Side by side
| Transcription | Translation | |
|---|---|---|
| What is made | An mRNA copy of a gene sequence | A polypeptide, a chain of amino acids |
| Template used | A DNA strand | The mRNA produced by transcription |
| Reading unit | One nucleotide added to the mRNA for every nucleotide read on the DNA | Three mRNA nucleotides, a codon, per amino acid added |
| Location in a eukaryotic cell | In the nucleus, after which the transcript is transported to the cytoplasm | On ribosomes in the cytoplasm and on the endoplasmic reticulum |
| Location in a prokaryotic cell | In the cytoplasm, since there is no membrane-bound nucleus | In the cytoplasm, on ribosomes |
| Main machinery | A promoter, a transcription bubble, and the enzymes that bind and unwind the DNA | Ribosomes with large and small subunits, tRNAs charged with amino acids, and enzymatic factors |
| Stages | Initiation, elongation and termination | Initiation, elongation and termination, guided by start and stop codons |
Two steps, two different alphabets
The central dogma states that genes specify the sequences of mRNAs, which in turn specify the sequences of proteins. The first step is the easier of the two, because the alphabets match: copying DNA to mRNA is relatively straightforward, with one nucleotide added to the mRNA strand for every complementary nucleotide read on the DNA. The second step is harder because the alphabets do not match. Four nucleotide letters have to specify 20 amino acids, so groups of three mRNA nucleotides correspond to one amino acid. The correspondence is still systematic and colinear: nucleotides 1 to 3 give amino acid 1, nucleotides 4 to 6 give amino acid 2.
Where students go wrong
The most useful thing to keep straight is the compartment. In eukaryotes the genes are bound in the nucleus, so transcription happens there and the mRNA transcript must be transported to the cytoplasm before it can be translated. Prokaryotes lack a membrane-bound nucleus, so both processes happen in the cytoplasm. A second error is thinking tRNA is a passive delivery service. Specific tRNAs bind sequences on the mRNA template and add the corresponding amino acid, which makes them the molecules that actually translate the language of RNA into the language of proteins, and each must be charged with its correct amino acid first.
Where this leads next
Translation is only doable once you can read a codon chart, since every amino acid added comes from looking up a triplet, so that page is the practical companion to this one. Underneath both sits the molecular difference between DNA and RNA, which is what makes a transcript a transcript rather than a second copy of the gene. Beyond this pair, eukaryotic mRNA is processed before it leaves the nucleus, with splicing and the addition of a 5′ cap and a poly-A tail, which is where gene expression proper begins.
Common mistakes
- Saying transcription happens in the cytoplasm of a eukaryotic cell. The genes are in the nucleus, so the transcript is made there and then exported.
- Reading transcription three nucleotides at a time. It is one for one; the triplet grouping belongs to translation.
- Calling the mRNA the protein. The mRNA is an intermediate; the ribosome builds the polypeptide from it.
- Treating tRNA as a passenger. Charged tRNAs pair with codons and deliver the matching amino acid, which is what makes translation possible.
Related concepts
- DNA vs RNA: DNA and RNA are both polynucleotides, but DNA uses deoxyribose and the bases A, T, G and C, and forms an antiparallel double helix.
- Codon chart: A codon chart maps each three-nucleotide mRNA codon to the amino acid it specifies.
- Prokaryotic vs eukaryotic cells: All cells share four components: a plasma membrane, cytoplasm, DNA and ribosomes.