Restriction Enzymes

Restriction enzyme activity can depend on proper buffer and temperature. Remember reaction conditions in practical questions.

The defining property of a restriction enzyme is sequence-specific cleavage of DNA. Each enzyme recognizes a particular short nucleotide sequence, often a palindromic sequence, and cuts the phosphodiester backbone at a characteristic position. This precise action makes restriction enzymes useful for isolating genes, opening cloning vectors, and constructing recombinant DNA molecules. Option A gives this essential summary. A pigment that colours protein, a mineral that freezes water, and a hormone that stores fat do not describe the nature or function of restriction enzymes. Thus A is the only scientifically correct option.

Questions on restriction enzymes may ask source sequence and sticky or blunt ends. Revise them together.

The student’s statement describes random cleavage, but restriction enzymes do not normally cut DNA at arbitrary positions. Their catalytic sites recognize particular DNA recognition sequences, frequently short palindromic sequences, and cleave the backbone at a defined location within or close to that sequence. The exact result may be sticky ends or blunt ends, depending on the enzyme. Therefore option B correctly corrects the misconception. Option A is wrong because these enzymes act on DNA, while C confuses nucleotide sequences with protein sequences and D refers to a cellular structure rather than a DNA substrate.

A restriction enzyme cuts DNA at a selected restriction site. This can be a starting step in cloning.

The direct answer is A: cutting both pieces with the same enzyme makes compatible ends. A restriction enzyme recognizes a particular DNA sequence. If the vector and insert are cut with the same enzyme, the cuts can expose complementary single-stranded overhangs on both pieces. These ends can base-pair, bringing the insert into the vector in a suitable position. DNA ligase then joins the sugar-phosphate backbones. Option A is correct because compatibility is the practical reason for using the same enzyme. Option B is wrong: DNA is not converted into water. Option C is wrong: cutting does not change the colour of DNA in the relevant cloning process. Option D is wrong: restriction digestion cuts DNA but does not generally make both molecules biologically inactive. Also, the same enzyme may sometimes cut at more than one site, so the exact fragments depend on the map; nevertheless, the stated main reason remains compatible ends. Memory cue: same enzyme gives matching ends; ligase completes the join.

DNA ligase and compatible ends are needed to join cut ends. Restriction enzyme only performs cutting.

DNA ligase forms phosphodiester bonds between the ends of DNA fragments cut by restriction enzymes, thereby joining the desired gene to vector DNA. DNA polymerase synthesises a new DNA strand but does not join cut DNA fragments. Exam tip: remember restriction enzymes as “cutting” enzymes and DNA ligase as the “joining” enzyme in cloning.

A blunt end is formed when a restriction enzyme cuts both DNA strands directly opposite each other at the same position. Therefore, no single-stranded overhang, or sticky end, is produced. In options B and C, the strands are cut at different positions, so an overhang is formed; these represent sticky ends. Exam tip: identify a blunt end by checking whether the two cuts are flush and directly opposite each other.

A sticky end has a single stranded overhang. This overhang can pair with a complementary sequence.

EcoRI is a restriction endonuclease that cuts both DNA strands at its specific recognition sequence, usually producing sticky ends. DNA ligase joins broken DNA fragments by forming phosphodiester bonds between them. Option B reverses the functions of the two enzymes. Exam tip: remember restriction enzymes as “cutters” and ligase as a “joiner.”

Specificity depends on which DNA sequence the enzyme recognizes. That is its recognition sequence.

A restriction enzyme requires its specific recognition sequence as a substrate. If that sequence is absent from the DNA molecule, the enzyme cannot bind at the expected position and therefore cannot produce the expected cleavage pattern. Double-stranded DNA is normally the appropriate substrate for many commonly used restriction enzymes, and a suitable reaction buffer supports enzyme activity. An active enzyme is also more likely to cut when its recognition site and required conditions are present. Hence option A is correct. The other choices describe conditions that do not prevent the expected cut in the stated context.

Direct answer: A, some sites may not cut. Restriction analysis depends on an enzyme cutting every accessible copy of its specific recognition sequence. Methylation changes the chemical pattern of DNA and can block recognition or cleavage by a methylation-sensitive restriction enzyme. Step by step: the enzyme is added, it searches for its target sequence, a methylated target is encountered, cleavage may fail, and the DNA fragments produced differ from the expected fragments. On a gel, this can give different band sizes or numbers, so interpretation becomes complicated. Option A is correct. Option B is impossible because DNA does not become sugar. Option C is wrong because ordinary methylation does not make the gel melt. Option D is wrong because an enzyme remains a protein catalyst and does not become a vitamin. Exam cue: unexpected restriction bands may result from methylation as well as from sequence changes.

Restriction enzyme names can relate to source genus species and strain. EcoRI is a common example.

The Roman numeral can show isolation order. In EcoRI I indicates the first enzyme from that source.

HindIII and EcoRI are both DNA-cutting restriction enzymes. Their recognition sequences are different.

HindIII is a restriction endonuclease that recognizes and cuts the palindromic sequence 5′-AAGCTT-3′. EcoRI, a common close distractor, recognizes GAATTC, not AAGCTT. Exam tip: Memorize major restriction enzymes as enzyme–recognition-sequence pairs.

BamHI is correct. This restriction endonuclease recognises the palindromic double-stranded DNA sequence 5′-GGATCC-3′ and cleaves it to form sticky ends. EcoRI is the closest distractor, but it recognises 5′-GAATTC-3′. SmaI and AluI recognise different specific DNA sequences. Exam tip: Learn common restriction enzymes along with their recognition sequences.

HaeIII recognizes the sequence GGCC and cuts it between GG|CC. This produces blunt ends. BamHI may look similar, but its recognition sequence is GGATCC. Exam tip: Memorize common restriction enzymes along with their recognition sequences and whether they produce sticky or blunt ends.

A restriction enzyme cuts DNA at its specific recognition sites. Therefore, when DNA has more such sites, more cuts can occur and generally more restriction fragments are produced. A single cut gives fewer fragments, while no recognition site or no enzyme treatment results in no cutting. Exam tip: More recognition sites usually mean more DNA cuts and more fragments.

A restriction enzyme cuts DNA only when it finds its specific recognition sequence, called a restriction site. If a mutation changes that sequence, the enzyme may no longer recognize it. The cut that normally occurred at that position is then absent. As a result, two neighboring fragments may remain joined and form one larger fragment. Therefore, option A, “one cut may be lost,” is correct. The word “may” is important because the exact effect depends on whether the mutation truly destroys recognition and on the enzyme’s sequence requirements.

For example, if a DNA molecule normally has three sites for an enzyme, it may produce four fragments when the molecule is linear. Removing one site can reduce the number of cuts to two and may produce three fragments, with one fragment larger than before. The mutation does not make the enzyme double, convert DNA into RNA, or determine a green gel color. Thus, the supplied answer correctly describes the likely change in the fragment pattern.

If a mutation creates a new sequence recognised by a restriction enzyme, the enzyme can cut the DNA at this additional site. This may produce an additional, usually smaller, DNA fragment and alter the band pattern on gel electrophoresis. Option B is incorrect because an extra cut can increase, not eliminate, the number of DNA fragments. Exam tip: a newly formed restriction site generally means an additional cleavage point.

In RFLP differences in lengths of DNA fragments made by restriction enzymes are observed. It is useful in DNA analysis.

The pattern of restriction fragments can differ among individuals. Thus comparison can help in identification.