This lesson will discuss what a GMO is, and how DNA plasmids are used in genetic engineering. The lesson will include these important features of plasmids: a multiple cloning site, an origin of replication, and a selectable marker.

Genetically Modified Organism

The lady standing beside you in the produce department puts her hands on her hips, and says, ‘Are you really going to buy that?! You know that’s a GMO, right? I wouldn’t feed that to my dog.’

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You aren’t quite sure how to reply. Truth be told, you usually just buy what’s on sale. But now you feel a little bit sheepish. You want to respond intelligently but you don’t really know that much about GMOs. You mutter a response and wander away, vowing to be better prepared for produce section confrontations.

GMO stands for genetically modified organism and refers to any artificial (think man-made) changes to an organism’s genetic material. This could be adding genes that the organism didn’t have before or in some way changing the genes that the organism does have.

In a world where GMOs are a hotly debated topic, it’s a good idea to know the basics of what it means to genetically engineer an organism.

Genetic Engineering

How do scientists do this genetic modification thing? Scientists use genetic engineering, which is the technical process of deliberately changing an organism by altering its genes. To engineer something is to design it, build it, and optimize it (make it work the way you want it to). Scientists can add genes from one organism into another, modify the genes an organism already has, or make entirely new genes from scratch and add them to an organism. These constructed or engineered pieces of DNA are called recombinant DNA.

Host and Vector

When scientists design and build their recombinant DNA, they need some way to move it into their experimental organism, or the host. The host could be a number of organisms including bacteria, plant cells, fungi, or animal cells. To move the recombinant DNA scientists use a vector, which is a physical mechanism to transfer recombinant DNA into the host. There are a few options for vectors, including viruses and big pieces of DNA called artificial chromosomes. But one of the most common vectors for recombinant DNA is called a plasmid.

Here we will use the example of a bacterial cell as the host and we will discuss how we could use a plasmid to engineer this bacterium, maybe for something useful like making a pharmaceutical.

DNA Plasmids

A DNA plasmid is a piece of circular DNA that is separate from the chromosomal DNA of the cell. Plasmids are commonly found in bacterial cells where they carry extra genes besides those found in the main chromosome. This is important because it means you can add some genes without messing with the rest of the genome (the chromosomal DNA). The chromosomal DNA contains all the genetic information required by the bacteria and altering the chromosome could have negative impacts or at the very least presents more challenges than simply adding some genes carried by a plasmid.

Bacterial cell containing a plasmid that is separate from the chromosomal (genomic) DNA.
image of bacterium with chromosome and plasmid

Plasmids occur naturally within some bacterial cells, but scientists got clever and customized their own plasmids! They used natural plasmids as their starting point but then added some additional features that made plasmids even better for genetic engineering.

Plasmids have three main features that make them great for genetic engineering:

Multiple Cloning Site: To get a gene into a plasmid you need to have a way to stitch the pieces of DNA together. A multiple cloning site is a region on the plasmid that can be cut open with enzymes creating an opening where you could attach an additional piece of DNA. It’s called ‘multiple’ because there are multiple enzymes that can be used to cut an opening. So you chop the plasmid open with your enzyme, cut (or engineer) your gene to have compatible ends, and then just stick the two together (called cloning). This recreates the complete circular plasmid with the new gene added in.

Origin of replication: Now this sounds fancy but it really isn’t. There is a spot on the plasmid where enzymes can attach to the DNA and start copying it. That spot is the ‘origin’ and that copying process is ‘replication’. This means the plasmid gets copied just like the chromosome as the cell grows and divides. If the plasmid didn’t get copied into new cells, you’d have to insert your DNA into every single cell.

In a bacterial cell the plasmid will be copied from the origin of replication. When the bacterial cell divides each new cell will receive a copy.
bacterial cell dividing

Selectable Marker: This feature is useful for propagating your cell and weeding out non modified cells. A selectable marker is a gene in the plasmid that, under certain specific conditions, will allow only the survival of cells that have the plasmid. So you can kill the cells that don’t have the plasmid and keep only the cells that do. Frequently this gene codes for resistance to an antibiotic. The bacterial cells carrying the plasmid will tolerate the antibiotic, and the ones without the plasmid will die.

Lesson Summary

A _GMO_ is a genetically modified organism or an organism whose genetic material has been changed in some way. Scientists accomplish these changes using genetic engineering. The organism that is getting modified is the host and the recombinant DNA, the new or modified genetic material, is delivered into the host using a vector. A common vector is a DNA plasmid which is a circular piece of DNA with a multiple cloning site for inserting new genes, an origin of replication for copying the plasmid, and a selectable marker for maintaining the new population of cells with the plasmid under selective conditions.

Now you know the basics of genetic engineering; you’re well on your way to developing your own well-informed opinion.