Gene Expression and Regulation process 5

Gene expression
Gene expression is the process by which the genetic code - the nucleotide sequence - of a gene is used to direct protein synthesis and produce the structures of the cell. Genes that code for amino acid sequences are known as structural genes .
The process of gene expression involves two main stages:
Transcription: the production of messenger RNA (mRNA) by the enzyme RNA polymerase, and the processing of the resulting mRNA molecule.
Translation: the use of mRNA to direct protein synthesis, and the subsequent post-translational processing of the protein molecule.
Some genes are responsible for the production of other forms of RNA that play a role in translation, including transfer RNA (tRNA) and ribosomal RNA (rRNA).
A structural gene involves a number of different components:


• Exons. Exons code for amino acids and collectively determine the amino acid sequence of the protein product. It is these portions of the gene that are represented in final mature mRNA molecule.
• Introns. Introns are portions of the gene that do not code for amino acids, and are removed (spliced) from the mRNA molecule before translation.
Gene control regions
• Start site. A start site for transcription.
• A promoter. A region a few hundred nucleotides upstream of the gene (toward the 5 end). It is not transcribed into mRNA, but plays a role in controlling the transcription of the gene. Transcription factors bind to specific nucleotide sequences in the promoter region and assist in the binding of RNA polymerases.
• Enhancers. Some transcription factors (called activators) bind to regions called enhancers that increase the rate of transcription. These sites may be thousands of nucleotides from the coding sequences or within an intron. Some enhancers are conditional and only work in the presence of other factors as well as transcription factors.
• Silencers. Some transcription factors (called repressors) bind to regions called silencers that depress the rate of transcription.
Note: The term gene expression is sometimes used to refer to the transcription phase alone.