# Transcription

> CIE A-Level Biology · Unit 6: Nucleic Acids and Protein Synthesis
> Source: https://www.owlsprep.com/study/cie-9700-u6-transcription/

Transcription is the first stage of protein synthesis, where genetic information from DNA is copied into mRNA. This module covers the steps of transcription in prokaryotes and eukaryotes, and key post-transcriptional modifications required to produce functional mature mRNA.

**Prerequisites:** [DNA structure and the genetic code](https://www.owlsprep.com/study/cie-9700-u6-dna-structure-genetic-code/); [Nucleic acid base pairing rules](https://www.owlsprep.com/study/cie-9700-u6-nucleic-acid-properties/)

## Learning objectives

- Describe the process of transcription in prokaryotes and eukaryotes
- Distinguish between pre-mRNA and mature mRNA in eukaryotes
- Explain the roles of key molecules involved in transcription
- Outline post-transcriptional modifications in eukaryotes

## Overview of Transcription and Key Components

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.

**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.

## Steps of Transcription in Prokaryotes

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

1. 1. **Initiation**: RNA polymerase binds to the promoter, unwinding the DNA double helix to separate the two strands.
2. 2. **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.
3. 3. **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.

## Transcription in Eukaryotes

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.

**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.

## Post-Transcriptional Modifications

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.

**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: $5' \text{ cap} - E_1-E_2-E_3-E_4 - \text{poly-A tail } 3'$

> **tip**
>
> Remember that introns are non-coding. If introns were not removed, translation would produce a non-functional polypeptide with extra amino acids.

## Common pitfalls

- **Wrong:** Confusing template and coding strands, claiming the coding strand is transcribed.
  - Why it fails: Only the template strand is read by RNA polymerase. The coding strand has the same sequence as mRNA but is never transcribed.
  - Correct: Remember: template = transcribed, coding = not transcribed, mRNA sequence matches coding strand (T→U).
- **Wrong:** Stating mRNA is synthesised in the 3'→5' direction.
  - Why it fails: Like all nucleic acid synthesis, RNA polymerase can only add nucleotides to the 3' end of the growing strand.
  - Correct: RNA polymerase moves along the template strand 3'→5', so mRNA is built 5'→3'.
- **Wrong:** Claiming introns are removed from DNA during transcription.
  - Why it fails: Introns are present in the DNA gene and transcribed into pre-mRNA. They are removed during RNA processing after transcription.
  - Correct: Introns are spliced out of pre-mRNA, not DNA, to produce mature mRNA.
- **Wrong:** Saying transcription occurs in the cytoplasm of eukaryotes.
  - Why it fails: Eukaryotic DNA is stored in the nucleus, so transcription must occur there. Only translation happens in the cytoplasm.
  - Correct: Prokaryotes: transcription + translation both in cytoplasm; Eukaryotes: transcription in nucleus, translation in cytoplasm.
- **Wrong:** Claiming thymine never appears in any RNA molecule.
  - Why it fails: While uracil replaces thymine in mRNA transcripts, thymine can appear in modified functional RNAs like tRNA.
  - Correct: For transcription, remember that DNA adenine pairs with RNA uracil, so mRNA has no thymine from transcription.

## 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 |

## 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.

- [Translation](https://www.owlsprep.com/study/cie-9700-u6-translation/)
- [DNA Replication](https://www.owlsprep.com/study/cie-9700-u6-dna-replication/)
- [The genetic code](https://www.owlsprep.com/study/cie-9700-u6-the-genetic-code/)

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