Gene expression is the process by which the heritable information in a gene (the sequence of DNA base pairs) is made into a functional product such as a protein or RNA. The basic idea is that DNA is transcribed into RNA, which is then translated into proteins. Proteins make many of the structures and all the enzymes in a cell or organism.
Several steps in the gene expression process may be modulated (tuned). This includes both the transcription and translation stages, and the final folded state of a protein. Gene regulation switches genes on and off, and so controls cell differentiation, and morphogenesis. Gene regulation may also serve as a basis for evolutionary change: control of the timing, location, and amount of gene expression can have a profound effect on the development of the organism.
Epigenetics[change | change source]
These changes may remain through cell divisions for the remainder of the individual's life, and may also last for more than one generation. However, there is no change in the underlying DNA sequence of the organism. Instead, non-genetic factors cause the organism's genes to behave (express themselves) differently.
The best example of epigenetic changes in eukaryote biology is the process of cellular differentiation. During morphogenesis, totipotent stem cells become the various cell lines of the embryo, which in turn become fully differentiated cells. In other words, a single fertilized egg cell – the zygote – divides and develops. The daughter cells change into the many cell types of the mature embryo. These include neurones, muscle cells, epithelium, blood vessels and so on. This happens by activating some genes while inhibiting others.
Epigenetic changes are long-term, and usually survive the process of cell division (mitosis). Changes occur in the chromatin, which is a combination of the DNA and its surrounding histone proteins in the chromosome. The details of how this happens are still being worked out, but it is fairly certain that the wrapping of the DNA and histone is a key feature.
Gene regulation[change | change source]
Up-regulation and down-regulation[change | change source]
Up-regulation increases the expression of one or more genes and as a result the protein(s) encoded by those genes. Down-regulation is a process resulting in decreased gene and protein expression.
Induction vs repression[change | change source]
Gene regulation can be summarized as:
- Inducible systems: an inducible system is off unless there is the presence of some molecule (called an inducer) that allows for gene expression.
- Repressible systems: a repressible system is on except in the presence of some molecule (called a corepressor) that suppresses gene activity. The molecule is said to repress expression.
Regulatory RNAs[change | change source]
There are a number of RNAs which regulate genes, that is, they regulate the rate at which genes are transcribed or translated. The following are two important examples
miRNA[change | change source]
Micro RNAs (miRNA) act by joining an enzyme and blocking mRNA (messenger RNA), or speeding its breakdown. This is called RNA interference.
siRNA[change | change source]
Small interfering RNAs (sometimes called silencing RNAs) interfere with the expression of a specific gene. They are quite small (20/25 nucleotides) double-stranded molecules. Their discovery has caused a surge in biomedical research and drug development.
Related pages[change | change source]
References[change | change source]
- Adrian Bird (2007). "Perceptions of epigenetics". Nature. 447: 396–398. doi:10.1038/nature05913. PMID 17522671. PMID 17522671
- Special report: 'What genes remember' by Philip Hunter | Prospect Magazine May 2008 issue 146
- Reik, Wolf (2007-05-23). "Stability and flexibility of epigenetic gene regulation in mammalian development". Nature. 447 (May): 425–432. doi:10.1038/nature05918. Retrieved 2008-04-05.
- Morris KV (2008). "Epigenetic regulation of gene expression". RNA and the regulation of gene expression: a hidden layer of complexity. Caister Academic Press. ISBN 978-1-904455-25-7.
- Hamilton A & Baulcombe D (1999). "A species of small antisense RNA in posttranscriptional gene silencing in plants". Science. 286: 950–2. doi:10.1126/science.286.5441.950. PMID 10542148. First description of siRNAs.
- Hannon G & Rossi J (2004). "Unlocking the potential of the human genome with RNA interference". Nature. 431: 371–8. doi:10.1038/nature02870. PMID 15372045.