Question

1- How is mRNA made from DNA? What enzyme catalyzes this reaction?

2- How does the enzyme know where to start and stop transcription?

Answer 1

1).

mRNA is made from DNA template strand during transcription. First the two DNA strands are separated and the tepmplate strand which runs from 3'-5' produces mRNA according to base pair rules as follows.

A=U

G=C

RNA polymerase is the enzyme that catalyzes the synthesis of mRNA from DNA.

2).

RNA polymerase enzyme recognizes an initiation site (TAC codon in DNA), a promoter sequence (usually containing a "TATA" box) in the DNA. Elongation of the RNA continues until a termination sequence( UAG or UGA or UAA codons) has been transcribed. Then transcription is teminated and the entire RNA polymer is freed from the DNA.