7 public questions tagged with this topic.
Restriction enzymes are used to cut DNA at specific recognition sites, which is essential in genetic_x000D_ engineering and cloning.
Ref: NCERT Class 12 Biology Chapter 10: Biotechnology and its Applications Agricultural Applications - Nutritional Enhancement - Golden Rice and GM Crops
Partial digestion generates a nested set of subpopulations where some sites are cut and others remain intact. For a linear end-labeled molecule with three internal sites, cleavage can stop after the first site, after the second, after the third, or include all three, producing fragments that extend from the labeled end to each site successively. With n sites, the number of distinct labeled species equals n+1. For three sites, four labeled fragments of increasing length appear on autoradiography, while internal unlabeled fragments remain invisible, enabling ordered map 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.
In circular DNA, two restriction sites placed diametrically opposite divide the circle into two equal halves. After complete digestion, two fragments of identical molecular weight are produced. During agarose gel electrophoresis, migration rate depends on size, not sequence content. Fragments of same length co-migrate, overlapping as a single intensely stained band despite being two distinct physical pieces. Asymmetric placement would give two bands of different sizes. Recognizing symmetry explains why band number may appear less than fragment number and intensity doubles.
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.
Circular DNA lacks free ends, forming a continuous loop. A single restriction cut converts the circle into a linear molecule of full length. Each additional cut divides one existing fragment into two. Consequently, in complete digestion the number of fragments generated equals exactly the number of recognition sites cleaved. With three distinct sites, three separate fragments appear whose sizes sum to the plasmid length. This contrasts with linear DNA where fragments are n+1. Understanding circular topology helps interpret plasmid mapping gels correctly during cloning validation experiments.
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.
A linear molecule possesses two distinct termini. Each restriction cleavage introduces an additional break in the phosphodiester backbone. With one site, the molecule separates into two pieces; the second site cuts one of those pieces further. Under complete digestion every recognition sequence is hydrolyzed efficiently, so total fragments equal number of cuts plus one. For two sites this yields three fragments of sizes determined by inter-site distances. Circular DNA behaves differently, requiring equal numbers of cuts and fragments, but linearity dictates n+1 rule universally taught in NCERT genetics.
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 covalently closed circular DNA, each restriction cut increases fragment count by one when digestion goes to completion. A single site yields one linear band, two sites yield two bands, three sites yield three bands, provided no partial digestion occurs. Observing three separate fragments of 1.5, 2.0, and 0.5 kb that sum exactly to 4 kb indicates three cleavage events occurred around the circle. Therefore three recognition sites are distributed around plasmid. Mapping distances between them using fragment sizes allows reconstruction of circular restriction map important for vector engineering and insert localization.
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 heterozygous individual carrying one wild-type allele retaining all three restriction sites and one mutant allele lacking recognition site between regions 2 and 3, digestion produces two different fragment lengths spanning probe region due to allele-specific cleavage. Southern blotting with a probe hybridizing between site 2 and 3 detects both alleles because probe sequence exists in both chromosomes regardless of cleavage status. Homozygous wild type would show one smaller band, homozygous mutant one larger band. Heterozygote displays both bands simultaneously, illustrating co-dominance of RFLP markers used in carrier screening and linkage analysis.
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.
