Gene Cloning and Vectors
When geneticists use small pieces of DNA to clone a gene and create a genetically modified organism (GMO), that DNA is called a vector.
What Vectors Have to Do With Genes and Cloning
In molecular cloning, the vector is a DNA molecule that serves as the carrier for the transfer or insertion of foreign gene(s) into another cell, where it can be replicated and/or expressed. Vectors are among the essential tools for gene cloning and are most useful if they also encode some kind of marker gene encoding a bioindicator molecule that can be measured in a biological assessment to ensure their insertion, and expression, in the host organism.
Specifically, a cloning vector is DNA taken from a virus, plasmid, or cells (of higher organisms) to be inserted with a foreign DNA fragment for cloning purposes. Since the cloning vector can be stably maintained in an organism, the vector also contains features that allow for the convenient insertion or removal of DNA. After being cloned into a cloning vector, the DNA fragment can be further sub-cloned into another vector that can be used with even more specificity.
In some cases, viruses are used to infect bacteria. These viruses are called bacteriophages, or phage, for short. Retroviruses are excellent vectors for introducing genes into animal cells. Plasmids, which are circular pieces of DNA, are the most commonly used vectors used to introduce foreign DNA into bacterial cells. They often carry antibiotic resistance genes that can be used to test for expression of the plasmid DNA, on antibiotic Petri plates.
Gene transfer into plant cells is commonly performed using the soil bacterium Agrobacterium tumefaciens, which acts as a vector and inserts a large plasmid into the host cell. Only those cells containing the cloning vector will grow when antibiotics are present.
The Major Types of Cloning Vectors
The six major types of vectors are:
- Plasmid. Circular extrachromosomal DNA that autonomously replicates inside the bacterial cell. Plasmids generally have a high copy number, such as pUC19 which has a copy number of 500-700 copies per cell.
- Phage. Linear DNA molecules derived from bacteriophage lambda. It can be replaced with foreign DNA without disrupting its life cycle.
- Cosmids. Another circular extrachromosomal DNA molecule that combines features of plasmids and phage.
- Bacterial Artificial Chromosomes. Based on bacterial mini-F plasmids.
- Yeast Artificial Chromosomes. This is an artificial chromosome that contains telomeres (disposable buffers at the ends of chromosomes which are cut off during cell division) with origins of replication, a yeast centromere (part of a chromosome that links sister chromatids or a dyad), and a selectable marker for identification in yeast cells.
- Human Artificial Chromosome. This type of vector is potentially useful for gene delivery into human cells, and a tool for expression studies and determining human chromosome function. It can carry a very large DNA fragment.
All engineered vectors have an origin of replication (a replicator), a cloning site (located where the insertion of foreign DNA neither disrupts replication or inactivation of essential markers), and a selectable marker (typically a gene that provides resistance to an antibiotic.)