Wikipedia:Reference desk/Archives/Science/2013 October 18

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October 18

DNA variability within a single organism

How much variability is there in the DNA sequence from cell to cell within an individual organism (say a human)? There is some variation due to copying errors when new cells are built, right? I imagine the error is very small as a percentage of the whole sequence. Can this error be measured? Sorry if this info is easy to find on WP- I don't know much at all about this. I found Human genetic variation but it doesn't seem to be what I want. Staecker (talk) 01:46, 18 October 2013 (UTC)

You might find these related topics of interest: mosaicism and chimerism. StuRat (talk) 02:58, 18 October 2013 (UTC)

somatic mutation, gene amplification; see also V(D)J recombination for one regulated instance I know of (which is extremely important to any adaptive immunity). I have a nagging suspicion that there are a lot more regulated instances of DNA alteration in somatic cells we haven't discovered, but of course, zero proof for that suspicion. Wnt (talk) 03:53, 18 October 2013 (UTC)

What about between organisms? I read a while ago that humans and chimps were 98% matching in DNA. Our nearest bioloigical species. A little bit farther down it said we share more than 70% with fruit flies. Kinda puts it all in perspective. (andecdotal BTW as I can no longer find the source).

The common estimate is that the error rate in DNA replication for animals is about 1 error per billion base pairs. Since we have 3 billion base pairs, you expect each cell division to produce three errors. Unfortunately, measuring this is difficult. You cannot, for instance, take two random cells from your body, sequence their DNA, and state with confidence the genetic differences between them. DNA sequencing technology typically pools the DNA from many cells to produce an aggregate signal that drowns mutations which are rare within the sample. Technology to sequence the DNA of a single cell exists, but such things introduce more detection errors than DNA replication produces mutations, so most of the differences you find between two samples would be artifacts. Someguy1221 (talk) 06:22, 18 October 2013 (UTC)

Exactly what I wanted to know- thanks! Staecker (talk) 13:39, 18 October 2013 (UTC)

I think that number is way too low. The data here have been evolving over time, but my understanding is that the most recent value for the mutation rate during human cell replication is on the order of 1 per 50 million base pairs, yielding on the order of 50 errors per replication event. See for example http://www.nature.com/scitable/topicpage/dna-replication-and-causes-of-mutation-409. Looie496 (talk) 17:21, 18 October 2013 (UTC)

Hot food kills good gut bacteria?

Does hot (Spicy) food like Thai Tom yum soups, or Tunisian Harisa, or Capsaicinoids and similar substances damage good gut bacteria \ gut flora? Thanks (I also wonder if there exists any typical foods who can..). many many thanks kind helpers ! Ben-Natan (talk) 02:24, 18 October 2013 (UTC)

Well, so the short answer is, I don't know. Our capsaicin article does not mention whether the effects on bacteria have been studied.

But if you're thinking that, simply because these substances provoke a strong response in us, that they must be somehow generally corrosive or hostile to other organisms, no, that doesn't follow at all. Capsaicin appears to be a defense against mammals specifically, and activates a particular kind of response in mammalian nerves to produce sensations of heat and pain. Bacteria are not mammals and do not have nerves, and I see no reason to think capsaicin should have any particular effect on them at all — which is not to say it doesn't, just that there's no obvious reason to think it does. --Trovatore (talk) 02:38, 18 October 2013 (UTC)

Not just mammals, see the 2nd paragraph here: Pain in invertebrates. StuRat (talk) 02:54, 18 October 2013 (UTC)

This http://npic.orst.edu/factsheets/Capsaicintech.pdf says it is toxic to some bacteria. That is as specific as saying it is toxic to some eukaryotes. (Paradoxically it is believed to help prevent ulcers, since it causes the stomach to react to protect itself.) μηδείς (talk) 02:50, 18 October 2013 (UTC)

Think of alcohol. Not a lot of stuff grows in it. Pretty good disinfectant. But the body metabolizes faster than it can really affect gut flora. Digestion is a complex process. I personally am not willing to test whether what goes in one end is the same as what comes out the other even if "fiery fiesta" describes both . Suffice to say it wasn't transformed it into ice cream on the way through. Stupid body. --DHeyward (talk) 05:36, 18 October 2013 (UTC)

Same question was asked here;the answer quotes a study: "no inhibition effect is found when the concentration of capsaicin is less than 0.0125mg/ml". Not sure how much 1 cubic cm of capsaicin would weigh, let's assume 3 gram. Tabasco is minimum 0.2% capsaicin, based on this, so 6 mg/ml; one drop about 1/30 ml; if you add 10 drops of tabasco to 200 ml of soup, the content would be 2mg capsaicin, or 0.01 mg per ml soup, close to the 0.0125mg/ml, so I assume at least some spicy eaters exceed that concentration regularly... Ssscienccce (talk) 12:04, 18 October 2013 (UTC)

These people would have gone over that limit. Count Iblis (talk) 19:11, 18 October 2013 (UTC)

Hydrocarbon smell

Do completely pure straight-chain hydrocarbons have any smell? I thought I remembered reading once that they do not, and that the smell is always due to traces of impurities, which are extremely hard to totally eliminate, but now I can't seem to find this confirmed anywhere. The Wikipedia articles for hexane through decane at least say "gasoline-like" smell, which of course would be expected, but for that feeling that I read somewhere it was actually a misconception. Maybe I'm misremembering it? 86.128.1.149 (talk) 02:45, 18 October 2013 (UTC)

Well, natural gas is mostly methane and is mostly odorless, because of this an odorant is added to help detect leaks. I can tell you from experience that butane does have a smell when it comes out of a lighter, but how "pure" it is, i don't know. Vespine (talk) 02:51, 18 October 2013 (UTC)

I would imagine that normal industrially produced samples of all such chemicals would be some way off 100% chemically pure. 86.128.1.149 (talk) 02:58, 18 October 2013 (UTC)

Smell is a chemical perception. It all depends on the individual but there is a "normal" normal olfaction. Odorants are usually added not because the chemicals are "odorlss" but because you can breath lethal quantities before your brain detects it. Odorants are added to natural gas because it reaches an explosive density before your body registeres enough of a smell. By contrast, the "rotten egg" odor is detected long before it's lethal. I believe most aromatic compounds have a dectable smell at some level (even if it's past the lethal leval). By contrast, nitrogen doesn't appear to have any receptors and is inert. Why people chose cyanide over nitrogen as a method aof execution is a mystery to me. Body no likee cyanide. body will breath nitrogen at normal heart and respiration rates until brain death. --DHeyward (talk) 05:58, 18 October 2013 (UTC)

It appears that H₂S gas is lethal at lower levels than HCN gas (but this seams to be the subject of some dispute) but I think you can smell H₂S at much lower levels. 122.111.240.138 (talk) 06:38, 18 October 2013 (UTC)

What does that have to do with my question? 86.160.217.227 (talk) 11:00, 18 October 2013 (UTC)

• Ok. I guess probably not! Although I know that all the AR grade hexane I have ever smelt smells a bitDie Antwoorde (talk) 12:13, 18 October 2013 (UTC)

Google search for "smell" and n-hexane, n-heptane, seems to return results that mainly mention "gasoline-like", so... Propane and n-butane also have a smell, but the odor treshold is quite high, about 1500 ppm. For longer chains, the smell is stronger, n-pentane treshold is 1.4, n-hexane 1.5. (source for the odor treshold, see google for the smell) Ssscienccce (talk) 12:23, 18 October 2013 (UTC)

I wonder about the purity of the n-alkanes used in this investigation was and what were the impurities. There are some possible impurities with quite low odor threshold values.122.111.240.138 (talk) 16:54, 18 October 2013 (UTC)

I would assume (hope) that laboratories determining the odor threshold of these substances would not neglect something as obvious as the possibility of impurities present in the samples. In other words, if they say the odor threshold of n-hexane is 1.4 ppm, that really means n-hexane and not some other compound mixed with it. The concentration of the compound was determined by gas chromatography, only in case of standard gases (like sulphur dioxide) was the concentration displayed on the bomb used. Ssscienccce (talk) 13:54, 19 October 2013 (UTC)

Confirmed the levels of the compound under investigation yes but it says here that all alkanes, all alkenes except ethylene and all alkynes except acetylene are odourless [[1]] and [[2]].Die Antwoorde (talk) 14:03, 19 October 2013 (UTC)

From my recollection parrafin oil etc does not really smell much. Die Antwoorde (talk) 04:53, 19 October 2013 (UTC)

But look at what it says about the composition of AR grade "hexane" at sigma

grade ACS reagent, description Mixture of hexane isomers plus methylcyclopentane, assay ≥98.5%, impurities Thiophene, passes test

Yes and I remember it usually shows quite a few peaks in a gc trace!Die Antwoorde (talk) 12:39, 18 October 2013 (UTC)

• Anyone know what this [[3]] smells like? Die Antwoorde (talk) 12:45, 18 October 2013 (UTC)

I work with it regularly -- it smells like gasoline, but not as strong. 24.23.196.85 (talk) 02:19, 19 October 2013 (UTC)

Ta but there is still about 3% of other stuff which could contribute. Die Antwoorde (talk) 04:43, 19 October 2013 (UTC)

• Well it says this - "Principal intermolecular interactions between alkane molecules are London dispersion interactions. Due to weak forces, the C1 to C4 alkanes are gases, C5 to C17 are liquids, those with more than 18 carbon atoms are solids at 298K. All are colourless and odourless." here [[4]]. Die Antwoorde (talk) 12:56, 19 October 2013 (UTC)

Nazi gas chambers

I've read just the other day that in gas chambers where carbon monoxide was the asphyxiant (such as the ones at Treblinka and Sobibor), quite a number of victims survived the gassing and had to be shot or burned alive; in the article about the Treblinka camp, it was claimed that pregnant women made up a disproportionally large percentage of these "survivors" (I put "survivors" in quotes because they were killed anyway, just not by gassing). My question is, why? What is it about pregnancy that helps one survive carbon monoxide poisoning (assuming, of course, that the article is correct)? 24.23.196.85 (talk) 04:02, 18 October 2013 (UTC)

Biochemical life giving majic maybe stem cells. --122.111.240.138 (talk) 04:12, 18 October 2013 (UTC)

Fetal hemoglobin (HbF) has a higher affinity for carbon monoxide than does the normal adult hemoglobin (HbA) that is circulating in the mother. So the fetus takes up carbon monoxide faster than the mother, and releases it more slowly...so it's sort of a case of the fetus taking a hit for the team. see Oxygen–haemoglobin_dissociation_curve#Fetal_hemoglobin - Nunh-huh 04:23, 18 October 2013 (UTC)

Pregnancy can cure disease. 122.111.240.138 (talk) 05:02, 18 October 2013 (UTC)

On the other hand, people with anemia are more susceptible to CO effects, and pregnant women have lower haemoglobin levels. It might just be a myth or observer bias? Ssscienccce (talk) 12:39, 18 October 2013 (UTC)

Thanks! I'm inclined to think the fetal hemoglobin explanation might account for this. 24.23.196.85 (talk) 02:40, 19 October 2013 (UTC)

How to make a real 3D movie?

https://www.youtube.com/watch?v=dVHP7Nhsn4E

Let's imagine a high-resolution LED cube has already been invented. It's at least good enough to display NTSC quality images or videos.

I think this cube can easily display 3D computer graphic images. It is also good for display of X-ray computed tomography.

It is a shame that the cube is transparent. You can see an object's inside.

How to make a 3D camera for the cube display?

I think its camera shall be a 3D scanner.

Ordinary 3D cameras are certainly not for this kind of display. -- Toytoy (talk) 11:03, 18 October 2013 (UTC)

See Light-field camera, although I picture the ideal display seeming more like a window that you can look through, and moving around lets you actually peek behind things, and your eyes would be able to focus on things in the display the same way as in real life. Katie R (talk) 13:05, 18 October 2013 (UTC)

I don't think a light-field camera would help much here. It does give you some depth information but that's about it. You'd be better off photographing the scene with ordinary cameras from a bunch of different angles. There has been a lot of research on automatically converting such photos into 3D scenes, but I don't know much about it. -- BenRG (talk) 22:18, 18 October 2013 (UTC)

And actually for your specific display example (I can't watch the video, but your description makes sense), you're right, you would want a 3D scanner. I don't know if there are real-time color systems that are capable of producing the data for a movie of the object being scanned. Katie R (talk) 14:11, 18 October 2013 (UTC)

How long from TIA to CVA (stroke)

Once I've heard a paramedic who said that every person who has TIA (It's a little stroke that goes away in some hours usually) he must get CVA (stroke) in three month from this day, if he doesn't treat of his little first stroke. So, I would like to cheek it out, Is it right setting \ assertion? 95.35.242.34 (talk) 12:44, 18 October 2013 (UTC)

See Transient ischemic attack#Prognosis. Rojomoke (talk) 12:49, 18 October 2013 (UTC)

now it's clear. only ten percent (or according other cite one third) of TIA will develop stroke (maybe I didn't remember well what he said). Thank you. 95.35.242.34 (talk) 12:57, 18 October 2013 (UTC)

How rare is it to have no viewable eclipse for 5 2/3 yrs?

Where a random point in the sky has an unvarying ~35% chance of being blocked by cloud (and remember, it only has to be peek out for seconds of an eclipse hours long to count). But not counting half the penumbral eclipses due to undetectability or ones that must be seen between say 1 and 5:30 in the morning because people that have jobs can't really do that. I don't know if latitude makes much difference (it's very important for total solar rarity though), but 40N. Sagittarian Milky Way (talk) 22:22, 18 October 2013 (UTC)

Those criteria are a bit vague to search with... Would any of these lists be useful:

CFL fixture problem

I have CFL bulbs in fixtures on either side of the porch. The fixtures each have a light sensor, motion sensor, and a timer, and are set to light the bulbs for a few minutes, if motion is detected, when it's dark out. Each works independently. One works properly. The other works when I first put a new bulb in, but, in short order, the bulb fails to light. Instead I get a faint red glow down by the base, and nothing more. The bulb does seem to be bad, though, as it doesn't work in another fixture, either. So, it looks like something is different about that fixture, causing it to destroy CFLs.

I could just put an incandescent bulb in there and not worry about it, but I'm curious about what could be causing the problem. The only thing obviously different between the fixture which works with CFLs and the one that doesn't is that the one which fries CFLs is closer to the indoor switch which provides power to both. Could there be some type of power surge coming from the switch which levels off over the longer distance ? StuRat (talk) 22:57, 18 October 2013 (UTC)

Some types of motion sensing fixtures have a dim down mode, where the light is dimmed to half brighness for a second or so before it is turned fully off. Such a feature will ruin CFL's. I have a fixture like this - it has a tiny switch on the base to enable or disable the dim mode. As all these motion sensing fixtures rely on a device called a triac to switch power to the globe, there is a possible failure mode where full voltage is never applied to the globe - again that will lead to failure of the globe. Triacs are designed to vary the power to an electrical load by delaying the start of each voltage cycle. They are so cheap they are used for on/off control as well. 58.167.234.97 (talk) 02:11, 19 October 2013 (UTC)

Thanks. So I had a bad triac in that fixture, and a fully functional one in the other ? StuRat (talk) 02:37, 19 October 2013 (UTC)

Not actually the triac itself, but the circuitry controlling the triac. If the triac itself fails, the globe will be permanently on or permanently off. 58.167.234.97 (talk) —Preceding undated comment added 04:56, 19 October 2013 (UTC)

Incandescent bulbs in general don't lose much life by being turned on and off a lot, but fluorescents only have so many starts before they fail. For good service life in a motion detector light, along with efficiency, a halogen might be a good choice. LEDs would also withstand frequent on and off cycles, but are still relatively expensive. They are a good choice for a bulb which is on a large percent of the day. Edison (talk) 03:43, 19 October 2013 (UTC)

True, but Stu said it fails "in short order". That implies the globe fails after at most a few dozen times maybe. Whiel CFL's are not desin=gned for short "on" times, it shouldn't fail that quick, so teh fault lies in the fixture circuitry. Or, he needs to turn the dim mode off, if fitted - he won't have noticed the CFL globe dimming, it it most likely will just go out. 58.167.234.97 (talk) 04:59, 19 October 2013 (UTC)

It has two 3-position switches, one of which controls the sensitivity to light, and the other of which controls the time it remains on after motion is detected. There's no switch to control whether it dims. It could just always dim, and not have a switch, but then the question comes up as to why the seemingly identical model on the other side of the porch doesn't seem to burn out CFLs. StuRat (talk) 02:54, 20 October 2013 (UTC)