RNA is the step between DNA and protein synthesis. With a few differences, RNA is chemically similar to DNA. RNA contains ribose as its sugar and has a nitrogenous base of uracil rather than thymine. DNA contains deoxyribose as its sugar. Each nucleotide along a DNA strand has deoxyribose as its sugar and A, G, C, or T as its base, while each nucleotide along an RNA strand has ribose as its sugar and A, G, C, or U as its base. RNA molecules are always single stranded.
Two major stages must occur in order to get from DNA to protein, transcription and translation. RNA helps in this process. Transcription is the synthesis of RNA. The information is transcribed from one molecule to the other. The RNA molecule that results is called messenger RNA (mRNA). Messenger RNA carries a genetic message from the DNA to the protein-synthesizing part of the cell. .
Following transcription is translation. Translation occurs under the direction of mRNA. The "language" changes in this stage, and the cell must translate the base sequence of an mRNA molecule into the amino acid sequence of a polypeptide. During translation, the genetic message (mRNA) is read as a sequence of base triplets, three-letter code words. These triplets, also called codons, specify the amino acid to be added at the corresponding position along a growing polypeptide chain. The end result is called the primary transcript. .
There are a few more important types of RNA. Transfer RNA (tRNA) serves as an adaptor molecule in protein synthesis, and translates mRNA codons into amino acids. Also, there is ribosomal RNA (rRNA), which plays important roles in the ribosomes. Two other types of RNA are small nuclear RNA (snRNA) and SRP RNA, and they are only found in eukaryotes. .
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