Coding genes are expressed through the synthesis of proteins, which involves two primary steps: transcription and translation. Transcription is the process by which the enzyme RNA polymerase synthesizes a messenger RNA (mRNA) molecule, which carries nitrogenous bases that are complementary to the DNA base sequence present in a gene. In eukaryotes, the newly formed pre-mRNA may be modified by RNA splicing. This process removes introns, regions that do not carry instructions for the particular protein, and may rearrange the coding regions (exons). The mRNA is then sent from the nucleus to the cytoplasm, where it is bound by a ribosome. Translation occurs at the ribosome and results in a chain of amino acids. During translation, transfer RNA (tRNA) binds an amino acid and brings it to the ribosome, which constructs a protein based on the message encoded in the mRNA base sequence. Because ribosomal RNA (rRNA) makes up the ribosome, RNA is both the message and the catalyst that decodes the message and may therefore have constituted the earliest life forms. Cells control how much protein is present in their cells, largely by controlling transcription. Regulatory DNA sequences interact with activators to increase gene expression or repressors to decrease gene expression in prokaryotes. Similar mechanisms in eukaryotes, including enhancers, carry out these same functions.
At A Glance
Transcription is the process by which the enzyme RNA polymerase forms messenger RNA (mRNA) with a nitrogenous base sequence that is complementary to the sequence of the gene on a DNA strand.
- Following transcription, modifications to the RNA may occur, including capping, poly-A tail addition, and RNA splicing, in which short segments of RNA are joined together to make a single mRNA message.
Translation is the process by which the mRNA strand is decoded in the ribosome and a polypeptide is made, via the assistance of transfer RNA (tRNA).
- Blocking transcription and translation is the main action of antibiotics; in addition, the earliest forms of life may have been based on RNA.
Transcription is regulated by regulatory DNA sequences, which interact with activators, repressors, and enhancers to increase or decrease the amount of mRNA made.
- Found in E. coli bacteria, the trp operon is a group of genes that function together to regulate the production of tryptophan.
- Found in E. coli bacteria, the lac operon is a group of genes that function together to regulate the transport and metabolism of lactose.