Chromosome walking is a method used to clone in an orderly fashion the DNA segments along the chromosome starting at any point for which we have a probe. To build up a series of overlapping cloned DNA fragments, it begins with one clone from a library, and then identifies a second clone whose insert overlaps with the insert in the first clone. This was the first method, called chromosome walking, devised for assembly of clone contigs. But there is a limitation to the speed of chromosome walking and that is because of the small size of the fragments that are to be cloned, another limitation is the difficulty of walking through the repeated sequence that are scattered through the gene. A more straightforward approach thus is to use the insert DNA from the starting clone as a hybridization probe to screen all the other clones in the library. Positive hybridization signals that are given by clones, whose inserts overlap with the probe, are used as new probes to continue the walk. There are about 96 clones that a library consists of and each clone contains a different insert. The insert from one of the selected clones is then used as a hybridization probe with all other clones in the library.
A probe may have a genome wide repetition of sequencesand this is a problem that is faced during the walk as then it will not only hybridize to overlapping clones but will also to non overlapping clone whose inserts also contain copies of repeat. This unwanted hybridization can be reduced by blocking the repeat sequence with pre-hybridization with unlabeled genomic DNA. But this isn’t that affective solution especially in the case when high capacity vectors such as BACs or YACs are used in the walk. (more…)
People require sinus endoscopy or sinoscopy when they have complex nasal problems which cannot be treated using basic medication. Professionals who operate on those who have sinus problems are called otolaryngologists and their expertise is mainly to deal with nasal, throat and ear problems.