Plasmid

A circular double-stranded DNA molecule, which is non-essential to the life of the cell. It is typically found in bacteria, yeast and a few superior eukaryotes, thus establishing a symbiotic or parasitic relationship with its host cell.

Plasmids can appear in different configurations that vary from fully intact to linear, when they have free ends. They can also exist in super-coiled structure, the most compact form of this molecule. They vary in size from 1 to 250 Kb.

There are two known replication strategies related to plasmids: non-integrative and integrative (fig.1). Non-integrative replication occurs, when a plasmid replicates independently, due to the possession of at least one DNA sequence, which is able to initiate replication. Other plasmids, called episomes, have the ability to insert themselves into bacterial chromosomes and they can maintain this form through several cell divisions. This is known as integrative replication.

 

Fig. 1 - Replication Strategies for (a) a non-integrative plasmid and (b) an episome

 Plasmids can be transferred between bacteria through host-to-host transfer, which is called horizontal gene transfer. This is an important process for the host cell, because it can provide selective advantages such as resistance to antibiotics.

They are important molecules in molecular genetics and are often used as a tool to clone, transfer and manipulate genes. Plasmids that are used experimentally for these purposes are called vectors.

 References:

Nature.com, (2015). plasmid / plasmids | Learn Science at Scitable. [online] Available at: http://www.nature.com/scitable/definition/plasmid-plasmids-28 [Accessed 29 Oct. 2015]

Brown, T. (2010). Gene cloning and DNA analysis: an introduction. (Chapter 2). John Wiley & Sons.

Watson, J., Gilman, M., Witkowski, J. (1992). Recombinant DNA. Nova Iorque: Scientific American Books

Thomas, C. M. and Summers, D. 2008. Bacterial Plasmids. eLS. 

Licenciatura em Bioquìmica / 2º Ano

Turno 3 Grupo 4

Ana Micaela Lavender

Bruna Araujo

Helena Vaz

Mário Azevedo

Nuno Santos