Components

• Besides the gene of interest, an effective expression vector must have a promoter sequence and a transcription terminator. The first allows the DNA to be made into RNA and the second signals where the process should stop. Both are important to ensure synthesis of the desired product. Expression vectors often have an enhancer sequence which increases the amount of protein or RNA produced.
  
  

Mechanism

• Except for the information encoded in the vector's DNA sequence, the machinery used in the synthesis of the protein belongs to the cell that has incorporated it. Since cells belonging to different groups (animals, plants, bacteria) use different mechanisms it is very important to use an appropriate vector. A vector that has been designed for use in animal cells will not work in bacteria.
  
  

Function

• Expression vectors are used mainly for two purposes. The first is the production of either protein or RNA in large quantities in order to be able to study them more easily or to use them for other purposes. One example of the latter is the synthesis of insulin needed by diabetic patients, a cheaper and safer process than the previous method of purifying it from slaughtered animals. Another use of expression vectors is to make a cell synthesize a protein they normally would not.
  
  

Types

• There are two main types of expression vectors: plasmids and viruses. Plasmids are small circular DNA sequences that derive from larger ones that occur naturally in bacteria. To be used as vectors they are modified as described above. Viral vectors are also modified, rendered incapable of multiplying by themselves and the gene of interest is added. Viral vectors are the ones of choice in gene therapy.
  
  

Reporter Genes

• Since not all cells take up the expression vector, reporter genes are included in them to distinguish those that have from those that have not. The most common ones are proteins that color the cells that express them and proteins that give them resistance to antibiotics. In the latter case, adding the right antibiotic will eliminate all cells that do not express the proteins encoded in the vector.