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Collaboration icon A Brief Introduction to Plasmids for the Non-Scientist

Basic research tools used in molecular biology have changed a tremendous amount over the last few centuries. In the mid 1800’s, an Austrian monk named Gregor Mendel, conducted various rudimentary experiments by breeding and studying pea plants. By specifically choosing to breed plants with certain characteristics, Mendel was able to produce offspring that had those same chosen characteristics as their “parent” plants. Today, we call those characteristics genes. He didn’t know it at the time, but Mendel would play a key role in what we consider molecular biology today.

In the early 1950’s, Joshua Lederberg, studying genetics at the University of Wisconsin, began using small bits of DNA in his research. In 1952 he coined the term “plasmid”, to describe any “extra-chromosomal genetic particles” 1 or any bits of DNA (separate from the genome) that carried some genetic material.

Since their discovery, plasmids have become a common basic research tool, especially in the last few years with the explosion of the new CRISPR-Cas9 technology. CRISPR-Cas9, which can be carried on and deployed from plasmids in many different species, has helped to progress the molecular biology field by allowing researchers to conduct research more efficiently and cheaply. But, after all the buzz around CRISPRs and other research tools, you may be wondering, “what exactly is a plasmid”?

What is a Plasmid and Why are They Useful?

A plasmid is a small, circular piece of DNA that usually contains genes. 2 Plasmids can be thought of as a way to store, manipulate, and study genes. If you think of a long, beaded necklace as a normal piece of DNA (the “bacterial DNA” in Figure A below), a plasmid could be thought of as a matching bracelet, or an independent copy of a small portion of that same beaded necklace.

bacterial dna cartoon
Figure A 3

Plasmids are very easy to use and manipulate because of their small size. Researchers can insert one or just a few DNA elements into a plasmid and study them separately from the complex happenings of the organisms they were isolated from. For example, if researchers wanted to study “Gene Z”, they might insert “Gene Z” into their plasmid and see how that plasmid reacts with its surroundings. By placing the plasmid containing “Gene Z” in different environments, the researcher would be able to obtain more information about that gene.

Plasmids are also a useful tool because they are easily replicated. That is to say that a researcher can obtain many copies of a plasmid (and the DNA elements it contains) by placing the plasmid in bacteria and allowing the bacteria to grow. Each time a bacterial cell replicates, it will copy the plasmid contained within it; as the bacteria grow so does the amount of plasmid available to the researcher.

Plasmids are also incredibly stable and can therefore be stored for long periods of time, as long as they are kept under the appropriate temperature conditions. In general, plasmids are stable because it is difficult to alter the structure or the contents of the plasmid without specifically intending to make a change. There are also many different kinds of plasmids from different species. This is an advantage because plasmids can be used for a variety of different purposes by a variety of different researchers and across many fields of study.

Basic Plasmid Elements

In most plasmids, there are three basic elements: the origin of replication, the antibiotic resistance gene and the multiple (multi)-cloning site . 4 Figure B (below) shows these elements within a plasmid.

plasmid map cartoon
Figure B

As previously mentioned, plasmids are easily replicated, allowing a researcher to obtain as many copies as they need from the original copy to conduct their experiments. The origin of replication is the component that prompts bacteria to make copies of the plasmid as they grow. The antibiotic resistance gene allows the researcher to isolate bacteria containing the plasmid of interest from all other bacteria. Specifically, the gene makes it so the bacteria with the plasmid will survive in an environment containing an antibiotic whereas the bacteria without the antibiotic resistance gene will die. Figure C (below) illustrates this process. Without this gene, it would be nearly impossible to specifically grow bacteria containing the plasmid as opposed to other bacteria from the surrounding environment.

bacteria antibiotic resistance cartoon
Figure C

The multi-cloning site is where the researcher “cuts and pastes”, or inserts, the gene of interest. This is one of the most critical parts of the plasmid as it will contain the gene the researcher is actually interested in studying.

Plasmids, which contain many different pieces, are in a way much like cellphones. Each piece in a plasmid, could be from a different source. For example, a cellphone’s software could be developed by Windows, the glass screen might be produced by Samsung, while the exterior body was made by Motorola. Each of these parts are interchangeable and could be switched out for a number of other pieces. The same concept applies to plasmids. In one plasmid, part of the multi-cloning site could be from one lab, while a fluorescent protein (which makes a certain component “light-up” or “fluoresce”) could be from another lab. Many pieces can be brought together to make a plasmid that is designed specifically for the researcher's desired experiment.

Plasmids & Technology Transfer

As a nonprofit plasmid repository, Addgene continues to fulfill its mission by helping organizations share these basic research materials with other scientists. Many labs design plasmids for a specific purpose, while other plasmids can be used as building blocks for scientists to adjust and to fit their specific needs. All kinds of plasmids, whether rudimentary or complex, can be deposited into the repository. After materials are deposited with Addgene, they are made available to other academic/nonprofit organizations. Addgene then facilitates the Material Transfer Agreements (MTAs) for each transfer and ensures that the proper paperwork is completed for each request. By doing so, Addgene helps labs fulfill grant sharing obligations, like the ones provided by the National Institutes of Health (NIH) 5 . Addgene’s goal is to make it as easy as possible for scientists to share basic research materials with their colleagues and to further scientific discovery around the world.