All RNA in a cell is made by DNA transcription, which has many similarities to the process of DNA replication discussed in Chapter 5. As in DNA replication, transcription begins with opening and unwinding a small portion of the DNA double helix. This allows the bases on each strand of the DNA to be exposed, and one of these strands acts as a template for synthesis of an RNA chain. An RNA chain is extended one nucleotide at a time by complementary base-pairing between incoming ribonucleotide molecules and the DNA template. When a match is found, the ribonucleotide molecule is covalently linked to the RNA chain in an enzymatic reaction. The resulting RNA is called an mRNA (m stands for messenger).
The four major types of RNAs produced by DNA transcription are tRNAs, uRNAs, rRNAs and mRNAs. All of these are polymer molecules that contain four different bases in their backbones-adenine (A), guanine (G), cytosine (C) and uracil (U). They differ from DNA only in that they lack the sugar deoxyribose, and they have the additional feature of being single-stranded. This feature, coupled with the fact that they can fold into complex three-dimensional shapes like a protein, enables them to perform various structural and catalytic functions.
The most abundant RNAs in the cells of eucaryotes are ribosomal RNAs (rRNAs), which comprise about 80% of all RNA in rapidly dividing cells. The rRNA genes are clustered together in a subcompartment of the nucleus, called the nucleolus. The nucleolus contains a variety of macromolecules including precursor rRNAs, mature rRNAs, specialized proteins that promote cleavage and processing of nascent RNA, snoRNPs and partly assembled ribosomes.