Gene expression
The way from gene to protein - Expression of genes
Sequencing of the human genome found far fewer genes than expected for the high complexity of the human organism. The increased diversity at the level of the primary mRNA and proteins can be accounted for by a process called alternative RNA splicing.
The genetic information of every known organism is stored in long chains of DNA (deoxyribonucleic-acid) molecules. The functional units of the genome are genes, which are arranged in succession on the DNA strands. Usually one gene codes for one protein, meaning that the sequence of the DNA determines the sequence of amino acids forming one specific protein. However, the information stored in the DNA cannot be translated into a protein directly; the DNA rather serves as a template which is copied into RNA (ribonucleic-acid) molecules. This process is called transcription and happens in the nucleus. The resulting product, the messenger-RNA (mRNA), is the working unit of the genome. It provides the link between the message stored in the genome and the gene product. After transcription the RNA is recognised by a large cellular machine - called the ribosome – which is able to decipher the information encoded by the RNA and translate it into a sequence of amino acids, that forms the protein molecule. This process is termed translation. The succession of events described is called gene expression; it is performed in the described order in every cell of every living organism known.
A gene is first transcribed into a pre-messenger RNA (pre-mRNA), which is a copy of the genomic DNA, containing intronic regions destined to be removed during pre-mRNA processing (RNA splicing), as well as exonic sequences that are retained within the mature mRNA. During the process of RNA splicing, exons can either be retained in the mature message or targeted for removal in different combinations to create a diverse array of mRNAs from a single pre-mRNA. This process is referred to as alternative RNA splicing.
Schematic overview of the gene-expression pathway in eukaryotic organisms.
The genome is located in the cellular nucleus where it is transcribed and the pre-mRNA is formed. After several RNA processing steps (including splicing) the mature mRNA is transported to the cytoplasm where protein production proceeds (translation).
Alternative splice events that affect the protein coding region of the mRNA will give rise to proteins which differ in their sequences and often in their activities. Alternative splicing within the non-coding regions of the RNA can result in changes in regulatory elements such as translation enhancers or RNA stability domains, which may have a dramatic effect on the level of protein expression. It is therefore important that the regulation of RNA splicing is as stringent as that observed for RNA transcription or translation. This is the case as RNA splicing occurs within a tightly regulated, multi-component molecular machine called the spliceosome, which is under the control of intra- and extra-cellular signalling pathways.
The accuracy of RNA splicing is also monitored by RNA proofreading mechanisms that are able to target incorrectly spliced mRNA for destruction or can correct the error. If mRNA is not spliced properly it will result in the defective protein and might lead to disease or could even be fatal. If you want to read about link between diseases and splicing, please refer to our section AS and health.
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