29 public questions tagged with this topic.
Transformation is the process of introducing recombinant DNA into a host cell for cloning.
Ref: NCERT Class 12 Biology Chapter 10: Biotechnology and its Applications Medical Applications - Molecular Diagnosis - PCR ELISA
A multiple cloning site (MCS) contains recognition sites for several restriction enzymes, making it easier to insert foreign DNA.
Ref: NCERT Class 12 Biology Chapter 9: Biotechnology: Principles and Processes Tools - Restriction Enzymes - Molecular Scissors
Inverse PCR enables amplification of sequences flanking a known region without prior knowledge of adjacent DNA. Genomic DNA is digested with restriction enzymes that cut outside the known segment, then circularized via dilute ligation favoring intramolecular joining. Primers designed in outward orientation relative to the conventional arrangement, facing away from the known core, now point toward each other on the circular template and amplify the unknown surrounding regions. Nested, touchdown or colony PCR serve different purposes like increasing specificity, optimizing annealing or screening bacterial colonies and cannot recover flanking unknowns. Inverse PCR is thus vital for mapping insertions and promoters.
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.
In inverse PCR mutagenesis, reaction mixture contains parental methylated plasmid propagated in Dam positive bacteria and newly synthesized unmethylated mutated copies produced by high-fidelity polymerase in vitro. To enrich mutants, mixture is incubated with DpnI restriction enzyme which recognizes methylated GATC sites present only in parental template, introducing double-strand breaks and fragmenting it. This selective digestion leaves unmethylated mutated amplicons intact, drastically lowering wild-type background upon transformation. Treatment therefore digests template DNA while retaining mutated copies, rather than cutting all DNA or degrading RNA.
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.
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.
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.
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.
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.
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.
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.
Restriction enzymes generating same overhang sequence can cross-ligate despite different recognition sites. BamHI cleaves 5'-G^GATCC-3' leaving 5' protruding GATC, while BglII cleaves 5'-A^GATCT-3' leaving identical GATC tetranucleotide. Because Watson-Crick pairing depends solely on overhang sequence, fragments produced by these enzymes anneal efficiently and are sealable by DNA ligase. The ligation product creates a hybrid sequence, either GGATCT or AGATCC, that is generally not recognized by either parental enzyme, a feature exploited to prevent re-digestion while enabling cloning when BamHI site is unavailable.
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.
Alkaline phosphatase hydrolyzes 5'-phosphomonoester bonds, converting 5'-phosphate termini of linearized plasmid DNA into 5'-hydroxyl groups. DNA ligase strictly requires a 5'-phosphate and adjacent 3'-hydroxyl to catalyze phosphodiester bond formation, so dephosphorylated vector molecules cannot self-circularize, thereby suppressing empty vector background. In cloning, the foreign insert retains chemically intact 5'-phosphates supplied by restriction digestion or polynucleotide kinase, enabling ligation at each junction to form a heteroduplex with two nicks that are repaired after transformation by bacterial repair machinery, greatly improving recombinant recovery.
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.
