Restriction endonucleases were discovered in the early 1970s and revolutionised molecular biology. Werner Arber, Hamilton Smith, and Daniel Nathans were awarded the Nobel Prize in Physiology or Medicine in 1978 for their discovery and application of restriction enzymes. The name "restriction" comes from the original observation that bacteria could "restrict" (limit) the growth of bacteriophages — they cut the incoming phage DNA at specific sites, preventing viral infection. The bacterium's own DNA is protected by methylation of the same recognition sequences (restriction-modification system). Smith isolated the first restriction enzyme that cut DNA at a specific site. Nathans used restriction enzymes to create the first restriction map of a virus (SV40).

Restriction enzymes recognise specific short nucleotide sequences called recognition sequences or restriction sites. These sequences are palindromic — they read the same in both directions on the two strands (5'→3'). This is similar to a word palindrome like "MALAYALAM" or "RACECAR." For example, EcoRI recognises 5'-GAATTC-3' (reading the top strand 5'→3') and the complementary strand reads 3'-CTTAAG-5', which is also 5'-GAATTC-3' when read 5'→3'. The recognition sequences are typically 4-6 base pairs long, though some Type IIS enzymes have 8-bp recognition sequences. The frequency of occurrence of a 6-bp palindrome in a random DNA is approximately once every 4⁶ = 4096 bp.

Restriction enzymes produce two types of cuts. Staggered cuts (sticky/cohesive ends): enzymes cut the two strands at different positions within or near the palindrome, generating short single-stranded overhangs. These overhangs are called "sticky ends" because they can base-pair with complementary single-stranded sequences. EcoRI produces 5' sticky ends: 5'-G↓AATTC-3' → produces 5'-AATT-3' overhang. HindIII produces 5'-A↓AGCTT-3' → 5'-AGCT overhang. PstI produces 3' sticky ends (3' protruding). Blunt cuts: enzyme cuts both strands at exactly the same position within the palindrome. SmaI cuts 5'-CCC↓GGG-3' → produces blunt ends (no overhang). Blunt-end ligation requires more enzyme (T4 DNA ligase) and is less efficient than sticky-end ligation.

Restriction enzymes are classified into three main types based on their subunit structure, cofactor requirements, and cleavage position. Type I enzymes: large, multi-subunit complexes. Recognise specific sequences but cut DNA at random sites far from the recognition sequence. Require ATP, Mg²⁺, and S-adenosyl methionine. Not useful in genetic engineering due to random cutting. Type II enzymes: most widely used in molecular biology. Cut DNA at or near the specific recognition sequence. Require only Mg²⁺. Examples: EcoRI, HindIII, BamHI, PstI, SalI, SmaI. These are the "molecular scissors" used in recombinant DNA technology. Type III enzymes: cut DNA 24-26 bp downstream of the recognition sequence. Require ATP and Mg²⁺. Intermediate between Type I and Type II in their properties. Type IIS enzymes (subtype): cut DNA outside the recognition sequence. Useful in Golden Gate Assembly cloning.

This distinction is essential for answering this question. Endonucleases cleave phosphodiester bonds within the DNA molecule (endo = within/inside). They can cut at internal positions. Restriction endonucleases are a specific type of endonucleases that cut at specific recognition sequences. Other endonucleases: DNase I (non-specific, used to make random cuts), S1 nuclease (cuts single-stranded DNA/RNA), RNase H (cuts RNA in RNA:DNA hybrid). Exonucleases remove nucleotides one at a time from the ends (exo = outside) of a DNA molecule. They require a free end to start degradation. Examples: Exonuclease III (removes nucleotides from 3' ends), Lambda exonuclease (removes nucleotides from 5' phosphorylated ends), Bal31 (has both 3'→5' and 5'→3' exonuclease activities). Statement D incorrectly describes exonuclease activity as belonging to restriction endonucleases — this is clearly wrong.

Restriction enzymes are central to recombinant DNA technology. The process involves: (1) Isolation of DNA from source organism (human insulin gene, for example). (2) Cutting DNA with the same restriction enzyme from both the gene source and the vector (plasmid). Both are cut with the same enzyme so they have complementary sticky ends. (3) Mixing the cut gene with the cut vector → hydrogen bonding between complementary sticky ends → annealing. (4) Sealing with DNA ligase (T4 DNA ligase) → forms recombinant DNA (rDNA). (5) Introducing rDNA into host cell (transformation for bacteria, transfection for eukaryotes). (6) Selecting transformed cells → culturing → gene expression → protein production. This technology has produced human insulin (first approved genetically engineered medicine, 1982), human growth hormone, Factor VIII, erythropoietin, and many other medical proteins.

Restriction mapping: digesting DNA with one or more restriction enzymes and determining the sizes of resulting fragments using gel electrophoresis. Creates a physical map showing positions of restriction enzyme sites relative to each other. Agarose gel electrophoresis: DNA fragments are negatively charged → migrate toward positive electrode. Smaller fragments move faster → further distance. Visualised with ethidium bromide (intercalates between base pairs, fluoresces under UV light). Bands represent fragments of specific sizes. DNA ladder (molecular weight marker) run alongside allows size estimation. Southern blotting: transfer DNA from gel to nitrocellulose membrane → probe with labelled complementary DNA → detect specific fragments. Uses: DNA fingerprinting, RFLP analysis, genetic diagnosis. Restriction Fragment Length Polymorphism (RFLP): variation in restriction sites between individuals → used in forensics, paternity testing, genetic mapping.