Transcription, Translation & the Central Dogma
Learn how DNA is transcribed into mRNA and translated into protein — codons, AUG, stop codons, and tRNA anticodons — with a full gene-to-protein walkthrough.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on Transcription, Translation & the Central Dogma, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
Every cell in your body carries the same DNA, yet a muscle cell builds different proteins than a pancreas cell does. The instructions get from the gene to the finished protein through a two-step information flow: transcription copies one gene into a messenger RNA, and translation reads that message three bases at a time to build a chain of amino acids.
In this lesson you will learn where each step happens, which enzymes and molecules do the work, and how to actually trace a written DNA sequence all the way to a protein. By the end you should be able to take a strand of DNA, write its mRNA, break the mRNA into codons, look each codon up in a codon chart, and name the tRNA anticodons that deliver the amino acids. That skill is the backbone of the next lesson on mutations, because you cannot predict what a mutation does until you can read the code correctly.
In this lesson you will learn where each step happens, which enzymes and molecules do the work, and how to actually trace a written DNA sequence all the way to a protein. By the end you should be able to take a strand of DNA, write its mRNA, break the mRNA into codons, look each codon up in a codon chart, and name the tRNA anticodons that deliver the amino acids. That skill is the backbone of the next lesson on mutations, because you cannot predict what a mutation does until you can read the code correctly.
The Central Dogma: One-Way Information Flow
The central dogma of molecular biology summarizes how genetic information moves in a cell:DNA is the permanent, protected master copy. It stays in the nucleus of a eukaryotic cell and is never shipped out to the cytoplasm. Instead, the cell makes a short, disposable RNA copy of just the gene it needs at that moment. That copy, messenger RNA (mRNA), leaves through a nuclear pore and is read by a ribosome.
Why bother with a messenger? Three reasons. First, DNA is far too large and too valuable to risk in the cytoplasm, where enzymes and mechanical stress could damage it. Second, one gene can be transcribed into many mRNA copies, so the cell can amplify production of a needed protein quickly. Third, mRNA is short-lived, which lets the cell shut off production simply by stopping transcription.
A common misconception is that a gene is a protein, or that DNA directly grabs amino acids. It does not. DNA stores a sequence of bases; that sequence only becomes a protein after being rewritten in RNA and then decoded by a ribosome.
Compare the two information-carrying molecules:
Because RNA uses uracil instead of thymine, every T in the DNA template pairs with A, and every A in the template becomes U in the RNA.
Why bother with a messenger? Three reasons. First, DNA is far too large and too valuable to risk in the cytoplasm, where enzymes and mechanical stress could damage it. Second, one gene can be transcribed into many mRNA copies, so the cell can amplify production of a needed protein quickly. Third, mRNA is short-lived, which lets the cell shut off production simply by stopping transcription.
A common misconception is that a gene is a protein, or that DNA directly grabs amino acids. It does not. DNA stores a sequence of bases; that sequence only becomes a protein after being rewritten in RNA and then decoded by a ribosome.
Compare the two information-carrying molecules:
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Strands | Double helix | Usually single |
| Bases | A, T, C, G | A, U, C, G |
| Location | Nucleus | Made in nucleus, works in cytoplasm |
| Lifespan | Permanent | Temporary |
Transcription: Writing the mRNA Copy
Transcription happens in the nucleus and is carried out by the enzyme RNA polymerase. The steps are straightforward once you know what the enzyme is looking for.
Initiation: RNA polymerase binds to a promoter, a specific DNA sequence just upstream of the gene. The promoter tells the enzyme where to start and which of the two DNA strands to read. The enzyme then unwinds and separates a short stretch of the double helix.
Elongation: RNA polymerase moves along the template strand and adds RNA nucleotides that are complementary to it. Pairing rules are the same as in DNA replication with one substitution: template A pairs with U, template T pairs with A, template C pairs with G, and template G pairs with C. Notice that the growing mRNA therefore has the same sequence as the non-template (coding) strand, except U replaces T.
Termination: When the polymerase reaches a terminator sequence, it releases the finished RNA and the DNA rezips into a double helix. Nothing about the DNA is used up — the same gene can be transcribed again immediately.
In eukaryotes the new RNA is processed before it leaves: non-coding introns are cut out, the coding exons are spliced together, and protective caps and a poly-A tail are added. Only then is it mature mRNA ready to exit the nucleus.
The most common student error here is copying the template strand instead of complementing it. If the template reads TAC, the mRNA is AUG, not AUC or UAC. Write the pairs vertically underneath each other until the habit sticks.
Initiation: RNA polymerase binds to a promoter, a specific DNA sequence just upstream of the gene. The promoter tells the enzyme where to start and which of the two DNA strands to read. The enzyme then unwinds and separates a short stretch of the double helix.
Elongation: RNA polymerase moves along the template strand and adds RNA nucleotides that are complementary to it. Pairing rules are the same as in DNA replication with one substitution: template A pairs with U, template T pairs with A, template C pairs with G, and template G pairs with C. Notice that the growing mRNA therefore has the same sequence as the non-template (coding) strand, except U replaces T.
Termination: When the polymerase reaches a terminator sequence, it releases the finished RNA and the DNA rezips into a double helix. Nothing about the DNA is used up — the same gene can be transcribed again immediately.
In eukaryotes the new RNA is processed before it leaves: non-coding introns are cut out, the coding exons are spliced together, and protective caps and a poly-A tail are added. Only then is it mature mRNA ready to exit the nucleus.
The most common student error here is copying the template strand instead of complementing it. If the template reads TAC, the mRNA is AUG, not AUC or UAC. Write the pairs vertically underneath each other until the habit sticks.
The Genetic Code: Reading Codons Three at a Time
An mRNA is read in blocks of three bases called codons. Each codon specifies one amino acid, or tells the ribosome to stop. With four possible bases in three positions there are codons, but only about 20 amino acids are used in proteins. That means the code is redundant (also called degenerate): several different codons can code for the same amino acid. For example, GGA, GGU, GGC, and GGG all specify glycine.
Redundancy does not mean the code is ambiguous. One codon never codes for two different amino acids, so a given mRNA always produces the same protein.
Three features anchor the reading:
The start codon AUG sets the reading frame. Bases before the AUG are not translated, and the ribosome counts off groups of three from the A of AUG onward. This is why grouping matters so much: the mRNA AUGCAUUGG read from the start codon gives AUG-CAU-UGG, but if you accidentally begin one base later you get UGC-AUU-GG and a completely different, meaningless answer.
When you use a codon chart, always read the mRNA codon, never the DNA and never the anticodon. Standard charts are written for mRNA, so plugging in a tRNA anticodon gives the wrong amino acid — a frequent mistake on homework.
Redundancy does not mean the code is ambiguous. One codon never codes for two different amino acids, so a given mRNA always produces the same protein.
Three features anchor the reading:
| mRNA codon | Meaning |
|---|---|
| AUG | Methionine; also the start codon |
| UAA, UAG, UGA | Stop; no amino acid added |
| UUC | Phenylalanine |
| GGA | Glycine |
| AAG | Lysine |
| CAU | Histidine |
| UGG | Tryptophan |
When you use a codon chart, always read the mRNA codon, never the DNA and never the anticodon. Standard charts are written for mRNA, so plugging in a tRNA anticodon gives the wrong amino acid — a frequent mistake on homework.
Translation: Building the Polypeptide at the Ribosome
Translation happens at a ribosome, either floating free in the cytoplasm or attached to the rough endoplasmic reticulum. The ribosome is built of ribosomal RNA and protein, and it holds the mRNA while transfer RNA (tRNA) molecules deliver amino acids.
Each tRNA is a folded single strand with two important ends: an anticodon of three bases at one end, and a specific amino acid attached at the other. The anticodon is complementary to the mRNA codon it reads. If the codon is AUG, the tRNA carrying methionine has the anticodon UAC.
Initiation: The ribosome assembles on the mRNA and locates the start codon AUG. The first tRNA, carrying methionine, base-pairs with it. Every polypeptide therefore begins with methionine (it is sometimes removed later).
Elongation: A second tRNA whose anticodon matches the next codon binds in the adjacent site. The ribosome forms a peptide bond between the two amino acids, releases the now-empty first tRNA, and shifts three bases down the mRNA. The empty tRNA is recharged with a new amino acid and reused. This cycle repeats, adding one amino acid per codon.
Termination: When a stop codon (UAA, UAG, or UGA) enters the ribosome, no tRNA matches it. The ribosome releases the finished polypeptide, which folds into its functional three-dimensional shape.
Two things students often get wrong: stop codons do not code for an amino acid, so do not write one in your protein chain, and tRNA is not consumed — it is a reusable shuttle, not a building block of the protein.
Each tRNA is a folded single strand with two important ends: an anticodon of three bases at one end, and a specific amino acid attached at the other. The anticodon is complementary to the mRNA codon it reads. If the codon is AUG, the tRNA carrying methionine has the anticodon UAC.
Initiation: The ribosome assembles on the mRNA and locates the start codon AUG. The first tRNA, carrying methionine, base-pairs with it. Every polypeptide therefore begins with methionine (it is sometimes removed later).
Elongation: A second tRNA whose anticodon matches the next codon binds in the adjacent site. The ribosome forms a peptide bond between the two amino acids, releases the now-empty first tRNA, and shifts three bases down the mRNA. The empty tRNA is recharged with a new amino acid and reused. This cycle repeats, adding one amino acid per codon.
Termination: When a stop codon (UAA, UAG, or UGA) enters the ribosome, no tRNA matches it. The ribosome releases the finished polypeptide, which folds into its functional three-dimensional shape.
Two things students often get wrong: stop codons do not code for an amino acid, so do not write one in your protein chain, and tRNA is not consumed — it is a reusable shuttle, not a building block of the protein.
Key terms
- Transcription.
- The process in the nucleus in which RNA polymerase uses one DNA strand as a template to build a complementary mRNA molecule.
- Translation.
- The process at the ribosome in which the codons of an mRNA are decoded to assemble a specific chain of amino acids.
- Codon.
- A group of three consecutive mRNA bases that specifies one amino acid or a stop signal.
- Anticodon.
- The three-base sequence on a tRNA that pairs with a complementary mRNA codon, ensuring the correct amino acid is delivered.
- Start codon (AUG).
- The codon that signals the ribosome where to begin translating; it sets the reading frame and codes for methionine.
- Stop codon.
- UAA, UAG, or UGA — codons that no tRNA reads, causing the ribosome to release the completed polypeptide.
- RNA polymerase.
- The enzyme that binds a promoter, separates DNA strands, and links RNA nucleotides complementary to the template strand.
- Template strand.
- The DNA strand that RNA polymerase reads; the mRNA produced is complementary to it, with U in place of T.
Worked example
A gene has the DNA template strand 3'-TAC AAG CCT TTC ATT-5'. Write the mRNA transcript, list the codons, give the tRNA anticodon for each amino-acid-carrying codon, and name the resulting amino acid sequence. Use the codon information from this lesson.
Step 1 — Transcribe. Pair each template base with its RNA partner: A with U, T with A, C with G, G with C.
Template: T A C A A G C C T T T C A T T mRNA: A U G U U C G G A A A G U A A
So the mRNA reads 5'-AUG UUC GGA AAG UAA-3'.
Step 2 — Find the start codon and set the reading frame. The first codon is AUG, so translation begins right there and the codons are already grouped correctly: AUG, UUC, GGA, AAG, UAA.
Step 3 — Look up each codon. AUG is methionine (start). UUC is phenylalanine. GGA is glycine. AAG is lysine. UAA is a stop codon, so it adds no amino acid and ends translation.
Step 4 — Give the anticodons. An anticodon is complementary to its codon, using RNA bases. AUG is read by UAC, UUC by AAG, GGA by CCU, and AAG by UUC. The stop codon UAA has no tRNA.
Step 5 — Write the protein. The polypeptide is methionine–phenylalanine–glycine–lysine, four amino acids long. Check your work: five codons, one of which is a stop, gives four amino acids. If you wrote five, you incorrectly translated the stop codon.
Template: T A C A A G C C T T T C A T T mRNA: A U G U U C G G A A A G U A A
So the mRNA reads 5'-AUG UUC GGA AAG UAA-3'.
Step 2 — Find the start codon and set the reading frame. The first codon is AUG, so translation begins right there and the codons are already grouped correctly: AUG, UUC, GGA, AAG, UAA.
Step 3 — Look up each codon. AUG is methionine (start). UUC is phenylalanine. GGA is glycine. AAG is lysine. UAA is a stop codon, so it adds no amino acid and ends translation.
Step 4 — Give the anticodons. An anticodon is complementary to its codon, using RNA bases. AUG is read by UAC, UUC by AAG, GGA by CCU, and AAG by UUC. The stop codon UAA has no tRNA.
Step 5 — Write the protein. The polypeptide is methionine–phenylalanine–glycine–lysine, four amino acids long. Check your work: five codons, one of which is a stop, gives four amino acids. If you wrote five, you incorrectly translated the stop codon.
Practice questions
An mRNA codon reads GCA. Which tRNA anticodon will pair with it?
- CGU
- GCA
- CGT
- GCU
Answer: CGU
An anticodon is complementary to the codon and is made of RNA, so pair G with C, C with G, and A with U, giving CGU. GCA simply repeats the codon. CGT contains thymine, which RNA does not use. GCU changes only one base and is not complementary at all. Remember that the amino acid (alanine, in this case) is looked up from the codon GCA, not from the anticodon.
An mRNA strand reads 5'-CCAUGCAUUGGUGAAC-3'. Determine the amino acid sequence of the protein it encodes and explain how you chose where to begin.
Answer: Methionine–histidine–tryptophan (translation begins at the AUG and ends at UGA).
Translation does not begin at the very first base. Scan for the start codon: the first AUG appears after the leading CC. From the A of that AUG, count off groups of three: AUG, CAU, UGG, UGA. AUG is methionine, CAU is histidine, UGG is tryptophan, and UGA is a stop codon that ends the process, so the trailing AC is never read. The protein is three amino acids long. Students who start counting at the first base get CCA-UGC-AUU-GGU-GAA-C, a completely different and incorrect chain — this is exactly why the start codon matters: it sets the reading frame.
Which statement correctly describes where each step of the central dogma occurs in a eukaryotic cell?
- Both transcription and translation occur in the nucleus.
- Transcription occurs in the nucleus; translation occurs at ribosomes in the cytoplasm.
- Transcription occurs at the ribosome; translation occurs in the nucleus.
- DNA leaves the nucleus and is read directly by the ribosome.
Answer: Transcription occurs in the nucleus; translation occurs at ribosomes in the cytoplasm.
DNA stays in the nucleus, so the copy step (transcription) must happen there. The mRNA then exits through a nuclear pore to a ribosome, where translation builds the polypeptide. The final choice describes a common misconception: DNA never leaves the nucleus, and ribosomes read mRNA, not DNA.
FAQ
- Why does RNA use uracil instead of thymine?
- Uracil pairs with adenine just as thymine does, so it carries the same information. Thymine is chemically uracil with a methyl group added, which makes DNA more stable and easier for repair enzymes to proofread. Because mRNA is a short-lived working copy rather than permanent storage, the cheaper, less stable uracil is sufficient. For your work, this just means every A in the DNA template becomes U in the mRNA.
- What is the difference between a codon and an anticodon?
- A codon is three bases on the mRNA; an anticodon is the complementary three bases on a tRNA. They base-pair with each other inside the ribosome. Codon charts are written for mRNA codons, so always look up the codon, not the anticodon, when naming an amino acid.
- Does every protein really start with methionine?
- Every polypeptide is initiated at an AUG start codon, so methionine is the first amino acid placed. Many finished proteins, however, no longer show it, because enzymes trim the leading methionine after folding. On classroom problems, include methionine as the first amino acid unless the question says otherwise.
- How do mRNA, tRNA, and rRNA differ?
- mRNA carries the coded message from the gene to the ribosome. tRNA is the shuttle: its anticodon reads one codon and its other end carries the matching amino acid. rRNA, together with proteins, forms the ribosome itself and helps catalyze the peptide bonds. All three are transcribed from DNA, but only mRNA is translated.
Learn this with a teacher, not a page
The Crimsora tutor teaches Transcription, Translation & the Central Dogma live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.