Transcription

CIE A-Level Biology· 15 min read

1. Overview of Transcription and Key Components★★☆☆☆⏱ 4 min

Transcription transfers genetic information stored in DNA into a messenger RNA (mRNA) molecule that can be translated into a polypeptide at the ribosome. Unlike DNA replication, transcription only copies one gene (or a group of related genes in prokaryotes) at a time, so only small sections of the genome are transcribed.

📘 Definition

Transcription

The process by which an RNA molecule is synthesised from a DNA template, resulting in a copy of genetic information from DNA to RNA

Example:

A gene coding for insulin is transcribed into mRNA, which is then translated to produce the insulin protein

- RNA polymerase: the enzyme that catalyses phosphodiester bond formation between RNA nucleotides
- Template (antisense) strand: the DNA strand read by RNA polymerase to produce complementary RNA
- Coding (sense) strand: the DNA strand with the same base sequence as RNA (T replaced by U)
- Promoter: DNA sequence upstream of the gene that binds RNA polymerase to initiate transcription
- Terminator: DNA sequence that signals the end of transcription

📐 Worked Example

A segment of the coding strand of a gene has the sequence 5' ATGCGATTA 3'. Write the sequence of the RNA transcript produced from this segment.

1
Recall that the RNA transcript has the same sequence and 5'→3' direction as the coding strand, with thymine (T) replaced by uracil (U).
2
Original coding strand: 5' ATGCGATTA 3'. Replace all T bases with U:
$5' \text{ATGCGATTA} 3' \rightarrow 5' \text{AUGCGAUUA} 3'$
3
This is the final sequence of the RNA transcript.

Exam tip:

Always confirm the 5'→3' direction when writing nucleic acid sequences; examiners commonly penalize incorrect direction.

2. Steps of Transcription in Prokaryotes★★☆☆☆⏱ 5 min

Transcription in prokaryotes occurs in three core stages, all taking place in the cytoplasm (prokaryotes have no nucleus):

1. Initiation: RNA polymerase binds to the promoter, unwinding the DNA double helix to separate the two strands.
1. Elongation: RNA polymerase moves along the template strand 3'→5', adding complementary RNA nucleotides, growing mRNA 5'→3'. Adenine on DNA pairs with uracil on RNA.
1. Termination: RNA polymerase reaches the terminator sequence, detaches from DNA, and releases the complete mRNA.

In prokaryotes, mRNA is ready for immediate translation, and translation often begins before transcription is complete.

📐 Worked Example

What effect would deletion of the promoter region of a prokaryotic gene have on transcription?

1
Recall the core role of the promoter region: it is the binding site for RNA polymerase to initiate transcription.
2
Without a promoter, RNA polymerase cannot recognise where the gene starts, so it cannot bind to the DNA template.
3
No mRNA can be transcribed from the gene, so no protein product can be produced.

3. Transcription in Eukaryotes★★★☆☆⏱ 4 min

In eukaryotes, transcription occurs in the nucleus, where DNA is stored. The core steps of initiation and elongation are similar to prokaryotes, but the initial product is not mature mRNA ready for translation.

📘 Definition

pre-mRNA

The primary transcript produced by transcription of a eukaryotic gene, before post-transcriptional modifications

Example:

Human pre-mRNA contains non-coding and coding regions that are processed before exiting the nucleus

📐 Worked Example

Why is transcription separated from translation in eukaryotes, but not in prokaryotes?

1
Eukaryotic DNA is enclosed in the nucleus, so transcription must occur where the DNA template is located.
2
Ribosomes, the site of translation, are only found in the cytoplasm and rough endoplasmic reticulum of eukaryotes.
3
This separation allows pre-mRNA to be processed before translation, enabling gene regulation and alternative splicing.

4. Post-Transcriptional Modifications★★★☆☆⏱ 5 min

Before mature mRNA can leave the nucleus to be translated, eukaryotic pre-mRNA undergoes three key modifications. Splicing is the most commonly tested modification for CIE exams.

📘 Definition

RNA Splicing

The process that removes non-coding introns from pre-mRNA and joins coding exons together to form mature mRNA

Example:

Alternative splicing of the same pre-mRNA can produce multiple different mature mRNA sequences, leading to different protein products

- 5' capping: A modified guanine nucleotide added to the 5' end to protect mRNA from degradation and help ribosome binding
- 3' polyadenylation: A poly-A tail of 50-200 adenine nucleotides added to the 3' end to protect mRNA from degradation

📐 Worked Example

A eukaryotic pre-mRNA has the structure $E_{1} - I_{1} - E_{2} - I_{2} - E_{3} - I_{3} - E_{4}$ , where $E$ = exon and $I$ = intron. What is the structure of mature mRNA after splicing?

1
Recall that splicing removes all introns, and keeps all exons joined in their original order.
2
Remove all three introns ( $I_{1}$ , $I_{2}$ , $I_{3}$ ) from the sequence.
3
Join the remaining exons, then add the 5' cap and 3' poly-A tail. The final mature mRNA sequence is: $5^{'} cap - E_{1} - E_{2} - E_{3} - E_{4} - poly-A tail 3^{'}$

5. Common Pitfalls

Wrong move:

Confusing template and coding strands, claiming the coding strand is transcribed.

Why:

Only the template strand is read by RNA polymerase. The coding strand has the same sequence as mRNA but is never transcribed.

Correct move:

Remember: template = transcribed, coding = not transcribed, mRNA sequence matches coding strand (T→U).

Wrong move:

Stating mRNA is synthesised in the 3'→5' direction.

Why:

Like all nucleic acid synthesis, RNA polymerase can only add nucleotides to the 3' end of the growing strand.

Correct move:

RNA polymerase moves along the template strand 3'→5', so mRNA is built 5'→3'.

Wrong move:

Claiming introns are removed from DNA during transcription.

Why:

Introns are present in the DNA gene and transcribed into pre-mRNA. They are removed during RNA processing after transcription.

Correct move:

Introns are spliced out of pre-mRNA, not DNA, to produce mature mRNA.

Wrong move:

Saying transcription occurs in the cytoplasm of eukaryotes.

Why:

Eukaryotic DNA is stored in the nucleus, so transcription must occur there. Only translation happens in the cytoplasm.

Correct move:

Prokaryotes: transcription + translation both in cytoplasm; Eukaryotes: transcription in nucleus, translation in cytoplasm.

Wrong move:

Claiming thymine never appears in any RNA molecule.

Why:

While uracil replaces thymine in mRNA transcripts, thymine can appear in modified functional RNAs like tRNA.

Correct move:

For transcription, remember that DNA adenine pairs with RNA uracil, so mRNA has no thymine from transcription.

6. Quick Reference Cheatsheet

Feature	Prokaryotic Transcription	Eukaryotic Transcription
Location	Cytoplasm	Nucleus
Initial product	Mature mRNA, ready for translation	pre-mRNA, requires processing
Post-transcriptional modification	None	Splicing, 5' capping, 3' polyadenylation
Introns in genes	Very rare	Common
Coupled to translation	Yes, translation starts early	No, processing completes first
RNA polymerase	Single type for mRNA	Multiple specialised types

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 22
Describe the process of transcription
2023 · 13
Compare transcription and DNA replication
2024 · 21
Explain the role of RNA splicing

Going deeper

syllabusCIE 9700 Syllabus 2024-2026Unit 6 learning outcomes

What's Next

Transcription is the first step of the central dogma of molecular biology, where genetic information flows from DNA → RNA → protein. After mature mRNA is produced, the next stage is translation, where ribosomes read the mRNA sequence to build a polypeptide chain. Understanding transcription also forms the foundation for learning about gene expression regulation, which is a common topic for extended response questions in CIE A-Level Biology exams. Comparing transcription to DNA replication also helps reinforce your understanding of nucleic acid synthesis more broadly.