How do I know if my gene’s inside 🤷‍♀️ with COLONY PCR it cannot* hide! 🧐 Polymerase Chain Reaction (PCR) is a way to amplify (make lots of copies of) short stretches of DNA from longer pieces of double-stranded (ds) DNA we call the TEMPLATE.  We choose what region to copy by designing short pieces of DNA called PRIMERS to bookend the start & stop of this region (1 per strand) so that a protein called DNA POLYMERASE (DNA Pol) can copy each strand.. But where does the template itself come from? That depends.

You’ve likely heard of PCR being used to test for paternity or presence at a crime scene. In this case, the template comes from cheek swabs, trace evidence, etc. My templates come from someplace a bit different… Most frequently I use PCR to make lots of copies of a gene to put into a vector to put into bacteria to make more of the gene to put into other bacteria to make a bacmid to put into insect cells to make a baculovirus to infect more insect cells to make more baculovirus to infect more insect cells to express my protein.

There are different variations of PCR and reasons to use it, but I mainly use PCR during the process of MOLECULAR CLONING -> I copy an (edited) gene for a protein I want to study from one template and put that gene INSERT into a circular piece of DNA called a PLASMID VECTOR that has the “bells and whistles” I want, like “tags” to help with purification and start signals for turning the gene into protein. Then I stick this RECOMBINANT plasmid into bacterial cells so the bacteria will make more of the DNA and/or protein.

But how do I know if the bacteria *really* have my gene in them? The plasmid vector has a selection marker – often an antibiotic resistance gene – so that if you grow the bacteria that should have it on food containing that antibiotic, only the bacteria that have the plasmid (and hence the resistance gene) are able to grow. These bacteria grow and replicate to form individual “colonies” on a bacterial plate. Each colony has lots of cells but they all have the same genetic makeup

BUT this only tells you if the *plasmid* is inside the bacteria not if your gene is inside that plasmid. To answer this latter question, you can use PCR with cleverly designed primers. You have a few options and presence/size of the copied produces (which you can tell by agarose gel electrophoresis) can tell you different things:

INSERT-SPECIFIC PRIMERS: both primers are in the INSERT (the gene you put in). This is a YES/NO for whether your insert’s present. If your gene’s not there there will be nothing for the primers to bind to -> no product. But if your gene is there the primers will latch on & Pol will copy between them -> product (note that by product I mean a defined, specific product, not “nonspecific products” that can come from primers binding incorrectly (mispriming)

  • tells you if your gene is present BUT NOT if your gene is where you want it…
  • advantage is that you can use this same set of primers to test for your insert in different plasmids

VECTOR-SPECIFIC PRIMERS: both primers are in the VECTOR, straddling the insertion site. As long as the plasmid’s present, you should get some sort of product, but it’s the SIZE of the product that gives you your answer (not a simple yes/no like above) – if your insert’s not in the vector the product will be really short but if your insert’s in there, the product should be bigger (that short length PLUS the length of your insert)

  • tells you if your gene (or something of that same size) is present IN YOUR VECTOR
  • useful because you can use the same pair of primers to test different constructs since the primers are specific for the vector not the insert
  • does NOT tell you whether your insert is inserted in the correct direction. for that you can use

ORIENTATION-SPECIFIC PRIMERS: one primer is in the insert & the other is in the vector

  • you’ll only get a defined product if your gene if facing the right way (not put in backwards so that the “start making protein here” message on the plasmid is next to the “stop making protein here” message on the DNA
  • tells you 1) is your plasmid present 2) is your gene present 3) is your gene in your plasmid and 4) is your gene “backwards”
  • downside is you have to design a specific primer

So we can use PCR as a secondary “screen” when cloning, but we still haven’t answered the question of how we get the DNA to screen. You can purify plasmid DNA out of bacteria – often using easy-to-use “mini prep kits” – they’re easy to use but if you have lots of bacteria to test, you don’t want to waste time purifying something “useless” so you can skip the purification (for now) and add a teeny bit of the whole bacterial cells into your PCR mix.

Just barely touch the colony with a sterile toothpick or pipet tip & swirl it around a bit in your PCR mix. (alternatively, you can resuspend a bit of it (pipet it up in down in some water) and add some of this to the PCR mix.

When the reaction heats up to MELT the DNA (separate the strands) it also LYSES the cells (breaks them open) so that the DNA “spills out” and DNA Pol can latch on.

If you get a positive result, you can then go ahead and grow up more of that colony and purify it. Then you can send it for sequencing to check that there aren’t any mutations hiding in the sequence (colony PCR will only tell you if it’s the right length, not if it’s the right sequence)

more on topics mentioned (& others) #365DaysOfScience All (with topics listed) 👉

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