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Bacteria often get a bad rap, but they’re some of our best partners in science and medicine! In this episode, we’ll explore what bacteria are doing with their DNA — including how they can trade it around. We’ll learn about chromosomes and plasmids, gene expression and recombinant DNA, and how E. coli are used to make insulin.

Introduction: The Microbiome 00:00
Prokaryotes & DNA 1:33
Plasmids & Horizontal Gene Transfer 2:44
Insulin 4:49
Gene Expression 6:30
Dr. Rebecca Lancefield 7:54
Review & Credits 9:05

This series was produced in collaboration with HHMI BioInteractive, committed to empowering educators and inspiring students with engaging, accessible, and quality classroom resources. Visit for more information.

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CC Kids:

I’m Dr. Sammy,   your friendly neighborhood  entomologist, and this is…me!

I’m a human, made up of around  37 trillion cells like these.  They form my skin, my spleen,  and all of what makes me, me.  But that’s not all. My body also plays host   to several trillion of these: bacteria. On average, humans have roughly as many   bacteria as they have cells, often more!

You truly do contain multitudes!  So many bacteria that it can be  described as a biome, the microbiome. There are bacteria in my gut helping  me digest food, in my mouth working   to prevent tooth decay, and on my skin  helping me maintain this healthy glow. And while there are definitely harmful bacteria  out there that you don’t want, most of the ones   hanging out inside us are doing some good,  or at least, they’re not hurting anything.

Bacteria are actually their own form of life. Like us, they have DNA, though they   organize and regulate it differently. And in fact, bacteria are some of our   best partners in science and medicine.

The study of bacterial DNA and genetics   has led to some major breakthroughs in  how we treat a whole host of diseases. So now that you’ve gotten to know me and my  bacteria, welcome to Crash Course Biology!  Now, let’s hear that sweet,  sweet theme music from—wait, no?  We’ve got the budget for  the bacteria-street boys?! [THEME MUSIC] Along with their distant cousins, archaea,  bacteria are part of a group called prokaryotes.  Their motto? “No nucleus, no problem.”  They’d probably get it on a T-shirt,   if we could make them x-tra, x-tra,  x-tra small..and if they had arms. Prokaryotes don’t have a defined cellular  nucleus to neatly store their DNA,   the way that eukaryotes like  plants, animals, and fungi do.  So they have to handle their genetic  information a little differently.

If we took a look inside an animal cell,   we’d see that the DNA in the nucleus  is wound up tight into chromosomes.  Animals can have a pretty wide  range of chromosome pairs per cell.  Like, fruit flies have four pairs,  while the Atlas blue butterfly has 225. On the other hand, if we peeked inside a bacterial   cell, things would look a lot different. Most bacteria only ever have one chromosome,   and it’s sort of circular.

Which would be more obvious   if it weren’t so twisted up with coils  and supercoils, stabilized by proteins. The chromosomes reside in an area  of the cell called the nucleoid.  While it’s similar to a nucleus — the name even   means ‘nucleus-like’— it lacks the  structured packaging of a nucleus.  It’s less like an enclosed clubhouse and more like  an open field where genetic materials run wild. In addition to that one chromosome,  bacteria can have a collection of   smaller circles of DNA called plasmids.

Plasmids duplicate separately from the main   chromosome, and their DNA isn’t always 100 percent  necessary for the bacteria’s everyday survival. They can come in handy though. For example, the genes carried   in bacterial plasmids can help make  the bacteria resistant to antibiotics.  That may sound like a bad thing to us humans,   but it’s all about perspective.

Bacteria gotta live, too. One of the really wild things about  plasmids is that they’re transferable.  Just because a bacterium starts with a certain  set of plasmids doesn’t mean it’s stuck with them.  Unlike plants and animals, which only get  genes that they inherit from their parents,   bacteria can transfer plasmids, exchange them,   and even collect new ones from the environment  in what’s called horizontal gene transfer. It’s actually a pretty popular mechanism to give   people superpowers in comics  that might sound familiar.  Like, radioactive animal “x” bites nerdy  person “y” and transfers its abilities.  That’s right, Spider-Man is a  product of horizontal gene transfer.

Sometimes, getting a new plasmid doesn’t do  much for a bacterium, but in other cases,   it can be like being bitten by a radioactive  spider and getting the power to climb walls,   and sense danger, and crack wise while  hanging upside down from a silken thread.  The bacterium ends up stronger, with new  abilities or resistances that it didn’t   have before, like the ability to survive in  high levels of a heavy metal, like mercury. Horizontal gene transfer also adds genetic  diversity to the bacterial gene pool in a similar   way that sexual reproduction in eukaryotes does. Which is important because, while eukaryotes often   reproduce using the DNA of two individuals,  bacteria basically just clone themselves.  So, without horizontal gene transfer, you’d  have a lot of pretty similar organisms.  That would make them super susceptible to being  all wiped out by the same thing in one fell swoop.

But because horizontal gene transfer  increases the genetic diversity of the   population, it becomes more resilient  overall, as individuals have different   vulnerabilities and strengths. Dang, science, that’s deep. In any case, our growing understanding of plasmids is leading to major leaps  in the world of medicine.  Take insulin for example, one of the most  important molecules in the human body.  It’s responsible for regulating how sugar  gets used, and without enough of it,   there’s often too much sugar in your  bloodstream and not enough in your cells.

So, for decades, doctors and researchers  have been working on ways to make insulin   for people with diabetes that can’t  produce enough of it on their own.  For a long time, this was done by purifying  insulin from cows and pigs for human use.  And although animal insulin is still used today,  it can cause allergic reactions in some people. Enter plasmid research. By the early 1980s,   a new type of insulin had hit the  market, made from an unexpected place:   a plasmid in E. coli bacteria.

In this method, scientists take   the gene that we humans use to make insulin,  and they insert it into an E. coli plasmid.  This hybrid is called a recombinant DNA molecule:  It’s a combo of both human and bacterial DNA. The plasmid is then put back into the E.  coli, and the bacterium carries on as normal!  As it reproduces, it makes more and  more copies of that plasmid — along   with more and more copies of the insulin  gene in a process called gene cloning. As time goes on, these bacteria read  that gene and start turning it into   proteins — specifically, into human insulin.

The molecule can then be harvested and purified   so it’s ready to be used in people. This new strategy for producing insulin   has helped save millions of lives, all  with the help of our bacterial buds. Besides insulin, gene cloning has  also been used to make cancer drugs   and human growth hormones, which  are used to treat diseases such as HIV.

And studying bacterial DNA has led  to other medical advances as well.  For example, some researchers focus  on how bacterial genes are expressed. Gene expression describes the way that cells can  turn their genes off and on, depending on their   needs, producing different proteins as a result. This helps prokaryotes and eukaryotes alike   adjust to the conditions around them.

Like, if the water that bacteria live   in suddenly gets too hot or salty, they can  make different proteins that help them adapt. As it turns out, gene expression in bacterial  cells doesn’t look that different than in our own.  A protein called an activator can bind  to DNA to get transcription to start,   like a little green “go” flag. When that happens, a section of DNA is copied into  an mRNA molecule, which is then carried off and   used as the instructions to build a protein.

On the flip side, proteins called repressors   can stop transcription from happening, raising a  “stop” sign to halt protein-making in its tracks. And understanding how gene expression works  in potentially harmful microbes turns out to   be… pretty useful for researchers,  such as Dr. Rebecca Lancefield.  Let’s take our seats and listen to  her story over in the Theater of Life… Meet Streptococcus.

A diverse type of bacterium. Some are harmless to humans,   and others can give us some pretty nasty  diseases like rheumatic fever or strep throat.  But for a long time, we could only guess why  some were so much more harmful than others. Enter American microbiologist  Dr.

Rebecca Lancefield.  She took up the mantle of strep  research in the pre-antibiotic era,   when there were few vaccines  available to prevent diseases.  And soldiers in World War I were frequently  getting sick from strep-induced infections. Using samples collected from those  soldiers, Lancefield and her team began   classifying bacteria based on specific  molecules displayed on their surface,   like little bacterial name tags. These are called antigens — and   which nametag a particular bacterium wears  is controlled by its gene expression.

Learning to read those name tags turned out to be  an indicator of which bacteria were dangerous,   which weren’t, and to whom. Like, for Streptococcus,   Lancefield realized there were two different  kinds of antigens that could be expressed:   one made of carbohydrate molecules,  and another made of proteins. She discovered that the carbohydrates determine  which species a strain of bacteria can infect,   while the proteins determine how dangerous  the strain is for that particular species.

Over six decades Lancefield’s antigen research  helped inform everything from how to prescribe   antibiotics, to how soldiers were housed  in military barracks during World War II.  Lancefield’s work showed us  that sometimes studying the   smallest thing can have the biggest impact! While we ourselves may be full  of bacteria, we have to remember   that as much as they’re a part of us –  they’re also very much their own thing.  And while there are definitely some that we  want to avoid, bacteria as a group aren’t as   malicious as they’re sometimes perceived. In fact, Earth is a microbial planet:   life started with them and life as we  know it could not survive without them.

And while we’ve only looked at a couple of  branches on the bacterial family tree today,   there are millions of others out there, and  probably millions more waiting to be researched. In our next episode, we’re moving on from  bacteria and into the rebellious world of viruses.  They play by their own rules. I’ll see ya then!

PEACE! This series was produced in  collaboration with HHMI BioInteractive.  If you’re an educator, visit for   classroom resources and professional development  related to the topics covered in this course. Thanks for watching this episode of Crash  Course Biology, which was filmed at our   studio in Indianapolis, Indiana, and was  made with the help of all these nice people.  If you want to help keep Crash  Course free for everyone,   forever, you can join our community on Patreon.