Transcription is the process by which the nucleotide sequence of DNA directs the synthesis of RNA. It is a fundamental step in gene expression, producing both protein-coding messages and RNAs that function without being translated. The principal enzyme responsible is RNA polymerase, which copies a selected DNA template into an RNA transcript. Unlike DNA replication, transcription normally copies particular regions rather than duplicating an entire genome. (genome.gov)
Templates, products, and directionality
Within a transcription unit, one DNA strand serves as the template. RNA polymerase reads this strand in the 3′-to-5′ direction while extending RNA in the 5′-to-3′ direction. Complementary pairing determines the sequence: DNA adenine directs RNA uracil, thymine directs adenine, guanine directs cytosine, and cytosine directs guanine. The RNA therefore resembles the opposite, coding DNA strand, except that uracil replaces thymine. Different genes can use different strands of the same DNA molecule as templates. (reach.cdc.gov)
The substrates are ribonucleoside triphosphates, which supply both the nucleotide units and chemical energy for chain growth. Polymerase forms phosphodiester bonds between successive units, releasing pyrophosphate. Unlike the enzymes responsible for DNA replication, cellular RNA polymerases can initiate a chain without a pre-existing primer. DNA is locally unwound during copying but is not consumed or permanently converted into RNA. (ncbi.nlm.nih.gov)
Products include messenger RNA (mRNA), which provides the template for translation into protein, and functional RNAs such as ribosomal RNA and transfer RNA. Transcription and translation are distinct processes: the former synthesizes a nucleic acid, whereas the latter assembles a polypeptide using an RNA message. (ncbi.nlm.nih.gov)
Initiation, elongation, and termination
Initiation establishes where RNA synthesis begins and which DNA strand is copied. A promoter is a DNA region recognized by the transcription machinery. Polymerase and associated factors assemble there, open a short segment of the double helix, and position the template in the enzyme’s active site. Early synthesis may produce short RNAs that are released before polymerase escapes the promoter and enters productive elongation. (ncbi.nlm.nih.gov)
Elongation is the repeated addition of nucleotides to the RNA’s 3′ end. A moving transcription bubble exposes the template, while a short RNA–DNA hybrid forms inside polymerase. Newly synthesized RNA separates from DNA, and the DNA strands rejoin behind the enzyme. Elongation is not necessarily continuous: polymerase can pause, and associated factors influence whether it resumes synthesis or terminates. (ncbi.nlm.nih.gov)
Termination ends synthesis and releases the transcription complex. In bacteria, intrinsic termination commonly involves an RNA hairpin followed by a uracil-rich sequence. A second major pathway uses the Rho protein, which travels along RNA and promotes disruption of the elongation complex. These are transcriptional termination mechanisms, not recognition of the stop codons used during translation. (ncbi.nlm.nih.gov)
For many protein-coding genes in eukaryotes, termination is coupled to cleavage of the emerging RNA and formation of its mature 3′ end. RNA polymerase II usually continues beyond the cleavage site before disengaging; consequently, the end of a mature message need not coincide with the position where transcription stops. (ncbi.nlm.nih.gov)
Differences among cellular organisms
Bacteria generally use one principal multisubunit RNA polymerase for their different RNA classes. Sigma factors confer promoter-recognition specificity. Because bacteria lack a membrane-enclosed nucleus, a ribosome can begin translating an emerging mRNA while its transcription is still underway. (ncbi.nlm.nih.gov)
Archaea also use a single principal RNA polymerase, but its architecture and basal initiation factors resemble those of eukaryotes more closely than those of bacteria. Archaeal initiation commonly involves TATA-binding protein and transcription factor B rather than bacterial sigma factors. Thus, the absence of a nucleus does not imply a bacterial-style transcription apparatus. (pmc.ncbi.nlm.nih.gov)
In eukaryotes, most transcription of nuclear genes occurs within the cell nucleus. Three major nuclear polymerases have specialized roles:
- RNA polymerase I produces the precursor of the major ribosomal RNAs.
- RNA polymerase II transcribes protein-coding genes and numerous noncoding RNA genes.
- RNA polymerase III produces transfer RNAs, 5S ribosomal RNA, and several other small RNAs.
Polymerase II requires general transcription factors to assemble an initiation complex and begin transcription at a promoter. (ncbi.nlm.nih.gov)
RNA processing
A newly synthesized transcript is not necessarily a mature functional RNA. In eukaryotes, precursor mRNA commonly receives a 5′ cap, undergoes RNA splicing, and acquires a poly(A) tail through polyadenylation. Splicing removes introns and joins exons; alternative splicing can generate different mature messages from the same precursor. Many processing events occur while transcription is still in progress, rather than only after polymerase has finished. Ribosomal and transfer RNAs also undergo specialized maturation. (ncbi.nlm.nih.gov)
Regulation and measurement
Transcriptional regulation determines when, where, and how strongly genes are expressed. Sequence-specific transcription factors can activate or repress transcription by influencing polymerase recruitment and activity. In eukaryotes, enhancers can act at a distance through interactions involving DNA looping. DNA packaging into chromatin changes access to regulatory sequences, while DNA methylation provides an additional layer of control. Regulation can affect initiation as well as later stages of synthesis. (ncbi.nlm.nih.gov)
The transcriptome is the collection of RNA transcripts present in a cell or sample. Measuring it reveals which genomic regions are expressed under particular conditions. However, RNA abundance reflects both production and degradation, so the amount of an RNA present is not identical to its transcription rate. This distinction separates measurements of accumulated transcripts from measurements of ongoing RNA synthesis. (genome.gov)