- Function: Absorption; secretion of mucus, enzymes, and other substances; ciliated type propels mucus (or reproductive cells) by ciliary action
Location: Non-ciliated type lines most of the digestive tract (Stomach to rectum), gallbladder, and excretory ducts of some glands; Ciliated variety lines small bronchi, uterine tubes and some regions of the uterus.
- Function: Secretion, particularly of mucus; propulsion of mucus by ciliary action
Location: non-ciliated type in male's sperm-carrying ducts and ducts of large glands; variety lines the trachea, most of the upper respiratory tract.
- Function: Protects underlying tissues in areas subjected to abrasion
Location: Nonkeratinized type forms the moist linings of the esophagus, mouth, and vagina; keratinized variety forms the epidermis of the skin, a dry membrane.
-Function: Stretches readily, permits stored urine to distend urinary organ
-Location: lines the ureters, bladder, and part of the urethra
When enhancers were initially found to influence transcription from many thousands of nucleotide pairs away from the promoters they control, two principal models were invoked to explain this action at a distance. In the “DNA looping” model, direct interactions between proteins bound at enhancers and promoters were proposed to stimulate transcription initiation. In the “scanning” or “entry-site” model, RNA polymerase (or another component of the transcription machinery) was proposed to bind at the enhancer and then scan along the DNA until it reached the promoter. These two models were tested using an enhancer on one piece of DNA and a B-globin gene and promoter on a separate piece of DNA (Figure Q8–9). The B-globin gene was not expressed when these two separate pieces of DNA were introduced together. However, when the two segments of DNA were joined via a linker (made of a protein that binds to a small molecule called biotin), the B-globin gene was expressed.
Does this experiment distinguish between the DNA looping model and the scanning model? Explain your answer.