Bacterial genetics. Carriers of genetic information. Plasmids and transposable elements- Structure and function
The bacterial genome contains the genetic information of the bacterium. It consists of essential and non-essential genetic elements. While the bacterial chromosome is the essential structure, non-essential genetic elements like plasmids and transposons are also carriers of genetic information.
Plasmids
- Structure:
- Plasmids are extrachromosomal genetic elements.
- They are typically smaller than the bacterial chromosome.
- Most commonly found as small, circular, double-stranded DNA molecules in bacteria.
- The DNA is supercoiled or twisted.
- An exception exists in Borrelia, which has linear plasmids.
- They range in size from 1,500 to 400,000 nucleotide pairs (5-120 Mdal), or 3 x 10^3 to 4.5 x 10^5 base pairs.
- Replication:
- Plasmids can replicate independently of the bacterial chromosome. They contain genes for their own replication.
- Large plasmids (20-120kb) replicate under strict control, often with only one copy per cell, and may attach to the cytoplasmic membrane for even distribution.
- Small plasmids (also called cryptic plasmids) replicate in many copies (100-1000 per cell) with weak replication control, and their distribution can be irregular as they don't attach to the membrane.
- Function:
- Plasmids are generally not essential for bacterial survival.
- However, they contain genetic information that provides a selective advantage, particularly in unfavourable environments.
- They can encode genes for:
- Antibiotic resistance (R-plasmids). R-plasmids are easily transferred and play a major role in spreading drug resistance. They can carry determinants for multiple resistances.
- Virulence factors. These can include exotoxins, adhesins, and other factors.
- Metabolic abilities (Metabolic plasmids), such as lactose degradation or the ability to degrade substances like detergents.
- Bacteriocins (Col plasmids), which are toxic proteins that kill sensitive bacteria.
- Sex pili (F-plasmids, also called Fertility plasmids, or R-plasmids), which are required for conjugation (transfer of DNA between bacteria by cell-cell contact). The F-factor is the prototype of a conjugative plasmid.
- Plasmids can be transferred between bacterial cells via transformation, transduction, or conjugation.
- They can also be used as vectors in genetic engineering.
- Plasmids play a crucial role in bacterial adaptation, resistance, virulence, and metabolism, contributing to microbial evolution.