Concepts of Biology · Molecular Biology

Transcription

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On this page 9 sections

In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

is the first step of gene expression: a cell copies the DNA sequence of a single gene into a molecule of RNA. It is the "copy the recipe onto a note card" step of the central dogma (DNA → RNA → protein). The enzyme reads one strand of the gene — the — and builds a complementary RNA copy, always in the 5′ to 3′ direction, using uracil (U) in place of thymine (T). Transcription proceeds in three stages — initiation, elongation, and termination — and in eukaryotic cells the raw product is processed into mature messenger RNA (mRNA) before it leaves the nucleus. Because cells control most gene expression at the level of transcription, this process is the key to understanding how cells decide which genes to use.

Why this matters

Gene control: Most decisions about which genes to express — and how strongly — are made at transcription (see How Genes Are Regulated).
Medicine: Antibiotics such as rifampin inhibit bacterial RNA polymerase, and drugs that alter transcription are used to treat cancer. Understanding RNA biology underlies mRNA vaccines and therapies.
Exams: Transcription is tested constantly because it combines sequence logic (template vs. , U for T), enzyme knowledge, and a clear prokaryote/eukaryote comparison.

The college version

Core Concepts

Genes are transcribed selectively

A gene is a stretch of DNA carrying instructions for one RNA product. During transcription, only one of the two strands of that gene is read — the template strand (read 3′ to 5′). The other strand, the coding strand, is not read; its sequence matches the RNA sequence (with U substituted for T). So the RNA is complementary to the template strand and identical to the coding strand — a distinction that shows up constantly in sequence-based exam questions.

Promoters: where transcription starts

RNA polymerase does not bind just anywhere. It attaches to a , a specific DNA sequence upstream of the gene, often with the help of accessory proteins (sigma factors in bacteria; general transcription factors in eukaryotes). The promoter determines where transcription begins and how strongly a gene is expressed. The bacterial −10/−35 elements and the eukaryotic TATA box are well-known examples — but many promoters lack them, so treat them as common patterns, not universal rules.

RNA polymerase builds RNA 5′ to 3′ without a primer

Unlike DNA polymerase, RNA polymerase can start a new strand from scratch — no primer needed. It adds ribonucleotides complementary to the template and rewinds the DNA behind it. Bacteria have one main RNA polymerase; eukaryotes have three: RNA polymerase I (ribosomal RNA), II (mRNA), and III (tRNA and other small RNAs).

The three stages: initiation, elongation, termination

Initiation: RNA polymerase binds the promoter with accessory proteins, unwinds a short stretch of DNA, and begins synthesis.
Elongation: The polymerase moves along the template, adding ribonucleotides 5′ to 3′; the growing RNA peels away as the DNA behind re-winds.
Termination: A stop signal ends transcription — in bacteria, commonly an RNA hairpin followed by a run of U's (rho-independent) or the rho protein; in eukaryotes, termination is coupled to processing of the RNA's 3′ end, such as the polyadenylation signal.

RNA processing in eukaryotes: primary transcript to mRNA

Bacterial mRNA is used almost immediately, often while transcription is still in progress. Eukaryotic mRNA is made in the nucleus and processed before export:

5′ cap: A modified guanine added to the front. It protects the RNA from degradation and helps ribosomes attach during translation.
3′ poly-A tail: A string of adenines added to the end. It stabilizes the mRNA and aids nuclear export.
: Most eukaryotic genes contain introns (noncoding segments) interrupted by exons (coding segments). The spliceosome removes introns and joins exons. Alternative splicing lets one gene produce several different mRNAs — and proteins — by joining exons in different combinations, a major reason we make more proteins than we have genes.

Types of RNA

Transcription produces the whole family of RNAs: mRNA carries protein-coding instructions; tRNA delivers amino acids during translation; rRNA is a structural and catalytic component of ribosomes; and other classes (such as microRNAs) regulate gene expression.

How It Works / Step-by-Step Process

Initiation: RNA polymerase (with accessory proteins) binds the promoter and unwinds a short stretch of DNA.
Elongation: It moves along the template, adding complementary ribonucleotides 5′→3′; the DNA behind re-winds.
Termination: A termination signal ends synthesis and releases the RNA.
Processing (eukaryotes): A 5′ cap and poly-A tail are added, introns are spliced out, and the mature mRNA is exported for translation.

Common Confusions

Do Not Confuse	With	Difference
Template strand	Coding strand	Template is read into complementary RNA; coding strand matches the RNA (U for T)
Transcription	Translation	Transcription makes RNA from DNA; translation makes protein from RNA
DNA polymerase	RNA polymerase	DNA→DNA, needs a primer, proofreads; RNA polymerase makes RNA, needs no primer
Intron	Exon	Introns are removed during splicing; exons are kept and joined
mRNA	tRNA / rRNA	mRNA carries the code; tRNA delivers amino acids; rRNA is part of the ribosome
Bacterial mRNA	Eukaryotic mRNA	Bacterial mRNA is used immediately and unprocessed; eukaryotic mRNA is capped, tailed, and spliced in the nucleus

Eli explains

The same idea, in plain words

Explain it like I’m 10

A gene is like one recipe on a page in a giant cookbook. Transcription is when the cell copies just that one recipe onto a sticky note (the RNA) so it can be carried to the kitchen counter (the ribosome) without lugging the whole book. In our cells, the sticky note even gets a protective cover and a handle added before it leaves the cookbook shelf (the nucleus).

Worked example

Pancreas cells must transcribe the insulin gene to make insulin. RNA polymerase II, guided by transcription factors, binds the insulin gene's promoter in the nucleus and moves along the template strand adding complementary ribonucleotides: where the coding strand reads A–T–G, the RNA contains A–U–G. The polymerase passes through introns and exons alike, producing a long primary transcript. The spliceosome cuts out the introns and joins the exons; the cell adds a 5′ cap and poly-A tail. The finished insulin mRNA exits through a nuclear pore to the cytoplasm, where ribosomes translate it into the insulin protein (see Translation). A skin cell, by contrast, keeps the insulin gene tightly shut — transcription is the switch that decides whether the recipe is ever read at all.

Key takeaways

Central dogma: DNA → RNA (transcription) → protein (translation).
RNA polymerase reads the template strand 3′→5′ and builds RNA 5′→3′; RNA uses U instead of T.
The coding strand matches the RNA sequence; the template strand is complementary to it.
Promoter = start signal; termination signals stop transcription.
RNA polymerase needs no primer (unlike DNA polymerase).
Prokaryotes: one RNA polymerase, no mRNA processing; transcription and translation happen together in the cytoplasm.
Eukaryotes: RNA polymerase II makes mRNA, which gets a 5′ cap, poly-A tail, and splicing (introns removed).
Alternative splicing → many proteins from one gene.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

Which DNA strand does RNA polymerase read, and in which direction is RNA synthesized?
Show answer
It reads the template strand 3′→5′ and synthesizes RNA 5′→3′.
How does the RNA sequence compare to the coding strand of the DNA?
Show answer
The RNA is complementary to the template strand and identical to the coding strand, except uracil (U) replaces thymine (T).
Why doesn't RNA polymerase need a primer?
Show answer
RNA polymerase can add the first ribonucleotide on its own; it does not need a free 3′ OH provided by a primer, unlike DNA polymerase.
List three processing steps that convert a eukaryotic primary transcript into mature mRNA.
Show answer
(1) Addition of the 5′ cap; (2) addition of the poly-A tail; (3) splicing — removal of introns and joining of exons.
What is alternative splicing, and what does it allow a cell to do?
Show answer
Alternative splicing joins exons in different combinations, so one gene can produce multiple different mRNAs and therefore multiple different proteins.
How does transcription differ between bacteria and eukaryotes?
Show answer
Bacteria have one RNA polymerase and no mRNA processing, and translation can begin while transcription is still happening; eukaryotes have three RNA polymerases (II makes mRNA), and the mRNA must be capped, tailed, and spliced in the nucleus before export.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Transcription: Copying a gene's DNA sequence into RNA
RNA polymerase: Enzyme that builds RNA from a DNA template
Template strand: The DNA strand read (3′→5′) to build complementary RNA
Coding strand: The non-template strand matching the RNA sequence (U for T)
Promoter: DNA sequence upstream of a gene where RNA polymerase binds
Intron / exon: Noncoding segment removed during processing / coding segment retained
5′ cap / poly-A tail: Modified guanine at the front / adenines at the end of mRNA
Splicing: Removal of introns and joining of exons by the spliceosome

Sources & references

openstax.org — Concepts Biology

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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