Transcription

The genetic information in DNA is not translated directly into
proteins, but is first transcribed into mRNA. Transcription involves
numerous enzymes that unwind a region of a DNA molecule,
initiate and end mRNA synthesis, and modify the mRNA after
transcription is complete. Unlike DNA replication, only one
or a few genes are exposed, and only one of the two DNA strands
is transcribed.
One of the important enzymes of this process is RNA polymerase.
After a section of DNA is unwound, RNA polymerase recognizes
a specific sequence of DNA nucleotides. RNA polymerase
attaches and begins joining ribose nucleotides, which are complementary
to the 3' end of the DNA strand. In RNA, the same complementary
bases in DNA are paired, except that in RNA, the base
uracil replaces the base thymine as a complement to adenine.
Newly transcribed mRNA, called the primary transcript,
must be modified before leaving the nucleus to carry out protein
synthesis. Some base sequences in newly transcribed mRNA do
not code for proteins. RNA splicing involves cutting out noncoding
regions so that the mRNA coding region can be read continuously
at the ribosome.