6. How Cells Read the Genome: From DNA to Protein
From DNA to RNA - Transcription
- Transcription is the name given to the process of producing RNA
molecules using DNA as guide.
- The enzymes that perform transcription are called RNA polymerases.
- RNA polymerases make about one mistake for every 104 nucleotides
compared with an error rate of about one in 107 nucleotides for DNA
- RNAs display some differences compared to DNA: RNAs have
ribonucleotides in their composition instead of deoxyribonucleotides;
RNAs contain the base uracil (U) instead of thymine (T); and RNAs are
- There are different types of RNAs: coding RNAs and noncoding RNAs.
Example of coding RNA is the messenger RNA (mRNA), which codes for
proteins. Noncoding RNAs comprise molecules that can perform
enzymatic, structural and regulatory functions. Examples of these are:
ribosomal RNA (rRNA), transfer RNA (tRNA), small nuclear RNA
(snRNA), small nucleolar RNA (snoRNA), microRNA (miRNA), small
interfering RNA (siRNA), piwi-interacting RNA (piRNA) and long
noncoding RNA (lncRNA).
- Prokaryotes have a single type of RNA polymerase while eukaryotes
have three types: RNA polymerase I, RNA polymerase II and RNA
polymerase III – each one is responsible for the transcription of specific
subsets of genes.
- Bacterial RNA polymerase associates with a transcription-initiation factor
called sigma (σ) factor to form the RNA polymerase holoenzyme, which
binds DNA in a special region called the promoter to start transcription.
RNA elongation continues until a second region called terminator is
reached by the polymerase complex, which consists of a string of A-T
nucleotide pairs and a sequence that will form hairpin structures.