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So, how is mRNA made?
Messenger RNA is synthesized in the cell nucleus by transcription of DNA, a process similar to DNA replication. As in replication, a small section of the DNA double helix unwinds, and the bases on the two strands are exposed. RNA nucleotides (ribonucleotides) line up in the proper order by hydrogen-bonding to their complementary bases on DNA, the nucleotides are joined together by a DNA dependent RNA polymerase enzyme, and mRNA results.
UNLIKE what happens in DNA replication where both strands are copied, only ONE of the two DNA strands is transcribed into mRNA (remember that RNA is a single-stranded molecule). The DNA strand that is transcribed is called the template strand (also known as the antisense strand), while its complement is called the informational strand (also called the coding or sense strand). Since the template strand and the informational strand are complementary, and since the template strand and the mRNA molecule are also complementary, it follows that the messenger RNA molecule produced during transcription is a copy of the DNA informational strand!
But how do the polymerase and helicase enzymes know where to begin? In other words, where does one gene start and stop and the next one begin? The starting point of a gene is marked by a certain base sequence which is called a promoter site. These sites are recognized by a factor called "SIGMA". It is sigma's job to recognize the promoter sites and "tell" the DNA dependent RNA polymerase where to begin transcription. Once the RNA polymerase has been directed to the start point of the gene by sigma, the sigma factor is released and the RNA polymerase carries out the process of transcription.
These promoter sites act as a "start sign" . Similarly, there are other base sequences at the end of a gene that signal a to mRNA synthesis. Just as there is a sigma factor to help signal the beginning of a gene, another factor called "RHO" aids in terminating the process of transcription. When the end of the gene is near, the rho factor binds to the mRNA (that's right, the mRNA, NOT the DNA) and interacts with the RNA polymerase. The interaction of rho with the RNA polymerase causes the enzyme to "fall off" the DNA template strand, thus stopping transcription!
CLICK HERE TO SEE ANIMATION OF THE PROCESS OF TRANSCRIPTION!
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