Biology · Introductory biology · Concept
Transcription and translation: DNA to protein
A gene’s information flows from DNA to RNA to protein. In transcription, RNA polymerase reads the template strand of a gene and builds a complementary messenger RNA from its 5′ end to its 3′ end, so the mRNA matches the coding strand with U in place of T. In translation, a ribosome reads the mRNA one codon, three bases, at a time from the start codon AUG; each tRNA brings the amino acid that matches the codon it pairs with, until a stop codon releases the finished protein.
From gene to protein
Most genes encode proteins. The DNA sequence is copied into RNA, and the RNA’s codons set the order of amino acids in the protein. Both steps rely on base pairing: in DNA, A pairs with T and G with C; in RNA, U takes the place of T.
Transcription
RNA polymerase binds a promoter at the start of a gene and unwinds the DNA. It reads the template strand from 3′ to 5′ and joins RNA nucleotides from 5′ to 3′, so the new RNA is complementary to the template and has the same sequence as the coding strand, with U for T. In eukaryotes the RNA is processed in the nucleus before it leaves: a cap is added at the 5′ end, a poly-A tail at the 3′ end, and introns are spliced out.
| Strand | Sequence |
|---|---|
| Coding strand (DNA) | 5′-ATG TTC GGA TGG TAA-3′ |
| Template strand (DNA) | 3′-TAC AAG CCT ACC ATT-5′ |
| mRNA | 5′-AUG UUC GGA UGG UAA-3′ |
| Protein | Met – Phe – Gly – Trp – stop |
The genetic code
A codon is three bases, so the four RNA bases make 64 codons. Of these, 61 code for amino acids and 3, UAA, UAG and UGA, are stop codons. AUG codes for methionine and also marks where translation starts. The code is redundant: most amino acids have several codons, which often differ only in the third base. It is also nearly universal, read the same way in bacteria, plants and people.
Translation
A ribosome assembles on the mRNA at the start codon. Each transfer RNA carries one amino acid and has an anticodon, three bases that pair with a codon. The ribosome joins each arriving amino acid to the growing chain with a peptide bond and moves along the mRNA one codon at a time, from 5′ to 3′. At a stop codon no tRNA matches; a release factor binds instead and the protein is freed.
Mutations change the message
A substitution changes one base. It is silent, or synonymous, when the new codon codes for the same amino acid; missense when it codes for a different one; and nonsense when it makes an early stop codon. Inserting or deleting bases in a number that is not a multiple of three shifts the reading frame, so every codon after it changes: a frameshift.
| Mutation | mRNA | Protein |
|---|---|---|
| None | AUG GCC AAA UGA | Met – Ala – Lys |
| Silent | AUG GCU AAA UGA | Met – Ala – Lys |
| Missense | AUG GCC GAA UGA | Met – Ala – Glu |
| Nonsense | AUG GCC UAA UGA | Met – Ala |
| Frameshift (one G inserted) | AUG GGC CAA AUG A… | Met – Gly – Gln – Met … |
Common mistakes
- Writing the mRNA as the complement of the coding strand: the mRNA matches the coding strand, with U for T, and is complementary to the template strand.
- Reading codons from the first base of a sequence: translation starts at the start codon, AUG.
- Translating a stop codon as an amino acid: UAA, UAG and UGA end translation.
- Expecting every substitution to change the protein: a change in the third base often gives a codon for the same amino acid.
- Pairing the tRNA anticodon with DNA: anticodons pair with mRNA codons.
Key terms
- Transcription
- Making an RNA copy of a gene’s DNA. RNA polymerase reads the template strand, so the RNA matches the coding strand, with U in place of T.
- Translation
- Building a protein from messenger RNA at a ribosome. From the start codon (AUG), each three-base codon adds one amino acid, until a stop codon ends the chain.
- Template strand
- The DNA strand that RNA polymerase reads to build the RNA. The RNA is complementary and antiparallel to it, so its bases pair with the template’s (A with U, C with G).
- Coding strand
- The DNA strand whose sequence matches the RNA transcript, except DNA uses T where RNA uses U. It is opposite the template strand.
- RNA polymerase
- The enzyme that builds RNA in transcription. It binds a promoter, reads the template strand from 3′ to 5′ and joins RNA nucleotides from 5′ to 3′, needing no primer.
- Messenger RNA
- The RNA copy of a gene that carries its coding sequence to the ribosome, where it is read 5′ to 3′, one three-base codon at a time.
- RNA splicing
- Cutting the introns out of a pre-mRNA and joining its exons. Joining exons in different combinations (alternative splicing) lets one gene make several proteins.
- Codon
- Three consecutive mRNA bases that stand for one amino acid or a stop signal: AUG codes for methionine and starts translation, UAA means stop.
- Anticodon
- The three bases of a tRNA that pair with an mRNA codon, complementary and antiparallel to it. They decide which codon a tRNA reads.
- Transfer RNA
- An RNA that carries one amino acid to the ribosome. Its anticodon pairs with an mRNA codon, so each amino acid joins the chain in the order the codons set.
- Start codon
- The codon where translation begins, usually AUG, which also codes for methionine. An AUG later in the gene simply adds another methionine.
- Stop codon
- A codon that ends translation: UAA, UAG or UGA in the standard code. It adds no amino acid; the finished chain is released.
- Genetic code
- The mapping from codons to amino acids or termination signals. The standard code is widely shared, but some organisms and organelles use variants.
- Reading frame
- The starting offset used to divide a sequence into consecutive triplets. Shifting the offset changes which bases form each codon.
- Synonymous substitution
- A substitution that changes a codon but not its amino acid, such as GCU to GCC (both alanine). It is also called a silent mutation.
- Missense mutation
- A substitution that changes one amino acid, such as GAG to GUG, glutamate to valine in sickle-cell hemoglobin. The effect depends on where it falls in the protein.
- Nonsense mutation
- A substitution that turns an amino-acid codon into a stop codon, so the protein ends early and usually cannot work.
- Frameshift
- An insertion or deletion of a number of bases that is not a multiple of three. Every codon after it shifts, which changes the amino acids and often reaches an early stop.
Work through an example
The coding strand of a short gene reads 5′-ATG TTC GGA TGG TAA-3′. Write the template strand and the mRNA, and translate the mRNA. A substitution then changes the G at position 12 to A. What happens to the protein, and what kind of mutation is it?
Translate a gene and test a mutation →Sources and scope
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