From Gene to Protein

The information stored in a gene is expressed in two steps: transcription copies the DNA into RNA, and translation reads that RNA in three-base codons to assemble a protein.

Definition

Gene expression is the process by which a gene's nucleotide sequence is transcribed into RNA and, for protein-coding genes, translated into a polypeptide whose amino acid sequence is specified by codons in the messenger RNA.

Scope

This topic covers transcription and the synthesis of messenger RNA, RNA processing in eukaryotes including splicing, capping, and polyadenylation, the structure and near-universality of the genetic code with its triplet codons, the role of transfer RNA and the ribosome in translation, and the stages of polypeptide synthesis. It traces the canonical gene-expression pathway; the regulation of when and how much a gene is expressed is treated in gene regulation.

Core questions

• How does RNA polymerase transcribe a gene, and how is the transcript processed in eukaryotes?
• What properties of the genetic code allow sixty-four codons to specify twenty amino acids and stop signals?
• How do transfer RNAs and the ribosome translate a codon sequence into a protein?
• Why is the genetic code described as degenerate and nearly universal?

Key concepts

• Transcription and messenger RNA
• RNA processing: splicing, capping, polyadenylation
• The triplet genetic code and codon-anticodon pairing
• Transfer RNA, ribosomes, and translation
• Degeneracy and near-universality of the code

Mechanisms

RNA polymerase synthesizes a complementary RNA copy of the template strand; in eukaryotes the primary transcript is capped, spliced to remove introns, and polyadenylated; the mature messenger RNA is then read codon by codon at the ribosome, where transfer RNAs deliver amino acids matched to each codon by anticodon pairing, building the polypeptide from start to stop codon.

Clinical relevance

Understanding expression explains how mutations in coding and splice sites cause disease, underlies messenger-RNA vaccines and antisense therapeutics, and grounds the interpretation of how a given sequence change alters a protein product.

History

Crick articulated the adaptor hypothesis and the central dogma in the late 1950s, Nirenberg and Khorana cracked the genetic code in the early 1960s by assigning codons to amino acids, and the discovery of split genes and RNA splicing in 1977 added a processing step unique to eukaryotes.

Key figures

• Francis Crick
• Marshall Nirenberg
• Har Gobind Khorana
• Sydney Brenner

Seminal works

• crick1958

Frequently asked questions

What does it mean that the genetic code is degenerate?

It means most amino acids are specified by more than one codon, so several different triplets can encode the same amino acid; this redundancy buffers many single-base changes against altering the protein.

What is the difference between transcription and translation?

Transcription copies a gene's DNA into a complementary RNA molecule, while translation uses that RNA as a template to assemble a protein, reading the sequence in three-nucleotide codons at the ribosome.