🎓 Prepared by students from Boğaziçi University

What Is Transcription?

Transcription is the process by which the enzyme RNA polymerase copies a gene's DNA sequence into a complementary messenger RNA (mRNA) strand. It is the first step of gene expression, converting genetic information stored in DNA into a message that ribosomes can later translate into protein.

Short answer

Transcription is the process where RNA polymerase reads a DNA template strand and synthesizes a complementary mRNA molecule, using base pairing (A-U, T-A, G-C, C-G).

Steps of Transcription

1
Initiation
RNA polymerase binds the promoter region and unwinds the DNA
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2
Elongation
RNA polymerase reads the template strand 3'→5' and builds mRNA 5'→3'
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3
Termination
RNA polymerase reaches a terminator sequence and releases the pre-mRNA
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4
RNA Processing
A 5' cap and poly-A tail are added, and introns are spliced out

Try it: interactive calculator

Mature mRNA length

1,000nt

= 1,500 - 500

Pre-mRNA (gene) length1,500 ntTotal intron length removed500 nt

Step-by-step worked examples

A gene's pre-mRNA transcript is 1,500 nucleotides long. If introns totaling 500 nucleotides are spliced out, how long is the mature mRNA?

M = L − I
= 1500 − 500
= 1000 nucleotides

The DNA template strand reads 3'-TACGGCTA-5'. What is the mRNA sequence transcribed from it?

RNA polymerase reads 3'→5' on the template and synthesizes 5'→3'
Base pairing: T→A, A→U, C→G, G→C, G→C, C→G, T→A, A→U
mRNA (5'→3') = AUGCCGAU

A mature mRNA is 900 nucleotides long, including a 150-nucleotide untranslated region (UTR). How many nucleotides code for the protein, and roughly how many amino acids can it encode?

Coding length = total mRNA − UTR length
= 900 − 150 = 750 nucleotides
750 ÷ 3 = 250 codons
So the protein has up to about 250 amino acids

Flashcards

Quick quiz

Q1.Which enzyme performs transcription?

Correct answer: B. RNA polymerase synthesizes mRNA from the DNA template strand.

Q2.In RNA, which base pairs with adenine?

Correct answer: B. RNA uses uracil instead of thymine to pair with adenine.

Q3.What is removed from pre-mRNA during splicing?

Correct answer: B. Introns are non-coding sequences cut out, leaving exons joined together.

Q4.Where does transcription occur in eukaryotic cells?

Correct answer: C. Transcription happens in the nucleus; translation happens later in the cytoplasm.

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Common mistakes

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Transcription and translation are the same process. — Correct: Transcription makes mRNA from DNA; translation makes protein from mRNA — two separate steps.

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RNA polymerase reads the coding (sense) strand. — Correct: RNA polymerase reads the template (antisense) strand to build mRNA matching the coding strand, with U instead of T.

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All of the pre-mRNA becomes mature mRNA. — Correct: Introns are spliced out; only exons remain in the mature, translatable mRNA.

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Transcription happens in the cytoplasm in eukaryotes. — Correct: In eukaryotes, transcription occurs in the nucleus; translation happens in the cytoplasm.

FAQ

What is transcription in biology?

It's the process of copying a gene's DNA sequence into a complementary mRNA molecule.

What is the transcription formula for mRNA length?

Mature mRNA length = pre-mRNA length − total intron length removed during splicing.

What are examples of transcription products?

Messenger RNA (mRNA), as well as tRNA and rRNA, all transcribed by RNA polymerase from DNA.

How do you calculate mRNA length from a gene?

Subtract the total length of spliced-out introns from the pre-mRNA (primary transcript) length.