The blood brain barrier (BBB) is formed from processes of manyMany substances cannot enter the BBB due to the.

The blood-brain barrier (BBB) is formed from processes of many endothelial cells. Many substances cannot enter the BBB due to the tight junctions and the high passage selectivity of these cells.

Explanation:

A researcher named Paul Ehrlich would introduce a dye into an animal's blood supply and later observe that the only point that was not stained was the central nervous system, and specifically the brain. The reason for this had to be related to a protection system that surrounded that area as if it were a membrane.

Later, another researcher, Edwin Goldman, would try the reverse process by staining the cerebrospinal fluid, observing that the only colored parts corresponded to the nervous tissue. These experiments reflect the existence of something that produces a high level of blockage between the nervous system and the rest of the organism, something that years later would be called the blood-brain barrier by Lewandowski and explored by a large number of experts.

step 1: initiation

initiation is the beginning of transcription. it occurs when the enzyme rna polymerase binds to a region of a gene called the promoter. this signals the dna to unwind so the enzyme can ‘‘read’’ the bases in one of the dna strands. the enzyme is now ready to make a strand of mrna with a complementary sequence of bases.

step 2: elongation

elongation is the addition of nucleotides to the mrna strand. rna polymerase reads the unwound dna strand and builds the mrna molecule, using complementary base pairs. there is a brief time during this process when the newly formed rna is bound to the unwound dna. during this process, an adenine (a) in the dna binds to an uracil (u) in the rna.

step 3: termination

termination is the ending of transcription, and occurs when rna polymerase crosses a stop (termination) sequence in the gene. the mrna strand is complete, and it detaches from dna.