Restriction Enzymes

Restriction enzymes are bacteria-linked specific DNA cutters. Remember them with recombinant DNA and cloning.

A unique restriction site is a single recognition site for a particular restriction enzyme within the vector. Cutting at this one defined position opens the vector in a predictable way and provides a suitable location for inserting foreign DNA. If the same enzyme cut at several sites in the vector, it could fragment or disrupt essential vector elements such as the origin of replication or selectable marker. Option A is correct because one unique site enables controlled opening and insertion. Host enlargement, gel cooling, and protein dissolution are not functions of restriction sites or cloning vectors.

The governing concept is the chemical structure of the DNA backbone. Nucleotides in one DNA strand are joined by phosphodiester bonds between the phosphate group of one nucleotide and the sugar of the next. A restriction endonuclease hydrolyses these backbone bonds at particular recognition sequences, producing defined DNA fragments with sticky or blunt ends. Therefore, option C is correct. Hydrogen bonds join complementary bases between the two strands, so they are not the primary bonds cut during restriction digestion. Peptide bonds belong to proteins, while glycosidic bonds connect a nitrogenous base to its sugar within one nucleotide.

BamHI is a type II restriction endonuclease, so its governing function is sequence-specific cleavage of double-stranded DNA. It recognizes the sequence GGATCC and cuts at a defined position, producing compatible sticky ends that can be useful in recombinant-DNA construction. Consequently, option A is correct. BamHI is not a chaperone that folds proteins, a ribosome that translates RNA, or a cellular organelle that stores lipids. In cloning, the enzyme is selected when its recognition site is suitably positioned in both the vector and the desired insert, while avoiding unwanted internal cuts.

Restriction enzymes are proteins whose activity depends on their correct three-dimensional structure and stable storage conditions. Prolonged exposure to room temperature can accelerate denaturation, aggregation, or loss of catalytic activity, especially for enzymes supplied frozen or requiring a cold chain. A weakened enzyme may cut DNA slowly or incompletely, causing unreliable digestion results. Thus option A is correct. Keeping the tube cold helps preserve activity, although the exact storage temperature must follow the manufacturer’s instructions. DNA does not form automatically, buffer cannot become protein, and enzyme storage does not strengthen an electrophoresis gel.

The governing laboratory principle is preservation of protein structure and enzymatic activity. Restriction enzymes are proteins, and many of them can lose activity when exposed for too long to room temperature, repeated warming, or unsuitable handling. Keeping the enzyme on ice during setup slows unwanted degradation and helps preserve its correct three-dimensional structure until the reaction begins. The reaction is later incubated at the enzyme’s specified working temperature, so cold storage is for handling and stability, not for performing digestion itself. Option A is therefore correct. DNA is not being frozen, buffer need not become solid, and agarose preparation is unrelated to restriction-enzyme handling.

The governing concept is sequence specificity of restriction enzymes. Each restriction enzyme has a particular three-dimensional active site that binds a defined recognition sequence, often a short palindromic DNA sequence. Only when the correct bases and local structure are present can the enzyme position the phosphodiester backbone properly and cleave at a characteristic site or distance. Consequently, DNA molecules are cut at predictable locations rather than at random positions. Being a protein does not prevent an enzyme from catalysing cleavage; in fact, the protein structure provides the catalytic activity. DNA does carry electrical charge, and water is a normal component of the reaction, so neither distractor explains the specificity.

The governing concept is the naming convention for restriction enzymes. In EcoRI, “Eco” refers to the bacterium Escherichia coli, while the following letter and Roman numeral provide information about the strain and the order in which enzymes from that strain were identified. The Roman numeral I therefore indicates that this was the first restriction enzyme discovered from the particular strain. It does not describe DNA length, a gel lane, or reaction temperature. Those properties may be recorded separately in an experiment, but they are not encoded by the Roman numeral in the enzyme name. Hence option A is correct.

The governing idea is the probability of finding a particular nucleotide sequence by chance. If bases are treated as approximately random, a specific sequence of length n has an expected frequency near 1 divided by 4 to the power n. Thus, a six-base recognition sequence has an approximate random frequency of 1/4⁶, whereas an eight-base sequence has 1/4⁸, which is much lower. Real genomes are not perfectly random, so the exact frequency varies with base composition and sequence bias, but the general trend remains: longer specific sites occur less often. Therefore option D is correct and such enzymes usually produce fewer cuts.

An enzyme unit is a standardized measure of catalytic activity under a defined assay condition, commonly related to the amount of enzyme needed to produce a specified extent of DNA cleavage in a specified time. Reaction planning uses this information to select enough units for the quantity and type of DNA present, while respecting the supplier’s recommendations and the buffer conditions. For example, a larger DNA mass or a more complex substrate may require proportionally more activity, although the exact requirement depends on the enzyme and protocol. Units do not describe smell, count gel pores, or directly read a DNA sequence. They estimate functional activity, making option B correct.