Science desk
< December 31	<< Dec \| January \| Feb >>	January 2 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

January 1

Positive vs negative selectable markers

A text which is discussing gene knockout, which I am trying to interpret, reads: one must design and construct and appropriate targeting vector in which the gene of intere4st has been disrupted with a positive selectable marker; in the most commonly used protocol, a negative selectable marker is also added at a position that flanks the gene sequence ... second step involves the introduction of the targeting vector into a culture of embryonic stem cells followed by selection for those cells in which the internal positive selectable marker has become integrate into the genome without the flanking negative selectable marker. Am I correct in my interpretation that the absence of the flanking negative selectable marker just allows discrimination between a) the cells in which integration has taken place (positive selectable marker present) and b) the cells in which the vector is still present extra-genomically or has otherwise inserted at some unintended site (positive selectable marker present)? --Seans Potato Business 01:47, 1 January 2008 (UTC)[reply]

[1] Spinningspark (talk) 14:19, 1 January 2008 (UTC)[reply]

I tried to "knock" the gene knockout article into a useful shape. I think you have it right, except I assume that when you wrote, "the absence of the flanking negative selectable marker just allows discrimination," you actually meant "the presence of the flanking negative selectable marker just allows discrimination." The desired integration is almost always a homologous recombination which will not insert the negative selectable marker. Sometimes the whole DNA construct gets inserted into a chromosome in a less specific way (as you say, "otherwise inserted at some unintended site"). See this textbook. --JWSchmidt (talk) 14:50, 1 January 2008 (UTC)[reply]

"the absence of the flanking negative selectable marker just allows discrimination" <-- Having read this a second time I guess you were talking about the "the absence" after a homologous recombination event. I first read it as a statement about the DNA constructs used for knockouts. --JWSchmidt (talk) 21:35, 1 January 2008 (UTC)[reply]

I've made an image that can be used in the knockout article. Have you any suggestions? :) --Seans Potato Business 01:47, 2 January 2008 (UTC)[reply]

~~I downloaded the file but I cannot open it.~~ On my second try I got it to open in GIMP. Comment: high entertainment value, but it will probably upset a few special interest groups. --JWSchmidt (talk) 15:02, 2 January 2008 (UTC)[reply]

Time since directly related

Hi everyone, and Happy New Year! You have probably heard the old saw that everyone is related. Although this may be true if you go back far enough, just how far back do you have to go before it is actually true? Thanks in advance for you answers. 71.100.3.166 (talk) 03:57, 1 January 2008 (UTC)[reply]

Some reading material, until someone gives you an exact date: The article on most recent common ancestor covers some of this, toward the bottom there's a subsection on time estimates. And you might be interested in the articles on mitochondrial Eve and Y-chromosomal Adam. ---Sluzzelin talk 04:51, 1 January 2008 (UTC)[reply]

Literal "Fire" Hose

I've heard a long time ago an anecdote about the friction from the water running through a fire hose can sometimes set it on fire. Although it sounds interesting (and ironic), I question the factuality of this story. Has this ever happened? Is it even possible by any remote degree? Chris16447 (talk) 05:06, 1 January 2008 (UTC)[reply]

It doesn't make sense for two different reasons. First, a heavy flow of water means that new water is flowing into the hose all the time and this won't be so hot it can't absorb any heat resulting from its own flow. Second, a fire hose isn't going to be made of material that will ignite at a temperature below the boiling point of water, and if the water somehow got hot enough to boil in the hose, the water wouldn't be left running. --Anonymous, 05:55 UTC, January 1.

Also, more importantly, there would be no air in the hose- and no burning. 24.56.161.10 (talk) 02:28, 2 January 2008 (UTC)[reply]

Ah, no, that's not a reason. If the fluid in a hose actually was hotter than the hose's ignition temperature, then it would heat the whole hose by conduction, and the outside would ignite. --Anonymous, 04:06 UTC, January 2, 2008.

The hose is designed to keep from burning - even when it isn't full of water. When trapped in a fire, the hose is used by firefighters to find their way back out. If it just burned up, there would be nothing to lead them out of the fire. Therefore, it is rather ridiculous to assume that a hose that can lay in open flames without burning under most normal conditions would burst into flames under even the rarest occurrences of water friction. -- kainaw ™ 04:10, 2 January 2008 (UTC)[reply]

If it's used to find the way out, is there any kind of direction indication on the hose? How do the firefighters know they're following it out and not in? moink (talk) 09:39, 4 January 2008 (UTC)[reply]

You carry it in on one side of your body and follow it out with it on the other. If you are separated from the hose and disoriented, you use your alarm (a locater on your suit) to get someone with a guide (a hose or rope) to come to you. Note that older hoses would burn much more readily than new ones. See Charleston Sofa Super Store fire. The hoses were too small and too old. They did burn. Nine firefighters were lost in the fire and died. -- kainaw ™ 13:04, 4 January 2008 (UTC)[reply]

The article on the Charleston fire says that hoses that were too small may have been used (there seems to be some dispute), but it certainly doesn't say anything about them hoses burning. --Anon, 09:05 UTC, January 5.

Urine tap

What muscle controls urine flow? -CarbonLifeForm (talk) 12:26, 1 January 2008 (UTC)[reply]

Mainly the detrusor urinae muscle and the sphincter urethrae. ›mysid (☎∆) 13:32, 1 January 2008 (UTC)[reply]

CCD exposure

Why does it take longer for a digital camera to process an image with a longer exposure time? ›mysid (☎∆) 13:30, 1 January 2008 (UTC)[reply]

In some digital cameras, if you expose for a prolonged period (e.g, 1 second), it will take another totally darkened frame (shutter closed; same exposure time) and use the dark exposure info to eliminate sensor artifacts such as hot spots formed due to prolonged exposure. See dark frame subtraction. -- Toytoy (talk) 14:18, 1 January 2008 (UTC)[reply]

Thanks! Makes perfect sense. ›mysid (☎∆) 14:43, 1 January 2008 (UTC)[reply]

You may be able to disable this feature, if you want. My Pentax K10D has a custom setting for it, called "Noise Reduction". -- Coneslayer (talk) 13:06, 2 January 2008 (UTC)[reply]

I would recommend taking a photo with noise reduction, and then without it, while keeping the lens cap on. Compare the difference and determine whether taking noise reduction off is acceptable; on most cameras it at high ISO sensitivities it makes greatly reduces the amount of non-random noise. --Bowlhover (talk) 00:48, 3 January 2008 (UTC)[reply]

Glass saw?

There is a multi-tool for rescurers to saw open a car windshield:

Victorinox Rescue Tool

What makes this saw blade capable of cutting laminated safety glass? I did a quick and dirty photo analysis. I noticed that the saw teeth are arranged in a L-S-S-L- pattern (large/small tooth) but the distances between two adjacent teeth are somewhat irregular.

Can a regular coarse wood saw be used to cut the laminated safety glass? -- Toytoy (talk) 13:51, 1 January 2008 (UTC)[reply]

I think so, if the saw is very coarse. Glass has a Mohs hardness of from about 5 to 7. Steel files are around 6.5, and plain steel saw teeth are apt to be softer than files. The discrepancy in hardness between the steel and the glass is not great. Western wood saws are push saws and are rather thin, and they would tend to bend while trying to cut glass. A pull saw (like the Victorinox) would stand a better chance, I think. Even if the teeth were softer than the glass, I'd expect them to break the glass away as they struck until they got worn down too much.

The Victorinox saw is curved to maintain good contact, is stiff, has unset teeth spaced and angled correctly to get a good bite, and is probably very hard. (Cool video, by the way.) --Milkbreath (talk) 17:28, 1 January 2008 (UTC)[reply]

I think the saw may exploit the fact that glass may not withstand strong shear force. The coarse tooth "bites" into a broken glass edge and pulls the edge to break it. I guess I may insert a pipe wrench into a windshield hole and use brute force to get equivalent results. However, the saw may save much efforts and time. It seems like the saw only has a veryshort life. It is replaceable! If Mohs hardness is a must, Victorinox could have used ceramics (regardless of the costs). They used steel instead. -- Toytoy (talk) 17:44, 1 January 2008 (UTC)[reply]

It comes with everything you need for rescue on the go! A "Large Locking Blade", a "Window Breaker", a "Crate Opener" a "Seatbelt Cutter", a Disc Saw for Shatterproof Glass aaand... a Toothpick! With one of these in your handbag, you have no excuse not rescue people (or make your gums bleed). --Seans Potato Business 19:27, 1 January 2008 (UTC)[reply]

Looking at this (excellent music on the video!!), I wonder if the mechanism of cutting is somewhat unusual. A wood saw effectively grinds away at the wood, but I think this saw might be working by impacting its teeth with the glass. Because of the way it is constructed, laminated safety glass can withstand large forces, but shatters spectacularly when it does break. As a result, a saw that progressively impacts the glass with its blades cuts at a reasonable rate. To cut by conventional means, a saw should have 3 teeth in contact with the material being cut - the pitch of the blades is clearly too large for that! —Preceding unsigned comment added by 212.69.54.32 (talk) 22:30, 1 January 2008 (UTC)[reply]

(A nit) A wood saw doesn't "grind away" at wood; it acts as a series of small chisels, each cutting (removing) a small chip of wood.

Atlant (talk) 13:31, 2 January 2008 (UTC)[reply]

Microwave plasma

There are many videos on Youtube of people playing with microwave ovens, such as this one [2]. They say they can obtain the fifth state of matter (plasma) that way. May somebody explain briefly the physics behind this? --Taraborn (talk) 14:00, 1 January 2008 (UTC)[reply]

Already being discussed above Wikipedia:Reference desk/Science#grapes in a microwave
Spinningspark (talk) 14:13, 1 January 2008 (UTC)[reply]

Oh, sorry. Thanks. --Taraborn (talk) 14:27, 1 January 2008 (UTC)[reply]

Efficiency of full vs emptry refridgerator

Mum says that a fridge running while empty uses more electricity than a fridge that is full of things. Is she misinformed? --Seans Potato Business 19:22, 1 January 2008 (UTC)[reply]

More electricity per item, technically, but not more electricity overall. Possibly even less, because you don't open the fridge quite as often when it's just about empty anyway. A fridgefull consumes more electricity total, but less electricity per item (again, technically, this also has something to do with frequency of opening the door, the door's insulation, the temperature of the air, etc. etc.). But if you want to store a large amount of things in as many refrigerators as you like, with maximal energy savings, put as many as you can into the most efficient one and unplug all the empty ones. Regardless, your parent(s) may have a vested interest in keeping you from starving. (;-) ~user:orngjce223 how am I typing? 20:06, 1 January 2008 (UTC)[reply]

If you left the door shut, the electricity consumption would stabilise at the same level. Also when you open the door, much of the cold air runs out, and needs to be heated again. The fullo fridge will have less air to waste heat!. Graeme Bartlett (talk) 20:12, 1 January 2008 (UTC)[reply]

If it was completely empty you could unplug it. :) Anyway, here is Cecil's whack at it. —Preceding unsigned comment added by TotoBaggins (talk • contribs) 20:24, 1 January 2008 (UTC)[reply]

This was discussed before, but I can't find it. I think it was essentially what Graeme said. — Daniel 21:05, 1 January 2008 (UTC)[reply]

I always understood that the reason a full fridge required less power to maintain temperature was that an empty one lost more heat through convection processes. A full one blocks the air flow and slows this down. I can't remember where I got this information from but I am "sure" it was a reliable source.Spinningspark (talk) 22:32, 1 January 2008 (UTC)[reply]

Also note that while a full fridge (or freezer) uses less energy for the reasons given above, it also uses more energy to initially cool the items in a full fridge than it does to cool the air of an empty one. So you have to remember that trade off too. —Pengo 23:36, 1 January 2008 (UTC)[reply]

IEs

Why does ionisation energy decrease as atomic number increases, and are there any anomalies to this generalisation? Daxert (talk) 21:06, 1 January 2008 (UTC)[reply]

To give you a simple answer from classical physics which entirely ignores quantum mechanics (a sin for which I will probably be jumped very quickly by the proper scientists out there). Consider the outermost electron. All the rest of the atom is beneath it and approximates to a point charge of +1. However, since a higher atomic number means the electron is in a higher orbit, it requires less force (and hence energy) to remove it - the coulomb attraction force is an inverse square relation to radius.

However, quantum mechanics completely messes up this simple picture because the electrons will interact with each other. It turns out that the Band IIX elements are much more stable (and difficult to ionise) than Band I. This certainly will cause anomolies. Hence He (Z=2) has an ionization energy about 2400 kJ/mol, Li (Z=3) is 520, but Be (Z=4) is 900. Spinningspark (talk) 23:03, 1 January 2008 (UTC)[reply]

I should also have mentioned that the ionisation data is in Wikipedia if you look up each element and there is an article on ionisation. Spinningspark (talk) 23:08, 1 January 2008 (UTC)[reply]