Athin extracellular layer upon which an epithelial rests is called"

Basement membrane

A thin extracellular layer upon which an epithelial rests is called a basement membrane.

Basement membrane is a specialised form of extracellular matrix on which a layer of epithelial cells rests . The basement membrane is made up of a mixture of collagens, and glycoproteins such as perlecan, heparan sulphate and entactin. The basement membrane supports the overlying epithelium by preventing any association between the epithelial cells and the other cells present within the tissue. The basement membrane also functions as a molecular filter as it permit only water and tiny molecules to move through.

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.