Transcription is the process cells use to copy genetic information from DNA into RNA. It is the first major step in using many genes to make proteins and other functional molecules.
The basic idea is straightforward: DNA stores genetic information, and transcription produces an RNA copy of the information needed for a particular cellular task. But the process is carefully controlled. Cells must identify the correct gene, copy the appropriate DNA strand, stop at the right place, and—especially in eukaryotic cells—process the resulting RNA before it can perform its function.
Understanding transcription provides a foundation for understanding gene expression, protein production, cell specialization, and many genetic diseases.
What is transcription?
Transcription is the synthesis of an RNA molecule using a DNA sequence as a template.
The enzyme responsible for transcription is RNA polymerase. It moves along the DNA template strand and joins RNA nucleotides together in a sequence complementary to the DNA template.
RNA uses four bases:
- A — adenine
- U — uracil
- C — cytosine
- G — guanine
DNA, by comparison, uses thymine (T) instead of uracil. During transcription, RNA bases pair with the DNA template according to these relationships:
| DNA template base | RNA base added |
|---|---|
| A | U |
| T | A |
| C | G |
| G | C |
The resulting RNA is therefore complementary to the DNA template strand. Its sequence is nearly identical to the DNA coding strand, except that RNA contains U where DNA contains T.
For example, if a DNA template contains:
3′-TACGGA-5′
the RNA produced from it is:
5′-AUGCCU-3′
The direction matters because RNA polymerase builds RNA only in the 5′ to 3′ direction.
Where does transcription happen?
In eukaryotic cells, such as human cells, transcription occurs primarily in the nucleus, where most DNA is located. The RNA produced there can later move into the cytoplasm, where some types of RNA participate in protein production or other cellular processes.
Bacteria do not have a membrane-bound nucleus. Their DNA and transcription machinery occupy the same general cellular compartment, allowing transcription and protein synthesis to occur much more closely together.
The distinction is important because eukaryotic RNA usually undergoes substantial processing between transcription and its eventual use.
DNA contains two strands, but only one is the template
A common source of confusion is the question of which DNA strand gets copied.
A gene’s two DNA strands have complementary sequences, but RNA polymerase uses only one of them as the template strand for a particular transcription event. The other strand is called the coding strand because its sequence corresponds to the RNA sequence, with T replacing U.
The template strand is read by RNA polymerase in the 3′ to 5′ direction, allowing the new RNA strand to be synthesized in the 5′ to 3′ direction.
Which DNA strand serves as the template depends on the gene. Different genes can be transcribed from different DNA strands.
The three stages of transcription
Transcription is commonly divided into three stages: initiation, elongation, and termination.
Initiation: finding the gene and starting RNA synthesis
Transcription does not begin randomly along a chromosome. RNA polymerase must be directed to the correct starting point.
Near the beginning of a gene is a region of DNA called a promoter. In eukaryotic cells, proteins called transcription factors help recognize regulatory DNA sequences and recruit or position RNA polymerase.
Once the appropriate machinery is assembled, the DNA strands separate over a short region. RNA polymerase then begins joining RNA nucleotides to form a growing RNA molecule.
The promoter is not itself copied into the RNA. Instead, it helps determine where transcription begins and influences whether and how strongly a gene is transcribed.
Elongation: building the RNA molecule
During elongation, RNA polymerase moves along the DNA template strand.
As it progresses, it temporarily opens the DNA double helix. RNA nucleotides pair with exposed DNA bases and are linked together to extend the RNA strand.
Only a small portion of the DNA needs to be separated at any one time. Behind the polymerase, the DNA strands can pair back together.
The RNA therefore grows one nucleotide at a time, with its sequence determined by the DNA template.
Termination: stopping transcription
RNA polymerase eventually encounters signals that cause transcription to stop.
The exact termination mechanism differs among organisms and types of RNA polymerase. In eukaryotic cells, transcription of protein-coding genes produces a precursor RNA that is cleaved after a sequence associated with termination, and the polymerase subsequently disengages.
Termination ensures that the RNA transcript does not continue indefinitely into neighboring DNA.
What happens to RNA after transcription?
For many human genes that encode proteins, the initial RNA molecule is not yet ready to be used. It is called pre-mRNA, or precursor messenger RNA.
Before it becomes mature messenger RNA (mRNA), it undergoes several important processing steps.
A 5′ cap is added
A modified nucleotide structure called a 5′ cap is added to the beginning of the RNA.
The cap helps protect the RNA from degradation and is important for later stages of gene expression, including the export of mRNA from the nucleus and recognition of the mRNA by the protein-making machinery.
Introns are removed
Many eukaryotic genes contain segments called introns that are transcribed but generally do not remain in the mature mRNA.
Other segments, called exons, are joined together during a process called RNA splicing.
The result is a continuous mature mRNA sequence that can be used to direct protein synthesis.
Splicing is not always identical for every transcript. Through alternative splicing, cells can combine certain exons in different ways, allowing a single gene to produce different RNA and protein products.
A poly(A) tail is added
For most eukaryotic protein-coding mRNAs, a stretch of adenine nucleotides called a poly(A) tail is added to the RNA’s 3′ end.
The poly(A) tail contributes to RNA stability and helps with other aspects of mRNA handling and translation.
These processing steps transform the initial transcript into a mature mRNA with the information and molecular features needed for its next stage.
Transcription and translation are different processes
Transcription and translation are closely related, but they are not the same thing.
Transcription: DNA → RNA
Translation: RNA → protein
During transcription, RNA polymerase reads DNA and produces RNA. During translation, ribosomes read the sequence of a mature mRNA and use it to assemble a chain of amino acids.
In a typical human protein-coding gene, the information therefore follows this general path:
DNA → pre-mRNA → mature mRNA → protein
Not all RNA is used to make protein. Cells also transcribe genes that produce functional RNAs, including ribosomal RNA, transfer RNA, and various regulatory RNAs.
How cells control transcription
Transcription is one of the major points at which cells control gene expression.
Almost every cell in the human body contains essentially the same genome, yet a neuron behaves differently from a liver cell or a muscle cell. One important reason is that different cells activate different sets of genes.
Transcription can be influenced by regulatory DNA sequences and proteins that interact with them. Transcription factors can increase or decrease the activity of particular genes. Other mechanisms, including changes to chromatin structure and chemical modifications associated with DNA and histone proteins, can affect whether transcription machinery can access a region of DNA.
This regulation allows cells to respond to developmental signals, hormones, environmental conditions, and other changes.
Gene regulation can also occur after transcription, but controlling how much RNA is produced in the first place is a particularly important part of the process.
Why transcription matters
Transcription connects the information stored in the genome with the molecules that actually carry out cellular functions.
A change in DNA can sometimes alter an RNA transcript. If the affected gene encodes a protein, that change may ultimately alter the protein’s sequence, amount, structure, or activity. Mutations can also affect regulatory DNA and change how much a gene is transcribed without changing the protein-coding sequence itself.
Transcription is therefore more than a simple copying mechanism. It is a regulated process through which cells selectively access genetic information and convert it into RNA molecules that can influence what the cell does.
At its core, however, the principle remains simple: RNA polymerase uses one strand of DNA as a template to build a complementary RNA molecule, and the cell then processes and regulates that RNA according to its function.
