Gene expression

Thursday, January 21, 2010

Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as rRNA genes or tRNA genes, the product is a functional RNA.

Several steps in the gene expression process may be modulated, including the transcription, RNA splicing, translation, and post-translational modification of a protein. Gene regulation gives the cell control over structure and function, and is the basis for cellular differentiation, morphogenesis and the versatility and adaptability of any organism. Gene regulation may also serve as a substrate for evolutionary change, since control of the timing, location, and amount of gene expression can have a profound effect on the functions (actions) of the gene in the organism.


The gene itself is typically a long stretch of DNA and does not perform an active role. It is a blueprint for the production of RNA. The production of RNA copies of the DNA is called transcription, and is performed by RNA polymerase, which adds one RNA nucleotide at a time to a growing RNA strand. This RNA is complementary to the DNA nucleotide being transcribed; i.e. a T on the DNA means an A is added to the RNA. However, in RNA the nitrogen-containing base Uracil is inserted instead of Thymine wherever there is an Adenine on the DNA strand. Therefore, the mRNA complement of a DNA strand reading "TAC" would be transcribed as "AUG".

RNA processing

Transcription creates a primary transcript of RNA at the place where the gene was located. This transcript is often altered before being translated. RNA processing, also known as post-transcriptional modification, can start during transcription, as is the case for splicing, where the spliceosome removes introns from newly formed RNA. Introns are RNA segments which are not found in the mature RNA, although they can function as precursors, e.g. for snoRNAs, which are RNAs that direct modification of nucleotides in other RNAs.
In some cases large aggregates of RNA and RNA processing factors are formed, notably in the nucleolus where ribosomal RNA is processed by snoRNAs and their partner enzymes. These cleave the primary ribosomal RNA transcripts into the correct segments and alter some of its nucleosides, for instance into pseudouridine.

RNA export

While some RNAs function in the nucleus, many other RNAs in eukaryotes are transported through the nuclear pores and into the cytosol, including all the RNA types involved in protein synthesis. In some cases RNAs are additionally transported to a specific part of the cytoplasm, such as a synapse; they are then towed by motor proteins that bind through linker proteins to specific sequences (called "zipcodes") on the RNA.

For most RNA, the mature RNA is the gene product (see non-coding RNA). In the case of messenger RNA however, the RNA is but an information carrier for the synthesis of a protein. Each triplet of nucleotides of the coding region of a messenger RNA corresponds to a binding site for a transfer RNA. Transfer RNAs carry amino acids, and these are chained together by the ribosome. The ribosome helps transfer RNA bind to messenger RNA and takes the amino acid from each transfer RNA and makes a structure-less protein out of it.
Some proteins have parts that should be within a membrane, these parts are moved into the membrane by the signal recognition particle which binds to the ribosome when it finds a signal sequence on the nascent amino acid chain.


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