Thread by @AdamRutherford

I'm seeing a lot of chat about PCR and COVID, some of it quite bonkers. So here’s a PCR 101.

DNA is very small and therefore not easy to detect. PCR is a tool for multiplying specific bits of DNA from a few to billions, thus making it easy to detect. 1/n

PCR is a standard tool in molecular biology, and has been for decades. It was, by the way, invented by Kary Mullis, who claims to have come up with the idea whilst tripping balls on LSD. Mullis was a bit of a jerk btw, an AIDS and Climate Change denialist. 2/n

Right, so DNA is a DOUBLE helix, meaning it has two strands made up of chains of just 4 ‘letters’ that pair up in a specific way (that is, ‘complementary’): A pairs with T, C pairs with G. a bit like this.

GAACTTAATTAA
CTTGAATTAATT

3/n

When you warm it up, they separate:

GAACTTAATTAA

CTTGAATTAATT

4/n

Add a short, specific bit of complementary DNA at either end (a primer), you can prompt the replacement of the missing strand, on each separate strand:

GAACTTAATTAA
CTT
GCC
CTTGAATTAATT

5/n

Cool it down again, and the enzyme polymerase (the P in PCR) fills in the rest, so you end up with two identical, photocopied versions of the original bit of DNA.

GAACTTAATTAA
CTTGAATTAATT

GAACTTAATTAA
CTTGAATTAATT

6/n

Repeat the cycle a few times, and you have billions of copies, and this specific bit of DNA is now easy to detect.

PCR is a signal booster. This is the technique on which all genetics is entirely reliant. Every geneticist on earth has done this thousands of times. 7/n

We used to do this by hand, but now the whole kit has shrunk to the size of phone now.
This is what the rapid turnaround COVID tests are. They detect the presence of COVID genes, indicating that the virus is active (I believe detecting the gene for the Spike protein) 8/n

Except there is an extra step. The COVID genome is not made of DNA, it’s actually RNA, which only comes in single strands.

So we add another enzyme, called Reverse Transcriptase, which replaces the missing strand and turns it into double stranded DNA for PCR. 9/n

You can add a dye to the process, such as SYBR Green, and this speeds up detection, making the whole thing possible in real time. 10/n

Anyway, that’s the very basics. It's a standard test. I am aware that some of the tests are not working particularly well for COVID mass detection, and there are technical and supply chain problems. 11/n

But there isn’t some grand conspiracy behind mass PCR-based testing. That would be like protesting against screwdrivers.
12/12

Addendum; Yes the sequence is why the film is called Gatacca

Addendum 2: flat head screwdrivers are a conspiracy. They make no sense whatsoever.

Addendum 3: Twitter is full of brilliant geneticists who work with cutting edge tech to do better science. I haven't held a pipette for years. But it pays to listen to experts. Translate my DNA sequence above.

Addendum 4: My demo sequence above was very short. Genes are typically thousands of base pairs (letters) long, and the primers typically 10-20 letters, thus making them utterly specific. If designed well, you can find one gene in a billion.

Addendum 5: I stupidly screwed up my primers trying to be clever with my sequence.

GAACTTAATTAA
CTT
TAA
CTTGAATTAATT

And if you really want to know how PCR and next gen sequencing works follow my friends @DrCJ_Houldcroft and @ewanbirney