vanish merely because its ends are incompatible, and an enzyme cannot be converted into sugar by this reaction. Compatible cohesive ends or specially prepared blunt ends can solve the problem.

Correct answer: A. Restriction enzymes

The direct answer is A, restriction enzymes. These enzymes are proteins made by many bacteria. They recognize particular short DNA sequences, called recognition sites, and cut the DNA at or near those sites. This is why they are often called molecular scissors. Step by step: first, the enzyme finds its specific DNA sequence; next, it breaks the sugar-phosphate backbone of both DNA strands; finally, the resulting fragments can be joined to another DNA fragment in recombinant DNA work. Option A is correct because restriction enzymes are specifically used for controlled DNA cutting. Option B, lipase, mainly breaks lipids or fats, not DNA. Option C, amylase, digests starch into smaller sugars, so it is not a DNA-cutting enzyme. Option D, cellulase, breaks cellulose, a plant cell-wall carbohydrate, and does not normally cut DNA. Remember: restriction enzyme means a sequence-specific DNA cutter; ligase joins DNA after cutting.

Correct answer: B. Restriction endonuclease

A restriction endonuclease cuts inside DNA at a specific sequence. It is essential in recombinant DNA technology.

Correct answer: D. DNA sequence recognized by the enzyme

A recognition site is the specific DNA sequence where an enzyme binds and cuts. Each enzyme may have a different site.

Correct answer: A. A sequence read the same on both strands in opposite directions

Many restriction enzymes recognize palindromic sequences. Remember it by symmetry of both strands.

Correct answer: C. Sticky ends

EcoRI is a restriction endonuclease that cuts the two DNA strands in a staggered manner rather than directly opposite each other. This produces short single-stranded overhangs called sticky ends. These overhangs can pair with complementary bases and make ligation by DNA ligase easier. Blunt ends do not have such overhangs. Exam tip: remember that staggered cuts produce sticky ends, whereas straight cuts produce blunt ends.

Correct answer: D. Straight DNA end without overhang

Blunt ends have no single-stranded overhang. They can ligate but may be harder than sticky ends.

Correct answer: A. Pairing with complementary ends becomes easier

Sticky ends can pair by complementary bases. Therefore they are useful in making recombinant DNA.

Correct answer: B. sticky ends with overhangs / Sticky ends with overhangs

A staggered cut cuts DNA strands at different positions. This forms sticky ends with overhangs.

Correct answer: C. Blunt ends

In a straight cut, both complementary DNA strands are cut at the same position, so no single-stranded overhang is formed. Such ends are called blunt ends. Sticky or cohesive ends result from staggered cuts that leave an overhang. Exam tip: same-position cut = blunt ends; staggered cut = sticky ends.

Correct answer: D. Because it cuts at a specific recognition sequence in DNA to produce the required fragments

A restriction enzyme recognizes a particular DNA recognition sequence and cuts at or near that site. This produces predictable DNA fragments, which are needed to isolate a desired gene and insert it into a vector during cloning. Option C describes the action of DNA ligase, not a restriction enzyme. Exam tip: restriction enzymes cut DNA, whereas ligases join DNA fragments.

Correct answer: A. DNA fragments

The direct answer is option A: DNA fragments. DNA is a long molecule made of nucleotides joined in a chain. During restriction digestion, restriction enzymes cut this long DNA at particular recognition sites. The original long molecule is consequently divided into smaller pieces, and each piece is called a DNA fragment. Option A is correct because it names exactly the products of DNA cutting. Option B, protein folds, refers to shapes or arrangements of proteins, not pieces of DNA. Option C, lipid droplets, are associated with fats and membranes, not restriction digestion. Option D, starch grains, are carbohydrate storage structures in plants and are unrelated to DNA cutting. The resulting fragments can be separated according to size by gel electrophoresis, where smaller pieces generally move farther through the gel. A useful memory cue is: restriction enzyme cuts DNA, so the immediate products are restriction fragments or DNA fragments.

Correct answer: B. Agarose gel electrophoresis

Agarose gel electrophoresis separates cut DNA fragments according to their size. Smaller DNA fragments move farther through the gel than larger fragments, producing distinct bands. Centrifugation is mainly used to separate particles based on density or size, not to resolve DNA fragments as bands. Exam tip: For size-based separation of DNA fragments, remember agarose gel electrophoresis.

Correct answer: C. To provide proper pH and salt conditions for the enzyme

A restriction-digestion buffer supplies the chemical environment required for the selected restriction enzyme to function. Its pH helps preserve the enzyme’s charge, folding, and active-site configuration, while salts control ionic strength and often provide essential cofactors such as magnesium ions needed for phosphodiester-bond cleavage. If the pH, salt concentration, or cofactors are unsuitable, digestion may be slow, incomplete, nonspecific, or absent. Therefore option C is correct: the buffer provides suitable pH and salt conditions for the enzyme. It does not stain DNA, transform protein into starch, or promote cell growth; those activities are unrelated to an in-vitro DNA-cleavage reaction.

Correct answer: D. To maintain proper enzyme activity

A restriction enzyme works most efficiently at its optimum temperature, so the reaction temperature is controlled. At an incorrect temperature, enzyme activity may decrease or the enzyme may denature, leading to incomplete or inefficient DNA cutting. Melting gel is not the purpose of restriction digestion. Exam tip: Many restriction enzymes have an optimum temperature of 37°C, but always check the enzyme-specific conditions.

Correct answer: A. About 37 degrees Celsius

Many common restriction enzymes work around 37 degrees Celsius. Always check the specific enzyme condition.

Correct answer: B. At a specific recognition site

Restriction enzymes are sequence-specific endonucleases, which is the governing concept here. Each enzyme recognizes a short DNA sequence called its recognition site, often a palindromic sequence, and cleaves the phosphodiester backbone at a defined position within or close to that site. This predictable cutting is why the enzymes are useful for making recombinant DNA fragments. Option B is therefore correct. The enzyme does not target a protein region, a cell membrane, or a ribosome: those choices describe structures or molecules outside the DNA substrate used in the digestion reaction.

Correct answer: D. Compatible ends can form

The governing principle is complementary end formation during recombinant-DNA construction. If the vector and the DNA insert are cut with the same restriction enzyme, the enzyme usually generates matching sticky ends, or compatible termini, on both molecules. Their exposed single-stranded bases can pair by hydrogen bonding, aligning the insert with the vector so DNA ligase can seal the remaining phosphodiester breaks. Option D is correct. The treatment does not guarantee degradation, produce RNA, or directly produce protein. Protein expression occurs later only if the inserted gene is correctly placed and expressed in a suitable host.