Key ideas
- Template strand. RNA polymerase reads one DNA strand, the template, and builds mRNA that is complementary to it. The mRNA then matches the other DNA strand, the coding strand, with U in place of T.
- RNA base pairing. DNA A pairs with RNA U, DNA T with RNA A, DNA G with RNA C, and DNA C with RNA G.
- Processing (eukaryotes). Before mRNA leaves the nucleus it gets a 5' cap and a poly-A tail, and splicing removes introns, keeping the exons.
- Codons. Each group of 3 mRNA bases is a codon. There are 64 codons: 61 code for amino acids and 3 (UAA, UAG, UGA) are stop signals. AUG codes for methionine and is the start codon.
- tRNA. Each transfer RNA carries one amino acid and has an anticodon that pairs with a codon on the mRNA.
| Transcription | Translation | |
|---|---|---|
| Makes | mRNA from DNA | Protein (a chain of amino acids) from mRNA |
| Where (eukaryotes) | Nucleus | Ribosomes in the cytoplasm or on the rough ER |
| Main machine | RNA polymerase | Ribosome, with tRNAs |
| Reads | The DNA template strand | mRNA codons, 5' to 3' |
| Starts at | A promoter | The start codon AUG |
| Ends at | A termination sequence | A stop codon (UAA, UAG or UGA) |
| Codon | Amino acid | Codon | Amino acid |
|---|---|---|---|
| AUG | Methionine (Met), start | GGC | Glycine (Gly) |
| CCU | Proline (Pro) | UUU | Phenylalanine (Phe) |
| AAG | Lysine (Lys) | GAG | Glutamic acid (Glu) |
| UGG | Tryptophan (Trp) | GUG | Valine (Val) |
| UAA, UAG, UGA | Stop | CAU | Histidine (His) |
Worked examples
Example 1: from template strand to protein
Problem A DNA template strand reads 3'-TAC GGA TTC ACT-5'. Write the mRNA and the amino acid chain.
- Transcribe: pair each template base with its RNA partner (T to A, A to U, C to G, G to C).
- Split into codons from the start codon: AUG, CCU, AAG, UGA.
- Look up each codon: AUG = Met, CCU = Pro, AAG = Lys, UGA = stop.
Answer mRNA 5'-AUGCCUAAGUGA-3', protein Met-Pro-Lys
Example 2: starting from the coding strand
Problem A DNA coding strand reads 5'-ATG TTT GGC TAA-3'. What is the mRNA and the protein?
- The mRNA matches the coding strand, with U instead of T: 5'-AUG UUU GGC UAA-3'.
- Translate: AUG = Met, UUU = Phe, GGC = Gly, UAA = stop.
Answer mRNA 5'-AUGUUUGGCUAA-3', protein Met-Phe-Gly
Example 3: the effect of a point mutation
Problem In a gene, the mRNA codon GAG (glutamic acid) changes to GUG. What happens to the protein? This is the change behind sickle cell disease.
- Look up the new codon: GUG = valine.
- One amino acid changes, glutamic acid to valine, and the rest of the protein is the same. This is a missense mutation.
- In hemoglobin, that single change makes the protein clump when oxygen is low, bending red blood cells into a sickle shape.
Answer Glutamic acid is replaced by valine (a missense mutation)
Common mistakes and how to fix them
- Using T in mRNA. RNA has uracil (U) instead of thymine.
- Translating the DNA directly. The codon chart is for mRNA codons. Fix: transcribe first, then translate.
- Starting at the first base instead of AUG. Fix: find the start codon and read in groups of three from there.
- Translating the stop codon as an amino acid. A stop codon ends the chain. It does not add anything.
- Mixing up codon and anticodon. The codon is on the mRNA. The anticodon is on the tRNA and is complementary to it.
Practice questions
What mRNA is made from the DNA template strand 3'-TACCAT-5'?
- 5'-AUGGUA-3'
- 5'-ATGGTA-3'
- 5'-UACCAU-3'
- 5'-TACCAT-3'
Show answer
Answer: 5'-AUGGUA-3'
Pair each template base with its RNA partner: T to A, A to U, C to G, and so on. RNA uses U, so the ATG choice is DNA, not mRNA.
Translate the mRNA 5'-AUG UGG CAU UAG-3'.
Show answer
Answer: Met-Trp-His
AUG = methionine, UGG = tryptophan, CAU = histidine, and UAG is a stop codon, so the chain ends after histidine.
Which codon is a stop codon?
- AUG
- UGA
- UGG
- GUA
Show answer
Answer: UGA
The three stop codons are UAA, UAG and UGA. AUG is the start codon, and UGG codes for tryptophan.
Where does transcription happen in a eukaryotic cell?
- Nucleus
- Ribosome
- Golgi apparatus
- Cytoplasm
Show answer
Answer: Nucleus
The DNA stays in the nucleus, so the mRNA copy is made there. It is processed and then exported to ribosomes in the cytoplasm.
An mRNA codon is 5'-AAG-3'. What is the tRNA anticodon that pairs with it?
- 3'-UUC-5'
- 3'-AAG-5'
- 3'-TTC-5'
- 3'-GAA-5'
Show answer
Answer: 3'-UUC-5'
The anticodon is complementary and antiparallel: A pairs with U and G with C. tRNA is RNA, so it uses U, not T.
Frequently asked questions
Why does the cell make mRNA instead of using DNA directly?
DNA stays protected in the nucleus, while ribosomes work in the cytoplasm. An mRNA copy can leave the nucleus, and the cell can make many copies of a gene it needs a lot of. The mRNA is also temporary, which lets the cell turn protein making up or down.
What does it mean that the genetic code is redundant?
Most amino acids have more than one codon. For example, glycine is coded by GGU, GGC, GGA and GGG. Because of this, some mutations in the third base of a codon do not change the protein at all. These are called silent mutations.
Is the genetic code the same in all living things?
Almost. The same codons stand for the same amino acids in bacteria, plants and people, which is strong evidence of shared ancestry. A few small exceptions exist, for example in mitochondria. This is also why bacteria can make human insulin from a human gene.
Sources
- OpenStax Biology 2e, 15.1 The Genetic Code, accessed October 1, 2026
- OpenStax Biology 2e, 15.3 Eukaryotic Transcription, accessed October 1, 2026
- OpenStax Biology 2e, 15.4 RNA Processing in Eukaryotes, accessed October 1, 2026
- OpenStax Biology 2e, 15.5 Ribosomes and Protein Synthesis, accessed October 1, 2026
