Gene expression is the process by which cells convert the DNA sequence of a gene to the RNA sequence of a transcript, and then decode the RNA sequence to the amino-acid sequence of a polypeptide.
The nearly universal genetic code consists of 64 codons, each one composed of three nucleotides. Of these codons, 61 specify amino acids, while 3--UAA, UAG, and UGA--are nonsense or stop codons that do not specify an amino acid.
The code is degenerate: More than one codon specifies every amino acid except methionine and tryptophan.
AUG in the context of a ribosome binding site is the initiation codon; it establishes a reading frame that determines the grouping of nucleotides into triplet codons.
The code is nonoverlapping. Within a reading frame, the first three nucleotides constitute one codon; the next three, the second codon; and so forth.
Gene expression based on the genetic code produces the colinearity of a gene's nucleotide sequence and a protein's sequence of amino acids.
Transcription is the first stage of gene expression. During transcription, RNA polymerase synthesizes a single-stranded primary transcript from a DNA template.
RNA polymerase initiates transcription by binding to the promoter sequence of the DNA and unwinding the double helix to expose bases for pairing.
RNA polymerase extends the RNA in the 5'-to-3' direction by catalyzing formation of phosphodiester bonds between successively aligned nucleotides.
Terminator sequences in the RNA cause RNA polymerase to dissociate from the DNA.
In prokaryotes, the primary transcript is the mRNA that guides polypeptide synthesis.
In eukaryotes, RNA processing after transcription produces a mature mRNA that travels from the nucleus to the cytoplasm to direct polypeptide synthesis.
RNA processing adds a methylated cap to the 5' end and a poly-A tail to the 3' end of the eukaryotic mRNA.
The spliceosome removes introns from the primary transcript and precisely splices together the remaining exons. Alternative splicing makes it possible to produce different mRNAs from the same primary transcript.
Translation is the stage of gene expression when the cell synthesizes protein according to instructions in the mRNA.
tRNAs carry amino acids to the translation machinery. Aminoacyl-tRNA synthetases connect amino acids to their corresponding tRNAs. Each tRNA molecule has an anticodon complementary to the mRNA codon specifying the amino acid it carries. Because of wobble, some tRNA anticodons recognize more than one mRNA codon.
Translation occurs on complex molecular machines called ribosomes. Ribosomes have two binding sites for tRNAs--P and A--and an enzyme known as peptidyl transferase that catalyzes formation of a peptide bond between amino acids carried by the tRNAs at these two sites.
Initiation: To start translation, part of the ribosome binds to a ribosome binding site on the mRNA, which includes the AUG initiation codon. Special initiating tRNAs with codons complementary to AUG carry the amino acid fMet in prokaryotes or Met in eukaryotes to the ribosomal P site. This amino acid will become the N terminus of the growing polypeptide.
Elongation: When the carboxyl group of the amino acid connected to a tRNA at the ribosome's P site becomes attached through a peptide bond to the amino acid carried by the tRNA at the A site, the ribosome travels three nucleotides toward the 3' end of the mRNA. This movement exposes the next codon and allows the next round of amino-acid addition. This mechanism of translation ensures that the 5'-to-3' direction in the mRNA corresponds to the N-terminus-to-C-terminus direction in the polypeptide under construction.
Termination: When the ribosome encounters in-frame nonsense (stop) codons, it ends translation by releasing the mRNA and disconnecting the complete polypeptide from the tRNA.
Posttranslational processing may alter a polypeptide by adding or removing chemical constituents to or from particular amino acids, or by cleaving the polypeptide into smaller molecules.
Mutations affect gene expression in several ways.
Mutations in a gene may modify the message encoded in a sequence of nucleotides. Silent mutations usually change the third letter of a codon and have no effect on polypeptide production. Missense mutations change the codon for one amino acid to the codon for another amino acid, and thereby direct incorporation of a different amino acid. Nonsense mutations change a codon for an amino acid to a stop codon, causing synthesis of a truncated polypeptide. Frameshift mutations change the reading frame of a gene, altering the identity of amino acids downstream of the mutation.
Mutations outside of coding sequences that alter signals required for transcription, mRNA splicing, or translation can also disrupt gene expression.
Mutations in genes encoding molecules of the gene-expression machinery are often lethal. Among the exceptions to this rule are mutations in tRNA genes that suppress mutations in polypeptide-encoding genes.
