Most genes contain the information needed to make functional molecules called proteins. (A few genes produce other molecules that help the cell assemble proteins.) The journey from gene to protein is complex and tightly controlled within each cell. Together, transcription and translation are known as gene expression.
Gene expression is the process by which the genetic code, the nucleotide sequence of a gene, is used to direct protein synthesis and produce the structures of the cell. Genes that code for amino acid sequences are known as ‘structural genes’. Some genes are responsible for the production of other forms of RNA that play a role in translation, including transfer RNA (tRNA) and ribosomal RNA (rRNA).
A structural gene involves a number of different components:
- Exons: Exons code for amino acids and collectively determine the amino acid sequence of the protein product.
- Introns: Introns are portions of the gene that do not code for amino acids, and are removed (spliced) from the mRNA molecule before translation.
Gene control regions
- Start site: A start site for transcription.
- A promoter: A region a few hundred nucleotides ‘upstream’ of the gene (toward the 5′ end). It is not transcribed into mRNA, but plays a role in controlling the transcription of the gene.
- Enhancers: Some transcription factors (called activators) bind to regions called ‘enhancers’ that increase the rate of transcription. These sites may be thousands of nucleotides from the coding sequences or within an intron.
- Silencers: Some transcription factors (called repressors) bind to regions called ‘silencers’ that depress the rate of transcription.
- Initiation: The DNA molecule unwinds and separates to form a small open complex. RNA polymerase binds to the promoter of the template strand.
- Elongation: RNA polymerase moves along the template strand, synthesizing an mRNA molecule.
- Termination: In prokaryotes there are two ways in which transcription is terminated. In Rho-dependent termination, a protein factor called “Rho” is responsible for disrupting the complex involving the template strand, RNA polymerase and RNA molecule. In Rho-independent termination, a loop forms at the end of the RNA molecule, causing it to detach itself. Termination in eukaryotes is more complicated, involving the addition of additional adenine nucleotides at the 3′ of the RNA transcript.
- Processing: After transcription the RNA molecule is processed in a number of ways; introns are removed and the exons are spliced together to form a mature mRNA molecule consisting of a single protein-coding sequence. RNA synthesis involves the normal base pairing rules, but the base thymine is replaced with the base uracil.
- Initiation: The small subunit of the ribosome binds at the 5′ end of the mRNA molecule and moves in a 3′ direction until it meets a start codon (AUG). It then forms a complex with the large unit of the ribosome complex and an initiation tRNA molecule.
- Elongation: Subsequent codons on the mRNA molecule determine which tRNA molecule linked to an amino acid binds to the mRNA. An enzyme peptidyl transferase links the amino acids together using peptide bonds. The process continues, producing a chain of amino acids as the ribosome moves along the mRNA molecule.
- Termination: Translation is terminated when the ribosomal complex reaches one or more stop codons (UAA, UAG, UGA). The ribosomal complex in eukaryotes is larger and more complicated than in prokaryotes. In addition, the processes of transcription and translation are divided in eukaryotes between the nucleus (transcription) and the cytoplasm (translation), which provides more opportunities for the regulation of gene expression.
- Post-translational processing of the protein