Cutting of DNA

Manipulated images can show false results. Accurate documentation is part of responsible science.

Some DNA stains may harm the skin, eyes, or environment. Therefore, appropriate PPE such as gloves, a lab coat, and eye protection when required should be used, and the stain and contaminated materials must be disposed of through the laboratory’s approved procedure. Pouring it directly into a sink is not automatically safe because many stains require chemical-waste disposal. Exam tip: for any hazardous reagent, prioritise PPE, label/SDS instructions, and approved disposal.

Waste should be disposed according to lab safety rules. Stain-containing waste may need special handling.

The cutting step is directly linked to ligation and verification. Clean compatible ends and correct records are needed.

Correct cutting makes insert and vector suitable for joining. Cleanup and ratio support ligation.

The goal of DNA cutting is to make controlled fragments and ends. Accurate setup gel analysis and records make results reliable.

When the cutting goal is clear enzyme choice becomes correct. The first step in cloning planning is understanding target site.

In recombinant DNA insert and vector must be prepared in a planned way. Predictable cutting increases cloning success.

The direct answer is A: the target molecule of a restriction-enzyme reaction is DNA. Restriction enzymes are sequence-specific nucleases. They recognize a particular short DNA sequence and break phosphodiester bonds in the sugar-phosphate backbone, producing either blunt or sticky ends depending on the enzyme. Step by step: DNA is placed in the reaction; the enzyme searches for its recognition sequence; it binds that sequence; it cuts the DNA backbone at a defined position; the resulting ends can be used in cloning. A is correct. B is wrong because proteins are not the normal substrate being cut in this reaction. C is wrong because starch is a carbohydrate and is not recognized by restriction enzymes. D is wrong because lipids are membrane-related molecules and are not the target here. The enzyme itself is a protein, but that does not make protein the target molecule. Memory cue: “restriction enzymes restrict DNA at specific sequences.”

In each DNA strand, successive nucleotides are connected through the sugar-phosphate backbone. The linkage between two consecutive nucleotides is a phosphodiester bond. Restriction endonucleases cut DNA at specific recognition sequences by breaking these bonds. Peptide bonds occur in proteins, not in DNA. Exam tip: Associate the DNA backbone with sugar-phosphate and phosphodiester bonds.

An endonuclease cuts internal phosphodiester bonds in nucleic acids. Restriction enzymes are in this group.

Restriction endonuclease cuts specific internal sites. Exonuclease removes nucleotides from ends.

Restriction enzymes are part of the bacterial defence system. They recognise and cut incoming foreign DNA, such as bacteriophage DNA, at specific sequences and can thereby prevent infection. Protein folding is performed by chaperone proteins, not restriction enzymes. Exam tip: remember that restriction enzymes cut DNA at specific recognition sites.

Host DNA methylation can protect its own recognition sites. Foreign unmethylated DNA can be cut.

The direct answer is A: host DNA methylation. In a restriction–modification system, modification means that the host cell adds methyl groups to particular bases in its own DNA. This methyl marking protects host DNA from being cut by the matching restriction enzyme. The restriction part cuts unprotected foreign DNA, while the modification part protects self-DNA. Option A is correct because methylation is the protective modification. Option B, DNA gel run, only separates DNA fragments and does not protect DNA. Option C, protein digestion, concerns proteins rather than DNA methylation. Option D, ethanol precipitation, is a method for collecting or concentrating nucleic acids, not a protective marking system. Remember: modification marks self, restriction cuts foreign.

Restriction enzyme names can relate to organism genus species strain and discovery order. This is an exam fact.

EcoRI is a restriction endonuclease. It recognises a specific DNA recognition sequence, such as GAATTC, and cuts both DNA strands, typically producing sticky ends. DNA ligase is a close distractor because it acts on DNA too, but it joins cut DNA fragments rather than cutting them. Exam tip: Names such as EcoRI, HindII and BamHI indicate restriction enzymes.

Many restriction enzymes recognize palindromic DNA sequences. This is a key feature of recognition specificity.

Restriction enzymes show sequence specificity: each enzyme recognizes a particular short DNA sequence, called its recognition or restriction site, and cleaves only when that sequence is present in a suitable context. If the site is absent from a DNA molecule, the enzyme has no matching target and therefore cannot make the expected cut at that position. The DNA does not automatically get cut elsewhere, become a protein, or prevent a gel from running. A reaction may still contain uncut DNA or cuts at other matching sites. Thus option A correctly states the direct consequence of absence of the restriction site.

Linear DNA has two free ends. A cut at one site divides it into two separate pieces, so two fragments are formed. One fragment would remain if there were no cut; in contrast, one cut in circular DNA usually produces one linear fragment. Exam tip: for linear DNA, number of fragments = number of cut sites + 1.

In linear DNA number of fragments is usually cut sites plus one. Three cuts make four fragments.

A single cut in circular DNA opens its closed loop and produces one linear DNA molecule. Two circular DNA molecules cannot be produced by merely making one cut; that would require replication and separation of DNA molecules. Exam tip: for circular DNA, the number of cuts generally equals the number of linear fragments formed.

When a circular plasmid DNA is cut at two different sites, it is divided into two DNA fragments. Therefore, two DNA fragments is correct. A single cut would generally produce one linear DNA fragment, making option B the closest but incorrect distractor. Exam tip: for circular DNA, the number of fragments generally equals the number of cut sites.

A restriction enzyme cuts DNA at particular recognition sites. The number of fragments produced depends on how many effective cuts occur and on whether the DNA is linear or circular. Before performing the digest, a restriction map or expected construct usually predicts the fragment pattern. Counting the bands after electrophoresis allows us to compare the observed result with that prediction.

If the fragment count and approximate pattern agree, the digestion is likely complete and the construct may have the expected arrangement. A missing, extra, or unexpected band can indicate incomplete digestion, an incorrect construct, or another experimental problem. Therefore option A is correct: fragment count helps verify the cut pattern. Enzyme colour, water smell, and protein production are unrelated to this verification.

The governing principle is movement of charged molecules through a porous agarose matrix under an electric field. DNA has a negatively charged phosphate backbone, so it migrates toward the positive electrode. For DNA fragments of similar shape, the gel primarily separates them according to length or size: smaller fragments pass through the pores more easily and generally travel farther or faster than larger fragments during the same run. Taste and smell have no role in electrophoretic separation, and the enzyme name does not determine migration. Therefore option A is correct, with the important qualification that the gel separates DNA mainly by size.