Friday, January 9, 2009
Lost in translation?
(from an article in El·lipse, the PRBB monthy newspaper, published in January 2008)
The essential information for life is encoded in the DNA. However, it is mostly the proteins that make cells work. How do nucleotides, the basic units that form the DNA, transfer the information to the amino acids that form the proteins? This happens through two steps: transcription, the transfer of information from DNA to mRNA which occurs in the nucleus of the cell, and translation, the transfer of information from mRNA to protein which occurs in the cytoplasm. Transcription was thought for years to be the main mechanism to control gene expression, but translation has now been found to be more important than expected.
The genetic code specifies that each codon (formed by three nucleotides) in the mRNA corresponds to a specific amino acid. The transfer RNAs (tRNAs) are the molecules that execute this code by carrying an amino acid on one of their sides and binding to the corresponding codon on the other side. All this happens within molecular machines called ribosomes, which catalyse the reaction. Translation finishes when the ribosome faces one of three existing ‘stop codons’ on the mRNA. When this occurs, no tRNA can recognize it and the amino acid chain is released.
Most of the energy that a cell consumes is dedicated to the making of ribosomes and to translation, an essential process for life. Thus, it is not surprising that translation is highly regulated. For example, cell stress and physiology are controlled by the activity of the translation initiation factor eIF2alpha, which is modified under stress and ultimately leads to a response that changes the transcriptional profile of cells. This is one of the many examples in which transcription is under translational control. Failure of this control system contributes to diseases such as diabetes, metabolic sindrome, osteoporosis and neurodegeneration.