How Bacterial Artificial Chromosome Bac Libraries are Made

Bacterial artificial chromosome (BAC) is a major breakthrough in genomic studies as it facilitates genetic sequencing of larger genomes, such as the human genome, in comparatively less time at a much lesser cost.

Mechanism of genetic cloning:

Genetic sequencing is the process in which DNA sequence related to a particular organism is recognized and stored in a database for many different scientific uses. Such sequences will enable researchers to recognize genetic variations, disease-causing genes, inheritance of certain characteristics as well as a ‘map’ for the use of future research. In order to determine a genetic sequence, researcher will require multiple copies of the same DNA sequence that they intend to interpret. ‘Cloning’, is the term used to describe this process of genetic replication.

In order to clone genetic material, a segment of the DNA needing interpretation is incorporated into a bacterial or viral DNA. Usually, the genetic material used for this purpose within the bacteria, is the ‘plasmid’. The genetic sequence of a plasmid contains a specific DNA sequence at the beginning of its genetic sequence along with a marker sequence, which is useful for its identification. Following incorporating the strings of DNA into a plasmid, the resulting ‘vector’ is placed within the bacteria, which allows it to replicate in a special form of culture media. Following the cloning process, antibiotics will destroy the bacteria, which do not contain the cloned plasmid. Remaining bacteria will facilitate the process of genetic sequencing.

Further reading: Common characteristics of bacteria

Limitations of using plasmid based genetic cloning:

Each plasmid used for the cloning process will have a limited capacity to hold the external DNA material. Thus, the usual amount of external DNA material accommodated by a plasmid is about 10 kilobase pairs. The usually human gene consists of about 27 kilobase pairs which means that cloning the full human genome can take ages. In order to avoid this drawback, scientists have come up with the new form of vector, known as the ‘Bacterial artificial chromosome’.

Using Bacterial artificial chromosome for genetic cloning:

The plasmid used to create the BAC vector is a special form of DNA sequence known as the ‘F’ plasmid, which enables bacteria to transfer its genetic material into another bacterium at times of absolute stress. However, not all bacteria will contain ‘F’ plasmid and special techniques will enable isolating these plasmids from the rest. According to scientists, the ‘F’ plasmids could hold up to 350 kilobase pairs and will contain all other DNA sequences needed for a successful replication process. Because of BAC vectors, the number of clones necessary to sequence the entire human genome, has reduced to millions from the earlier calculation of almost 2 billion clones.

‘Shotgun cloning’:

In a relatively newer method of cloning to determine the human genome, scientists were able to abandon the traditional linear sequencing, thanks to the BAC vector. In this process, several segments of DNA from the human genome are incorporated into the BAC vector, which will create a ‘library of genetic material’. This will greatly reduce the number of clones necessary to complete the human genome sequencing as well as interpreting the genetic sequence of other organisms and agriculturally important plants.


Producing the bacterial artificial chromosome is a major determinant of the speed and the effectiveness of sequencing the human genome. However, the technology is not restricted to sequencing the human genome but would also be useful in other genetic research.