What is RNA?
RNA is a single-stranded molecule which has as its 5-carbon sugar ribose instead of the deoxyribose found in DNA. Also, uracil is substituted for thymine as a nitrogen base. RNA is built from a DNA template in a process called transcription. The RNA then delivers the coded specifications to the cytoplasm upon which protein molecules are built. While DNA does not synthesize proteins directly, the protein structure is ultimately determined by the DNA code.
The process of transcription is similar to DNA replication. An enzyme, RNA polymerase, binds to DNA and causes separation of the complementary strands. Next, complementary base-pairing occurs between the bases of the DNA strand and complementary RNA nucleotides floating in the nucleus. Only one of the DNA strands, the template strand, is transcribed. The complementary DNA strand is called the coding strand.
Transcription begins at specific segments called initiation sites, to which RNA polymerase attaches. RNA polymerase catalyzes the transcription of DNA in only one direction, on the 3' position of the molecule. RNA polymerase catalyzes sugar-to-phosphate bonds between the RNA nucleotides until it reaches the sequence of amino acids that serves as a termination signal, when the completed RNA molecule is released.