Transcription: From DNA to mRNA Revision Notes | A Level AQA Biology

1. Why Transcription Matters

Your DNA holds the instructions for every protein your cells make — but DNA never leaves the nucleus. So how does the ribosome in the cytoplasm get those instructions? That is exactly the problem transcription solves.

Transcription copies a gene's sequence from DNA into a mobile messenger molecule: messenger RNA (mRNA). This mRNA can then travel out of the nucleus to the ribosome, where translation converts the message into a protein. Without transcription, the genetic code stays locked away and no proteins are made.

There is a crucial twist, though. The way transcription works — and what it produces — differs between prokaryotes and eukaryotes. That difference is a favourite exam topic, so we will build up both pictures carefully.

2. The Mechanics of Transcription

Picture a stretch of double-stranded DNA inside the nucleus. One strand runs in the 3' to 5' direction — this is the template strand (sometimes called the antisense strand). RNA polymerase reads along this strand and builds a complementary RNA copy.

Here is how the process unfolds, step by step. RNA polymerase binds to a specific region of DNA called the promoter, which signals the start of a gene. The enzyme unwinds and separates the two DNA strands, exposing the template strand. Free ribonucleotides — carrying the bases adenine (A), uracil (U), guanine (G), and cytosine (C) — then line up opposite their complementary bases on the template strand. Remember: in RNA, uracil replaces thymine, so where the DNA template has adenine, the mRNA carries uracil.

RNA polymerase catalyses the formation of phosphodiester bonds between adjacent ribonucleotides, joining them into a growing mRNA chain. The enzyme moves along the template strand in the 3' to 5' direction, so the mRNA strand is built in the 5' to 3' direction. Once the gene has been fully transcribed, RNA polymerase reaches a terminator sequence and detaches, releasing the mRNA transcript.

Worked example: Tracing the Sequence from DNA to mRNA

A section of the DNA template strand reads: 3'-TACGGA-5'. Determine the sequence of the mRNA produced during transcription.

1. Template strand: 3'-T-A-C-G-G-A-5' — RNA polymerase reads the template strand in the 3' to 5' direction, so we read left to right here.
2. T pairs with A, A pairs with U, C pairs with G, G pairs with C — In RNA, uracil (U) replaces thymine (T). There is no thymine in RNA.
3. DNA template: 3'-T A C G G A-5' mRNA: 5'-A U G C C U-3' — Each ribonucleotide is complementary to the corresponding DNA base. The mRNA is built 5' to 3'.
4. mRNA: 5'-AUGCCU-3' — Notice AUG — this is the start codon. Examiners sometimes use this sequence deliberately to test whether you recognise it.

Answer: mRNA sequence: 5'-AUGCCU-3'

The sequence above illustrates the central role of RNA polymerase: it does not merely unzip DNA, it actively catalyses the phosphodiester bonds that link each ribonucleotide to the next. No RNA polymerase means no mRNA — and therefore no protein synthesis.

3. Transcription in Prokaryotes

Prokaryotes — bacteria and archaea — have no nucleus. Their DNA sits freely in the cytoplasm, and transcription takes place there too. This has a significant consequence: the mRNA produced is used directly, without any further processing.

In prokaryotes, transcription results directly in the production of mRNA from DNA. The mRNA transcript is immediately available for translation — in fact, ribosomes can begin attaching to the 5' end of the mRNA and start translating it even before RNA polymerase has finished transcribing the 3' end. This simultaneous transcription and translation is only possible because there is no nuclear membrane separating the two processes.

This direct route from DNA to functional mRNA is one of the key features that distinguishes prokaryotic gene expression from eukaryotic gene expression — and it is a comparison that appears regularly in AQA exam questions.

4. Transcription in Eukaryotes: Pre-mRNA and Splicing

In eukaryotes, the story does not end when RNA polymerase detaches. The initial transcript — called pre-mRNA — contains sequences that will not be translated. These non-coding sequences are called introns, and the coding sequences that will be kept are called exons.

Splicing takes place in the nucleus before the mRNA leaves through the nuclear pore. During splicing, the introns are precisely cut out and the exons are joined together. The result is a shorter, continuous coding sequence — the mature mRNA — which then moves into the cytoplasm for translation.

To summarise the eukaryotic sequence: DNA is transcribed to produce pre-mRNA in the nucleus; the pre-mRNA is then spliced to remove introns and join exons; the mature mRNA exits the nucleus and is translated at the ribosome. Each stage is spatially and temporally separated, unlike in prokaryotes.

A useful memory anchor: think of exons as the sequences that are EXported and EXpressed. Introns are INterrupting sequences that are cut out and discarded. This distinction is worth locking in precisely because examiners frequently swap the two terms to test whether you know which is which.

5. Comparing Prokaryotic and Eukaryotic Transcription

Now that both processes are clear individually, it is worth placing them side by side. AQA mark schemes reward precise comparative language — words like 'whereas' and 'however' signal to the examiner that you are genuinely comparing, not just describing each in turn.

The table above captures the core contrasts. Notice that the key structural difference — the nuclear membrane — is what drives almost every other difference on the list. Eukaryotes have a nucleus, so transcription and translation are physically separated; prokaryotes do not, so the two processes can overlap in time.

6. Exam Technique: Getting Full Marks on Transcription Questions

Transcription questions on AQA papers tend to use the command words 'describe', 'explain', and 'compare'. Each demands a different type of response, and confusing them is one of the most common ways students lose marks.

A 'describe' question wants a sequence of events — what happens, in order, without explanation. A 'explain' question wants the mechanism behind each step — use 'because' to link cause and effect. A 'compare' question requires you to address both organisms in the same sentence or adjacent sentences, explicitly noting similarities and differences.

Transcription — from the binding of RNA polymerase at the promoter to the production of a mature mRNA ready for translation — is the essential first step in converting genetic information into protein. In prokaryotes, this produces functional mRNA directly; in eukaryotes, it produces pre-mRNA that must be spliced before it can be used. Both routes depend on the same enzyme, RNA polymerase, and the same base-pairing rules — but the eukaryotic pathway adds a layer of processing that gives cells far greater control over which proteins are ultimately made.