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Transcription
The first step in transcription is the partial unwinding
of the DNA molecule so that the portion of DNA that codes for the
needed protein can be transcribed. Once the DNA molecule is unwound
at the correct location, an enzyme called RNA polymerase helps line up
nucleotides to create a complementary strand of mRNA.
Since mRNA is a single-stranded molecule, only one of the two strands
of DNA is used as a template for the new RNA strand.
The new strand of RNA is made according to the rules of
base pairing:
- DNA cytosine pairs with RNA guanine
- DNA guanine pairs with RNA cytosine
- DNA thymine pairs with RNA adenine
- DNA adenine pairs with RNA uracil
For example, the mRNA complement to the DNA sequence TTGCAC
is AACGUG. The SAT II Biology frequently asks about the sequence
of mRNA that will be produced from a given sequence of DNA. For
these questions, don’t forget that RNA uses uracil in place of thymine.
After transcription, the new RNA strand is released and
the two unzipped DNA strands bind together again to form the double
helix. Because the DNA template remains unchanged after transcription,
it is possible to transcribe another identical molecule of RNA immediately
after the first one is complete. A single gene on a DNA strand can
produce enough RNA to make thousands of copies of the same protein
in a very short time.
