DNA strands run in 1 of 2 directions depending on which end of the strand is free and which end has a phosphate bond. One direction is 5' to 3' () and the other direction is 3' to 5' (downstream). polymerase binds to the DNA at the TATA box and unzips the double helix. Working along the DNA chain, RNA polymerase reads the DNA nitrogenous bases and matches RNA nitrogenous bases from the nucleus (uracil instead of thymine). As it moves, RNA polymerase re-zips the DNA behind it and lets the mRNA separate from the DNA. It continues to do this until it reaches a sequence downstream called a signal. In other words, RNA polymerase copies the DNA sequence downstream of the TATA box into an mRNA strand.

The answers in the blank spaces:

1. Upstream

2. RNA

3. Terminator

Explanation:

This question is describing the process of transcription, which is the process whereby DNA template is used to synthesize an mRNA molecule. Transcription involves three stages viz: initiation, elongation and termination.

- DNA strands run in 1 of 2 directions depending on which end of the strand is free and which end has a phosphate bond. One direction is 5' to 3' (UPSTREAM) and the other direction is 3' to 5' (downstream). An enzyme called RNA polymerase binds to the DNA at the TATA box and unzips the double helix (INITIATION).

- Working along the DNA chain, RNA polymerase reads the DNA nitrogenous bases and matches RNA nitrogenous bases from the nucleus (uracil instead of thymine). As it moves, RNA polymerase re-zips the DNA behind it and lets the mRNA separate from the DNA (ELONGATION).

- It continues to do this until it reaches a sequence downstream called a TERMINATOR signal (TERMINATION). In other words, RNA polymerase copies the DNA sequence downstream of the TATA box into an mRNA strand.

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.

answer; the tunica intima also called tunica interna, consists of elastic fibers,connective tissue and smooth muscle cells;