Yes, cDNA made from messenger RNA typically contains a poly(A) sequence, but with an important detail: the first strand of cDNA is actually a poly(T) stretch, since it’s the complementary copy of the mRNA’s poly(A) tail. Once the second strand is synthesized, the resulting double-stranded cDNA has a poly(dA/dT) region at its 3′ end. Whether your cDNA includes this sequence at all depends on how it was made.
How the Poly(A) Tail Ends Up in cDNA
Most eukaryotic mRNAs carry a poly(A) tail of roughly 200 adenine bases at their 3′ end. The most common method for converting mRNA into cDNA uses an oligo(dT) primer, a short string of thymine bases that binds directly to that poly(A) tail through standard base pairing. Reverse transcriptase then extends from the primer, copying the mRNA sequence into a single-stranded cDNA.
Because the oligo(dT) primer anneals at the poly(A) tail, the first-strand cDNA begins with a poly(T) sequence at its 5′ end. When second-strand synthesis converts this into double-stranded cDNA, one strand carries poly(T) and the complementary strand carries poly(A). Researchers often refer to this region as a poly(dA/dT) tract. So the poly(A) information from the original mRNA is preserved in the cDNA, just in DNA form rather than RNA.
Why the Primer Choice Matters
Not all cDNA is made with oligo(dT) primers. The two main alternatives, random hexamers and gene-specific primers, behave very differently when it comes to the poly(A) region.
- Oligo(dT) primers bind specifically to the poly(A) tail. The resulting cDNA will almost always include a poly(dA/dT) tract of varying length. This is the most widely used approach and has been the standard since the early days of recombinant DNA technology.
- Random hexamers are short six-base sequences that bind at many locations throughout an RNA molecule. Because they prime at random positions, some cDNA fragments will include the poly(A) region and others won’t. Random hexamers are useful when you want to capture the full length of a transcript, including regions far from the 3′ end, or when working with RNA that lacks a poly(A) tail entirely.
- Gene-specific primers target a known sequence within a particular gene. These produce cDNA only from that gene and typically do not extend through the poly(A) region unless the primer happens to sit downstream of it.
When cDNA Won’t Have a Poly(A) Sequence
Some RNA molecules don’t carry a poly(A) tail in the first place. Bacterial mRNAs, ribosomal RNAs, many long non-coding RNAs, and certain other transcripts are not polyadenylated. If you use oligo(dT) priming on a sample containing these RNAs, they simply won’t be captured. The resulting cDNA library will miss them entirely.
This is a real limitation in sequencing experiments. Most commercial cDNA sequencing kits rely on oligo(dT) priming or poly(A) selection, which means they exclude non-polyadenylated transcripts by design. To work around this, researchers use ribosomal RNA depletion combined with random priming, which captures transcripts regardless of their polyadenylation status. These protocols detect 30% to 50% more reads from intronic regions compared to poly(A)-specific methods, reflecting their ability to pick up a broader range of RNA species.
The Internal Poly(A) Problem
One quirk of oligo(dT) priming is that poly(A) stretches don’t only exist at the very end of an mRNA. Some transcripts contain internal runs of adenine bases within their sequence. Oligo(dT) primers can bind to these internal sites just as easily as the true 3′ poly(A) tail, which creates truncated cDNAs that start from the wrong position. This produces two types of artifacts: shortened cDNAs that begin at an internal poly(A) site, and cDNAs that start correctly at the 3′ end but terminate prematurely when they hit an internal poly(A) stretch.
This internal priming problem is well documented and is one reason why some experiments use anchored oligo(dT) primers (oligo(dT) with one or two non-T bases at the 3′ end) to reduce false priming at internal sites.
What This Means for Sequencing and Cloning
In RNA sequencing, the poly(dA/dT) tract in cDNA is both useful and problematic. On the useful side, poly(A) capture during cDNA library preparation gives very clean results: about 99% of sequencing reads map to exons, and non-coding RNAs that lack poly(A) tails are correctly excluded rather than showing up as noise. This improves accuracy for measuring gene expression levels and detecting alternative splicing events.
On the problematic side, long homopolymer stretches (runs of the same base) are difficult for both reverse transcriptase and sequencing platforms. Reverse transcriptase moves much faster through poly(A) stretches than through mixed sequences, reaching speeds of up to 29 bases per second on poly(A) compared to about 7 bases per second on varied sequences. This speed difference can affect the quality of cDNA synthesis near the tail. Many cloning and sequencing protocols trim the poly(dA/dT) region or use strategies to minimize its length for this reason.
If you’re working with cDNA in a practical setting, assume it contains a poly(dA/dT) tract when oligo(dT) priming was used, and plan your downstream steps accordingly. If poly(A) sequences are undesirable for your application, random hexamer priming or enzymatic trimming after synthesis are straightforward alternatives.