🎓 Prepared by students from Boğaziçi University

What is Transcription?

Transcription is the process by which the genetic information in a gene's DNA is copied into a messenger RNA (mRNA) molecule. It is the first step of gene expression, turning a DNA blueprint into an RNA message that can be translated into protein.

Short answer

Transcription is the synthesis of mRNA from a DNA template by RNA polymerase, which reads the template strand 3'→5' and builds a complementary RNA strand 5'→3', substituting uracil (U) for thymine (T).

Steps of Transcription

1
Initiation
RNA polymerase binds the gene's promoter region, and the DNA double helix unwinds locally.
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2
Elongation
RNA polymerase moves along the template strand 3'→5', synthesizing mRNA 5'→3' by adding complementary RNA nucleotides (A, U, C, G).
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3
RNA processing (eukaryotes)
A 5' cap and poly-A tail are added, and introns are removed by splicing to form mature mRNA.
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4
Termination
RNA polymerase reaches a terminator sequence, releasing the completed mRNA transcript.
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5
Export
In eukaryotes, mature mRNA exits the nucleus through nuclear pores to reach the ribosomes for translation.

Step-by-step worked examples

A DNA template strand reads 3'-TACCGGATT-5'. What mRNA sequence is transcribed?

RNA polymerase pairs A-U, T-A, C-G, G-C (uracil replaces thymine in RNA).
Template 3'-TACCGGATT-5'
mRNA 5'-AUGGCCUAA-3'

The coding (non-template) strand of a gene reads 5'-ATGGCTAAC-3'. What is the mRNA sequence?

The mRNA has the same sequence as the coding strand, except U replaces T.
Coding strand 5'-ATGGCTAAC-3'
mRNA 5'-AUGGCUAAC-3'

A gene's primary transcript (pre-mRNA) is 1,200 nucleotides long. After splicing removes 350 nucleotides of introns, how long is the mature mRNA (before considering the poly-A tail)?

Mature mRNA length = pre-mRNA length − intron length
= 1,200 − 350
= 850 nucleotides

Flashcards

Quick quiz

Q1.Transcription produces:

Correct answer: B. Transcription copies DNA into a complementary mRNA molecule.

Q2.Which base pairs with adenine in RNA?

Correct answer: B. RNA uses uracil instead of thymine, so it pairs with adenine.

Q3.What removes introns from pre-mRNA?

Correct answer: C. The spliceosome removes introns and joins exons during RNA splicing.

Q4.RNA polymerase reads the DNA template strand in which direction?

Correct answer: B. It reads the template 3'→5' while synthesizing the new mRNA 5'→3'.

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Common mistakes

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Thinking transcription produces a protein directly. — Correct: Transcription only produces mRNA; translation (at the ribosome) makes the protein.

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Forgetting that RNA uses uracil instead of thymine. — Correct: Wherever DNA would pair with thymine, RNA uses uracil instead.

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Assuming the mRNA sequence matches the template strand. — Correct: mRNA matches the coding (non-template) strand's sequence, with U replacing T.

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Believing introns are translated into protein. — Correct: Introns are removed during splicing and never appear in the mature mRNA used for translation.

FAQ

What is transcription in biology?

The process of copying a gene's DNA sequence into a complementary mRNA molecule, carried out by RNA polymerase.

What is the formula or rule for transcription base pairing?

DNA template bases pair with RNA bases as A-U, T-A, C-G, and G-C — RNA uses uracil in place of thymine.

What are examples of transcription and mRNA synthesis?

Every gene expressed in a cell — from insulin in pancreatic cells to hemoglobin in red blood cell precursors — starts with transcription producing its mRNA.

How to determine the mRNA sequence from a DNA template?

Read the DNA template strand 3'→5' and write the complementary RNA bases 5'→3', replacing thymine (T) with uracil (U).