During transcription, RNA polymerase uses one strand of DNA as a guide to make an RNA molecule. That DNA strand is called the template strand. The other DNA strand is called the coding strand because its sequence matches the RNA sequence, with one important difference: RNA contains uracil (U) where DNA contains thymine (T).
The key point is simple:
The template strand is transcribed. The coding strand is not directly transcribed.
Understanding why the two strands have different names becomes much easier once you keep track of which strand RNA polymerase reads, which direction it moves, and how the resulting RNA sequence relates to each DNA strand.
What is the template strand?
The template strand is the DNA strand that RNA polymerase reads during transcription. It provides the sequence information needed to build a complementary RNA molecule.
RNA polymerase reads the template strand in the 3′ to 5′ direction. At the same time, it builds the RNA strand in the 5′ to 3′ direction.
This directional relationship follows from complementary base pairing. As RNA polymerase moves along the DNA template, it adds RNA nucleotides according to these pairings:
- DNA A pairs with RNA U
- DNA T pairs with RNA A
- DNA C pairs with RNA G
- DNA G pairs with RNA C
For example, if the template DNA strand contains:
3′-TAC GGA CTT-5′
the RNA produced is:
5′-AUG CCU GAA-3′
The RNA is therefore complementary to the template strand.
What is the coding strand?
The coding strand is the DNA strand that is not used as the template during transcription of that particular gene. It is sometimes called the non-template strand or sense strand.
Its sequence is almost identical to the RNA produced from the gene. The important difference is that DNA uses thymine (T), whereas RNA uses uracil (U).
For example:
Coding DNA: 5′-ATG CCT GAA-3′
RNA: 5′-AUG CCU GAA-3′
Because the coding strand and RNA have corresponding sequences, the coding strand is often easier to use when predicting the RNA sequence. Simply replace each T in the coding-strand sequence with U.
The coding strand is not read by RNA polymerase to determine the RNA sequence. Instead, its sequence corresponds to the RNA because it is complementary to the template strand.
Template strand vs. coding strand
The relationship can be summarized as follows:
| Feature | Template strand | Coding strand |
|---|---|---|
| Also called | Antisense strand, noncoding strand | Sense strand, non-template strand |
| Used directly by RNA polymerase as the transcription guide? | Yes | No |
| Direction relative to the RNA sequence | Complementary and antiparallel | Same sequence orientation as RNA |
| Sequence relationship to RNA | Complementary | Nearly identical |
| Contains T or U? | T, because it is DNA | T, because it is DNA |
The terms coding and noncoding can cause confusion. Calling the template strand the “noncoding strand” does not mean that it lacks genetic information or is unimportant. Both DNA strands contain sequence information. The distinction refers specifically to which strand’s sequence corresponds to the RNA product for a particular gene.
How can you tell which DNA strand is transcribed?
If a problem gives you two DNA strands and asks which one is the template, look at their orientation and the RNA sequence.
The template strand is the one that runs 3′ to 5′ relative to the direction in which the RNA sequence is being synthesized. RNA polymerase reads that strand 3′ to 5′ while producing RNA 5′ to 3′.
Consider this example:
Template DNA: 3′-TAC GGT CAA-5′
Coding DNA: 5′-ATG CCA GTT-3′
RNA: 5′-AUG CCA GUU-3′
Here, the template strand is easy to identify because it is complementary to the RNA. The coding strand has the same sequence as the RNA except for T in DNA instead of U in RNA.
If a question gives only one DNA sequence without indicating its orientation or whether it is the template or coding strand, you generally cannot determine the RNA sequence unambiguously from the sequence alone. The strand designation or 5′/3′ orientation matters.
Why does the template strand change from gene to gene?
There is not one permanent “template strand” for an entire chromosome.
For one gene, RNA polymerase may use one DNA strand as the template. For a different gene, it may use the opposite strand. Which strand serves as the template depends on the location and orientation of the gene and its promoter.
This means that the same physical DNA molecule can function as a template for one gene and as the coding strand for another.
That distinction is important: template and coding are gene-specific descriptions, not permanent labels assigned to the two strands of a chromosome.
Why is the coding strand useful if it is not transcribed?
Although RNA polymerase does not use the coding strand as its direct template, its sequence provides a convenient representation of the RNA sequence.
Suppose the coding strand is:
5′-ATG GCA TTT-3′
The corresponding RNA is:
5′-AUG GCA UUU-3′
Because the coding strand and RNA have the same sequence pattern, apart from T versus U, the coding strand is often the sequence shown when researchers or textbooks describe a gene.
The template strand for the same region would be:
3′-TAC CGT AAA-5′
Reading that strand in the 3′ to 5′ direction and applying complementary base pairing produces the same RNA.
Transcription direction is the key to avoiding mistakes
A common mistake is to assume that RNA polymerase reads DNA from 5′ to 3′. It does not. RNA polymerase reads the template DNA strand from 3′ to 5′ and synthesizes RNA from 5′ to 3′.
The two DNA strands themselves are antiparallel, meaning that they run in opposite directions:
Template: 3′ → 5′
Coding: 5′ → 3′
RNA: 5′ → 3′
This arrangement explains why the coding strand and RNA have matching sequence orientation while the template strand has the opposite orientation.
When solving a transcription problem, first identify the strand being used as the template and make sure its direction is clear. Then write the RNA in the 5′ to 3′ direction.
Is the coding strand ever transcribed?
For a particular gene, the coding strand is not the DNA strand used as the transcription template. However, the opposite DNA strand can serve as the template for another gene located elsewhere, depending on that gene’s orientation.
So it is more accurate to say that a DNA strand is the coding or template strand relative to a particular gene. Neither strand is inherently the “template strand” for the entire DNA molecule.
The simplest way to remember the difference
The template strand is the strand RNA polymerase reads.
The coding strand is the strand whose sequence matches the RNA except that DNA has T where RNA has U.
If you remember those two relationships, the rest follows naturally:
Template DNA → complementary RNA
Coding DNA → same sequence as RNA, with T replaced by U
The template strand is therefore the DNA strand that is transcribed, while the coding strand provides the sequence that corresponds directly to the RNA product.
