What Are Plasmids? – Definition, Types, Function & Characteristics

Learn about plasmids and how their DNA works to allow their host to adapt to harsh environments. Discover how plasmids are useful in modern-day genetic engineering.

Introduction to Plasmids

In the news lately, there is a lot of talk about antibiotic-resistant bacteria. This bacteria is causing major health problems throughout the world, especially in hospital settings. But how does bacteria become antibiotic resistant?

Definition of a Plasmid

Some bacteria contain additional double-stranded DNA molecules in the form of plasmids. Imagine that you were taking a test and you didn’t know all of the answers. Luckily, your teacher allowed you to rummage through your textbooks during the test to find the right answer. Because of your ability to access these references when needed, you were able to ace the test! This is similar to how hosts containing plasmids work.

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One way that bacteria can become antibiotic-resistant is when plasmids give extra genetic information to the bacterium as needed. These genes supplement the regular chromosomal DNA, and plasmid DNA replicate independently of the chromosomal DNA. They are individual double-stranded DNA molecules that form a circle. Plasmids are not unique to bacteria, but they’re found in many bacteria, archaea, and eucarya hosts.

The plasmid doesn’t contain DNA needed for basic functions or for the bacteria to survive. Yet the genes on the plasmid can be referenced when needed. If a bacterium is in a stressful environment, they can look to their plasmids for extra survival skills. Much like we would look up the answers to a test in our textbooks, bacteria have access ranging from a few to over a thousand helpful genes in their plasmids.

Transmission of Plasmids

Plasmids are able to replicate independently from the normal chromosomal DNA. However, not all hosts contain plasmids. Through a process called conjugation, cells can transfer the genetic information of plasmids if they’re touching, just like sending text messages in class. As a result, a host that didn’t previously contain the plasmid now has an exact copy of the plasmid in its own cell.

Resistance Plasmids

A very important type of plasmid is called the R or resistance plasmid, which encodes for resistance genes. Resistance genes are R genes that give bacteria bonus survival traits. R genes can carry resistance to antibiotics, such as streptomycin and tetracycline. It isn’t uncommon for bacteria to contain plasmids that allow them to survive in environments containing heavy metals, such as mercury and arsenic.

Cell hosts can transfer these plasmids to each other through conjugation by the resistance transfer factor (RTF). RTF are the genes that serve the function of permitting the plasmid to replicate and conjugate. Bacterial hosts that contain R plasmids are commonly found where antibiotics are frequently used, such as hospitals and animal farms. In these situations, antibiotic exposure can kill the bacteria subgroup without the R plasmids, yet still select for bacteria containing the R plasmid DNA because they could survive. Surviving resistant bacteria will transmit R plasmids to new bacteria, promoting antimicrobial resistance though a new bacterial population.

Plasmid Characteristics

It’s sometimes the plasmid DNA of bacteria, instead of its chromosomal DNA, that we associate with common traits. For example, E. Coli causes severe stomach ailments to a person when it contains a plasmid that encodes for genes that produce a toxin and attach it to the intestine. So think of it this way: E. Coli without this plasmid is harmless to you.

Other bacteria that contain harmful plasmids are Clostridium tetani, which secrete a neurotoxin causing tetanus. And Bacillus anthracis should be familiar to those who remember headlines about dangerous letters and white powder back in 2001: it causes the illness known as anthrax.

Plasmids in Genetic Engineering

Among plasmids, there are some with unique capabilities that are utilized in genetic engineering. For example, the bacterium Agrobacterium tumefaciens can transfer its plasmid genes into plant cells in a process similar to conjugation. The plant cells incorporate the plasmid into their chromosomal DNA. The host plant develops tumors as a result of this infection, yet can synthesize a novel compound called opine, which neither host parent could produce. Genetic engineering experiments can use the plasmid from Agrobacterium tumefaciens to introduce novel genes into plant hosts.

Lesson Summary

Bacteria often contain circular double-stranded DNA called plasmids, yet plasmids can be found in all domains of life, including bacteria, archaea, and eukarya. They’re self-replicating and contain different genes than regular chromosomal DNA. Just as we might reference a textbook for answers to a test, plasmids aren’t relevant to the host’s survival under normal circumstances, but they can still be used when they’re needed. R, or resistance plasmids, confer resistance to bacteria. Resistance is promoted through the replication and transfer of plasmids to a new host, called conjugation. Plasmids are transferred through the resistance transfer factor (RTF), which are the genes that serve the function of permitting the plasmid to replicate and conjugate. Plasmids are even useful in modern-day genetic engineering projects to introduce novel genes into hosts, such as Agrobacterium tumefaciens, which transfers its plasmid DNA into plants.

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