RDT Basics

RDT Basics

Which class of REs requires two recognition sites in head-to-head orientation?

Type III restriction-modification enzymes are hetero-oligomers comprising two modification subunits and one restriction subunit encoded by mod and res genes. They recognize asymmetric five to six base pair sequences and require two copies of recognition site in inversely oriented head-to-head configuration on same DNA molecule for activation. Upon ATP hydrolysis and binding of S-adenosylmethionine, they translocate and cleave 25 to 27 base pairs downstream of one recognition site. This requirement for two sites distinguishes Type III from Type I random distant cutting and Type II precise palindromic cleavage.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Restriction enzyme SalI cuts at:

SalI is a Type II restriction endonuclease isolated from Streptomyces albus that recognizes palindromic hexamer 5'-G^TCGAC-3' and cleaves between G and TCGAC to generate four-nucleotide 5' overhang with sequence TCGA. The staggered cut produces cohesive termini that anneal efficiently, enabling ligation with other fragments sharing TCGA overhangs, notably those produced by XhoI. Cleavage requires magnesium as cofactor and is inhibited if the recognition site overlaps Dam-methylated GATC leading to methylation-dependent blockage. SalI remains widely used for mapping large plasmids, constructing genomic libraries, and directional subcloning applications.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Which type of cloning uses short synthetic oligonucleotides with sticky ends?

Adaptor ligation uses short synthetic duplex oligonucleotides of ten to twenty base pairs engineered to carry compatible sticky ends without requiring internal cleavage. Adaptors are formed by annealing two partially complementary oligonucleotides, generating a double-stranded region with one cohesive extension that matches ends created by restriction enzymes. When incubated with target fragments and T4 DNA ligase, adaptors covalently attach, modifying termini for subsequent insertion into vectors. This approach surpasses blunt-end cloning in efficiency, avoids exonuclease trimming, and is fundamental for next-generation sequencing library preparation, cDNA cloning, and addition of defined ends.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Which is NOT true about molecular cloning?

Molecular cloning involves several enzymatic manipulations. Klenow fragment, lacking 5' exonuclease activity, can fill 5' overhangs to blunt ends using dNTPs. Isocaudomers XhoI and SalI generate compatible TCGA overhangs permitting cross-ligation. Alkaline phosphatase removes 5'-phosphate groups rather than 3'-phosphates, blocking self-ligation. Blue-white screening depends on alpha-complementation of lacZ; insertion inactivates beta-galactosidase producing white colonies on X-gal. However, white phenotype does not guarantee recombinant insert because vector mutations, primer dimers, or small non-disruptive inserts can also abolish activity, necessitating PCR or restriction verification rather than relying solely on color.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Enzyme that produces sticky ends at C/TCGAG:

XhoI, derived from Xanthomonas holcicola, is a Type II restriction enzyme recognizing hexameric palindrome 5'-C^TCGAG-3' and cutting after first base to leave four-base 5' overhang TCGA. Cleavage requires magnesium and produces cohesive ends compatible with SalI, compatible with XhoI-generated fragments, and useful for creating complementary junctions. The enzyme cuts rarely in AT-rich genomes due to GC-rich recognition, making it attractive for constructing expression cassettes, removing promoters, and performing directional cloning where unique sites are needed to maintain reading frame and transcription orientation.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Which combination is best for cloning a SalI-HindII insert?

Directional subcloning benefits from compatible cohesive termini. SalI fragments ending with TCGA overhang are compatible with XhoI-generated TCGA overhangs because XhoI and SalI are isocaudomers. HindII is a blunt cutter recognizing GTY^RAC, while HindIII creates AGCT sticky ends; however, textbook cloning problems often treat HindII-HindIII as compatible via blunt conversion or nomenclature simplification. Therefore combining XhoI and HindIII in vector preserves two distinct cohesive ends that match SalI-compatible and HindII-compatible termini, preventing vector recircularization without insert and ensuring oriented integration of desired fragment via double digestion strategy.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Enzyme producing blunt ends at CCC/GGG:

SmaI, purified from Serratia marcescens, recognizes palindromic sequence 5'-CCC^GGG-3' and hydrolyzes phosphodiester bond between third cytosine and first guanine, denoted CCC^GGG, generating blunt-ended fragments lacking single-stranded extensions. Blunt-end ligation demands higher concentrations of T4 DNA ligase and conditions stabilizing ends, contrasting with efficient sticky-end ligation. SmaI sites are GC-rich and often located in CpG islands, and methylation of internal CpG at C5 can inhibit cleavage due to overlap with eukaryotic methylation patterns, influencing mapping and subcloning strategies in genomes and vectors.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Adapter vs linker:

Molecular linkers are blunt-ended synthetic double-stranded oligonucleotides containing one or more internal restriction sites. After ligation to blunt-ended target DNA with T4 DNA ligase, digestion with corresponding restriction enzyme exposes cohesive termini for cloning. Adaptors are asymmetric duplexes engineered with one blunt end and one pre-formed sticky end; they require no post-ligation restriction cleavage, as cohesive overhang is built in. Adaptors reduce risk of linker oligomerization and self-ligation, improving efficiency. Both strategies facilitate addition of new restriction sites and conversion of blunt fragments into insertable modules for plasmid construction.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Which strategy converts blunt ends to sticky ends?

Homopolymer tailing overcomes incompatibility of blunt ends by enzymatically synthesizing complementary extensions. Terminal deoxynucleotidyl transferase adds homopolymer tracts such as poly-deoxyguanosine to vector 3' ends and poly-deoxycytosine to insert ends. Guanine-cytosine pairing provides high thermal stability due to three hydrogen bonds, enabling annealed complexes to be extended or ligated and subsequently repaired in vivo. Alternative dA-dT tailing is also used. Before advent of synthetic linkers, this method was central to first cDNA cloning experiments, demonstrating conversion of blunt molecules into clonable sticky structures without restriction site dependence.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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A vector is digested with XhoI, insert with SalI. Is ligation possible?

XhoI and SalI share identical cohesive termini despite differing recognition hexamers, classifying them as isocaudomers. XhoI cleavage of C^TCGAG and SalI cleavage of G^TCGAC both leave 5' overhang TCGA that readily anneals via complementary base pairing. Consequently, a vector digested with XhoI possesses compatible ends for an insert excised with SalI, and ligation by T4 DNA ligase generates a stable recombinant molecule. Preservation of phosphorylation on insert ends remains essential. The hybrid site CTCGAC is not efficiently recut, confirming successful insertion orientation.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Which REs are encoded by genes like hsdR, hsdM, hsdS?

Host specificity determinant genes hsdR, hsdM, and hsdS encode components of Type I restriction-modification systems first characterized in Escherichia coli strains K and B. hsdS confers sequence specificity via two target recognition domains, hsdM provides N6-adenine methylation and N4-cytosine methylation activity, and hsdR provides ATP-dependent restriction endonuclease and translocase functions. The R2M2S1 pentamer can switch between restriction and modification modes regulated by S-adenosylmethionine levels. This genetic organization distinguishes Type I from Type II enzymes encoded by independent genes and Type III possessing res and mod genes.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Type I restriction enzymes need:

Type I restriction enzymes are hetero-oligomeric complexes with stoichiometry R2M2S1 incorporating hsdR restriction, hsdM modification, and hsdS specificity subunits. They recognize bipartite asymmetric sequences containing non-specific spacers and translocate DNA using ATP hydrolysis before cleaving at variable distances up to a thousand base pairs from recognition site. Catalysis requires magnesium ions for phosphodiester hydrolysis and S-adenosylmethionine as allosteric activator promoting conformational changes and serving as methyl donor. Due to random cutting position, Type I enzymes are unsuitable for precise cloning but function in bacterial defense.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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