Restriction Enzymes

HaeIII is a restriction endonuclease whose recognition sequence is 5′-GGCC-3′, with the complementary strand also carrying GGCC in the corresponding orientation. It cuts symmetrically in the recognition site, so the two DNA strands end at the same position and no single-stranded overhang is left; the result is a blunt end. Thus option A is correct. EcoRI, BamHI and PstI are also restriction enzymes, but they are commonly associated with staggered cleavage and sticky ends rather than this specific GGCC blunt-end example. The question tests recognition of a named enzyme and its cleavage pattern.

A neoschizomer is a pair or group of restriction enzymes that recognize the same DNA recognition sequence but cleave at different positions within or around that sequence. Because the cleavage positions differ, the enzymes can generate different end structures, even though their binding specificity is the same. Option A therefore states the defining concept precisely. This term should not be confused with enzymes that synthesize DNA, change colour or store fat. Some neoschizomer pairs may produce different sticky ends, while others can be compared with an enzyme producing a blunt end; the essential criterion is identical recognition and different cleavage position.

A DNA ladder, also called a molecular-size marker, contains DNA fragments of known lengths. During agarose-gel electrophoresis, DNA molecules move through the gel according to size: smaller fragments generally travel farther than larger fragments. The bands produced by restriction digestion are run beside the ladder, and their positions are compared with marker bands to estimate fragment sizes, usually in base pairs. Therefore option A is correct. Chlorophyll colour, soil particles and protein taste provide no calibrated DNA-length reference and cannot be used to interpret restriction fragments on a gel. The ladder is a comparison standard, not an enzyme or a digestion product.

When a plasmid sample is treated with a restriction enzyme, complete digestion at the relevant site should convert every suitable plasmid molecule into the expected cut form. If a band corresponding to uncut plasmid remains, at least some molecules were not cleaved. This may indicate incomplete digestion caused by insufficient enzyme, unsuitable buffer, incorrect incubation conditions, too little time, or a missing recognition site in that plasmid preparation. Thus option A is the best interpretation. The observation does not demonstrate photosynthesis, RNA translation or protein digestion; those processes are unrelated to restriction analysis. Additional controls can help distinguish incomplete cutting from different plasmid conformations.

For a circular DNA molecule, the number of fragments produced by complete digestion generally equals the number of distinct cut positions, provided the cuts are at different sites. Here there are two different restriction sites and both are cleaved. The two cuts divide the circular DNA ring into two separate arcs, so two DNA fragments form. Option A is therefore correct. The one-fragment answer would describe an uncut circular molecule or an inappropriate assumption, while zero and five have no basis in the stated number of cuts. This counting rule is useful in restriction mapping, although repeated sites or incomplete digestion would change the observed pattern.

An overhang is the short single-stranded extension left when a restriction enzyme cuts the two DNA strands at staggered positions. Such an extension is characteristic of a sticky or cohesive end. If two DNA fragments have complementary overhangs, their bases can pair temporarily through hydrogen bonds, bringing the fragments into alignment; DNA ligase can then seal the sugar-phosphate backbone. Therefore option A is correct. A blunt-end cut does not leave a single-stranded extension, and the other options are unrelated biological structures or materials. The term describes DNA-end structure, not a separate molecule or enzyme.

The governing concept is that restriction enzymes cut DNA only at their specific recognition sites. If a mutation changes the nucleotide sequence so that a new recognition site appears, the corresponding enzyme can cut at this additional position. For a linear DNA molecule, one extra internal cut generally increases the number of fragments by one; for circular DNA, the new cut can alter the fragment count and sizes in the resulting pattern. Thus option A is correct. The mutation does not convert DNA into water, change an enzyme into a vitamin, or transform gel into paper. In practice, such a site change can be detected as a different restriction-fragment pattern on a gel.

The governing concept is restriction-enzyme fidelity. Under unsuitable reaction conditions, such as an incorrect pH or salt concentration, excessive glycerol or enzyme concentration, prolonged incubation, or an unsuitable buffer, some restriction enzymes may lose their normal sequence specificity. They can then cut at sites that resemble, but are not identical to, the canonical recognition sequence. This relaxed, non-specific cleavage is called star activity, so option A is correct. Photosynthesis is light-driven carbohydrate production, osmosis is movement of water across a membrane, and translation is protein synthesis; none describes inaccurate restriction digestion caused by poor reaction conditions.

The governing concept is the definition of isoschizomers in molecular biology. Isoschizomers are different restriction enzymes, often obtained from different organisms, that recognize the same nucleotide sequence. Depending on the enzyme pair, they may cut at the same position or may generate different ends if their cleavage positions within that shared recognition sequence differ; therefore identical cutting behaviour should not automatically be assumed. The essential expected feature is shared sequence recognition, making option A correct. Colour production, vitamin identity, and being unrelated to DNA have no connection with the defining property of restriction enzymes.

Restriction-modification systems protect bacterial DNA through a paired mechanism. The restriction enzyme recognizes a particular sequence and can cleave unprotected foreign DNA, whereas the modification enzyme adds methyl groups to selected bases in the same recognition sequence of the host genome. This methylation changes the recognition site sufficiently to prevent or reduce cleavage by the corresponding restriction enzyme. Therefore option C is correct. Modification does not mean burning DNA, translating it into protein, or washing an enzyme. The methyl mark is a chemical protection signal, not a change in the DNA sequence itself.

The governing concept is restriction mapping: fragment length is determined by the distance between successive cleavage sites on the DNA molecule. If a linear DNA molecule is cut at positions measured along its length, each interval between adjacent sites becomes one fragment; the fragment sizes therefore add up to the length of the original molecule. For circular DNA, the cuts similarly define segments around the circle, with the number and arrangement of sites determining the products. Option B is correct. Enzyme taste, water smell, and tube material do not determine sequence-specific cleavage distances under a controlled reaction. Reaction conditions can affect whether digestion is complete, but they do not replace the basic rule that the locations of recognition and cut sites establish fragment size.

Restriction mapping infers the arrangement of restriction sites from the sizes and number of DNA fragments observed after digestion. The method therefore requires the enzyme to cut every accessible copy of each target site under the selected conditions. If digestion is partial, some molecules are cut at only a subset of sites, producing larger intermediate fragments in addition to the true complete-digest fragments. These extra bands can be mistaken for genuine map products and lead to an incorrect estimate of site order or distance. Thus option A is correct. DNA is not burned, gel electrophoresis is normally essential for observing sizes, and the protein nature of the enzyme does not explain the requirement. Appropriate enzyme amount, incubation, buffer, and DNA quality help obtain complete digestion.