Staggered cuts by restriction enzymes create ends with short single-stranded overhangs. These overhangs are sticky because they can pair with complementary overhangs on another DNA fragment, guiding correct base-pairing before the backbone is sealed by ligase. This cohesive pairing makes joining fragments efficient and directional, which is why these ends are called sticky ends. Blunt ends, in contrast, have no overhangs and must be ligated without the guiding base-pairing, making the process less efficient and less specific. The term double-stranded ends isn’t the standard way to describe the outcome of staggered cuts, and circular ends refer to circular DNA rather than the overhangs created by staggered cuts.

To study promoter regions across the genome, you need access to the DNA sequences that make up the genome, including regulatory elements upstream of genes. A genomic DNA library is built from fragments of the entire genome and cloned into vectors, so it represents all genomic regions, including promoters, enhancers, introns, and intergenic sequences. This makes it the best resource for examining promoter architecture on a genome-wide scale. In contrast, a cDNA library comes from expressed mRNA and reflects only the transcribed portions of the genome, lacking promoters and other noncoding regulatory regions. A protein library contains expressed proteins, not DNA sequences, and an RNA interference library is designed to perturb gene expression rather than survey genomic sequences. Therefore, the genomic library best fits the goal.

The concept here is sticky ends. Some restriction enzymes cut DNA in a staggered way, leaving short single-stranded overhangs called sticky ends. When the same enzyme cuts both the human DNA fragment and the plasmid, the overhangs they generate have complementary sequences. Those complementary overhangs can base-pair with each other, aligning the fragments precisely. Once aligned, DNA ligase seals the sugar-phosphate backbone to form a continuous, recombinant DNA molecule. This base-pairing guidance makes the joining efficient and specific. If the ends were blunt, there would be no such base-pairing guidance, making ligation much less efficient.

Overlapping sequences provide homologous ends that guide the joining of DNA fragments in Gibson assembly. An exonuclease chews back the fragment ends to reveal single-stranded overlaps that are complementary between pieces. These overlaps let the fragments anneal to one another, creating a continuous template. A DNA polymerase fills in any gaps, and a DNA ligase seals the remaining nicks, all in one isothermal reaction. This seamless junction formation is the core reason overlaps are essential in Gibson assembly. It’s not about restriction enzymes cutting anything, and the overlaps are actively used in the assembly rather than being ignored.