Transcription and translation

How RNA polymerase copies DNA into RNA, and how the ribosome reads the genetic code to build a protein.

Undergrad

In a nutshell

Every protein in your body is made from instructions stored in DNA, but DNA never leaves the nucleus and is never read directly by the protein-making machinery. Instead, the cell works in two steps. In transcription, an enzyme called RNA polymerase copies a gene into a messenger RNA molecule. In translation, a large molecular machine, the ribosome, reads that message three letters at a time and joins the matching amino acids into a protein chain. Each three-letter codon, such as AUG, specifies one amino acid or a stop signal, and this genetic code is nearly the same in bacteria, plants and people. That universality is what lets scientists put a human gene into bacteria and have them produce human insulin.

Transcription

DNA is copied into RNA by RNA polymerase, which needs no primer and unwinds the duplex itself. It reads the template strand 3′→5′ and builds RNA 5′→3′ from ribonucleoside triphosphates, releasing pyrophosphate at each step; hydrolysis of that pyrophosphate pulls the reaction forward. The transcript therefore matches the coding (non-template) strand, with U in place of T. For example, a coding strand 5′-ATGGCA-3′ gives mRNA 5′-AUGGCA-3′.

Initiation. In bacteria, a \sigma factor directs the polymerase core to a promoter, recognising conserved sequences centred about 10 and 35 base pairs upstream of the start site. Their match to consensus and their spacing set promoter strength. The promoter's orientation defines which strand is the template, so genes on either strand can be transcribed. Elongation proceeds in a moving bubble of about 12–14 unwound base pairs after \sigma is released. Termination occurs at a GC-rich hairpin followed by a run of U (intrinsic), or with the help of the Rho helicase.

In eukaryotes, three polymerases share the work, general transcription factors assemble at promoters such as the TATA box, and the pre-mRNA is processed: a 7-methylguanosine 5′ cap, a poly(A) tail at the 3′ end, and splicing by the spliceosome to remove introns and join exons. Alternative splicing lets one gene encode several proteins.

The genetic code

Four bases read in triplets give 4^3 = 64 codons for 20 amino acids plus stop signals, so the code is degenerate: most amino acids have several codons, typically differing at the third position. AUG codes for methionine and is the usual start codon; UAA, UAG and UGA are stops. A sequence has three possible reading frames on each strand; an open reading frame runs from a start codon to an in-frame stop. Organisms differ in codon usage, which matters when expressing a gene in a foreign host.

Translation

Transfer RNAs carry amino acids and recognise codons through a three-base anticodon. Aminoacyl-tRNA synthetases attach each amino acid to its correct tRNA via an aminoacyl adenylate, at the cost of ATP, and proofread to keep error rates low. Wobble pairing at the codon's third position (e.g. G·U, or inosine in the anticodon) lets one tRNA read several codons.

The ribosome (in bacteria 70S, of 30S and 50S subunits) has A, P and E sites. Initiator tRNA binds the start codon (in bacteria positioned by the Shine–Dalgarno sequence). In each elongation cycle an aminoacyl-tRNA enters the A site, the peptidyl transferase centre, made of rRNA so the ribosome is a ribozyme, catalyses attack of the A-site amino group on the P-site ester, and translocation shifts the tRNAs, all assisted by GTP-hydrolysing elongation factors. A stop codon recruits a release factor that hydrolyses the finished chain. Several ribosomes can translate one mRNA at once (a polysome).

Key points

RNA polymerase reads the template strand 3′→5′ and synthesises RNA 5′→3′ without a primer.
The mRNA matches the coding strand with U for T.
Bacterial promoters are recognised by \sigma factor; eukaryotic pre-mRNA is capped, polyadenylated and spliced.
The triplet code has 64 codons for 20 amino acids and three stops; it is degenerate, with wobble at position three.
The ribosome's rRNA catalyses peptide bond formation; tRNAs move through the A, P and E sites.

Check yourself

5 questions

1Which molecule carries the genetic message from DNA to the ribosome?
Messenger RNA is the transcript that the ribosome translates.
2A coding strand reads 5′-ATGCCT-3′. What is the mRNA sequence?
mRNA has the same sequence as the coding strand, with U replacing T.
3Why can there be several codons for one amino acid?
Redundancy, mostly at the third position, accommodates 64 triplets coding 20 amino acids plus stop.
4What catalyses peptide bond formation in the ribosome?
The catalytic centre is made of rRNA, so the ribosome is a ribozyme.
5How can a single tRNA recognise more than one codon?
Relaxed geometry at the third position allows pairs such as G·U and inosine pairs.

Recent preprints on this topic

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Matched by keyword from ChemRxiv via Crossref. Not peer-reviewed; read critically.