Wikipedia:Reference desk/Archives/Science/2020 April 21

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April 21

What is this common insect (Sweden)?

I have recently been observing a lot of these insects in my apartment in Linköping, Sweden:

• http://privat.rejbrand.se/insekt.jpg
• http://privat.rejbrand.se/insekt2.jpg
• http://privat.rejbrand.se/insekt3.jpg
• http://privat.rejbrand.se/insekt4.jpg
• http://privat.rejbrand.se/insekt5.jpg

The animal's body length is about one centimetre (0.4 inches). What kind of insect is it? --Andreas Rejbrand (talk) 10:26, 21 April 2020 (UTC)

A non-biting midge (Chironomidae)? Mikenorton (talk) 11:05, 21 April 2020 (UTC)

Agree, probably a male with those fluffy antennae, so not likely to bite. Richard Avery (talk) 13:28, 21 April 2020 (UTC)

Thank you very much! --Andreas Rejbrand (talk) 13:32, 21 April 2020 (UTC)

Resolved

RNA to DNA conversion

In case of coronavirus diagnosis for example, why its RNA should be converted to DNA during real-time reverse transcription polymerase chain reaction, if it's seemingly more simple to search and detect its RNA directly? Thanks. 212.180.235.46 (talk) 19:25, 21 April 2020 (UTC)

I am not a PCR expert... that said, looking at our articles, I can see a possible explanation. Remember that all PCR techniques are dependent upon specific enzymes, such as DNA polymerase, working under the conditions of the PCR process. The polymerase is the thing actually copying DNA enough to be detected. Even when working just with DNA, not just any DNA polymerase can be used. You need one that is stable under thermal cycling, since the PCR process involves thermal cycling, and we thus use a very specific DNA polymerase (Taq polymerase, isolated from a thermophilic bateria). RNA-dependent RNA polymerase is not nearly as widespread among biological organisms as either DNA polymerase or RNA polymerase (which itself needs a DNA template to make RNA, and so wouldn't be useful in an application where you are starting from RNA). It's mainly found in viruses, and highly conserved among viruses, so there may not be a thermally stable version of it that could work for PCR application. Rather than trying to make out own (an incredibly difficult task), it is far easier to use a reverse transcriptase to make complementary DNA from the sample RNA material, and then use existing PCR techniques with DNA polymerase to amplify the material. --OuroborosCobra (talk) 20:06, 21 April 2020 (UTC)

It was once done directly by Northern blot but such a method was inefficient, not sensitive enough and prone to errors. Ruslik_Zero 20:40, 21 April 2020 (UTC)

But since the virus replicates, making more copies of itself, isn't the amount of RNA in the infected person sufficient for detecting and diagnosing directly, without the need for any polymerase? 212.180.235.46 (talk) 21:55, 21 April 2020 (UTC)

You are probably overestimating the amount of RNA that is recovered from a sample, and underestimating the amount of amplification your get from PCR. While it is possible to detect single molecules of RNA (for instance), doing that robustly, reproducibly, fast, and in a clinical setting is something different altogether. 30 cycles of PCR would theoretically increase the signal 2³⁰-fold, or over a billion times. Fgf10 (talk) 22:42, 21 April 2020 (UTC)

Almost all NAT tests require replication in order to have detectable amounts. Beyond that, the replication technique itself is a step in determining a positive detection. We don't merely "detect the presence RNA." You have tons of RNA in your body right now, your own RNA. You need to detect RNA of a specific sequence unique to the target organism. This is why PCR uses primer sequences, basically, a sequence of genetic material that is unique to your target organism, and highly conserved within it (not likely to change due to mutation). Genetic material matching that sequence is what is replicated and amplified. That means successful amplification not only gives you a measurable signal, but is also confirmation that the target organism or virus was in fact present in the sample. Otherwise, all RNA strands in a given sequence would need to be individually sequenced in order to determine their origin. In any human sample, you would have human RNA, you'd have symbiotic and commensal bacteria, etc. That's a LOT of sequencing to do. It is much simpler to have a primer that will only amplify the target organism or virus, and therefore will only work if the target organism or virus is present. --OuroborosCobra (talk) 15:15, 22 April 2020 (UTC)

Thanks. 212.180.235.46 (talk) 12:30, 23 April 2020 (UTC)