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During DNA replication, RNA primase puts an RNA primer in the lagging strand. What is the function of this RNA primer? Why can't the enzymes put DNA fragments directly?
DNA polymerases need a primer oligonucleotide (RNA or DNA) - their substrates are an existing 3'-OH group and a dNTP. The primase however is a typical RNA polymerase, capable of initiating polynucleotide synthesis de novo by positioning a complementary ribonucleoside 5'-triphosphate opposite its complementary DNA base. The primase makes an RNA primer that the DNA polymerase can then use for chain extension. The RNA primer is ultimately degraded and replaced by a DNA polymerase.
The rationale for this difference is that DNA polymerases have an active site that is geared towards proofreading and that primerless initiation would be an error-prone process. By having the primers 'tagged' by virtue of them being RNA, it is possible for the replication machinery to use them but then replace them with a high fidelity DNA copy of the template strand.
Edit in response to OP comment: Synthesis of the leading strand consists of extending an existing DNA. However the leading strand is also originally initiated, at the ori element, with an RNA primer. Once that first initiation event has taken place the synthesis of the leading strand is simply a process of extending that original primer.
Some viruses employ ingenious variations on this theme such as using tRNA primers , or proteins - see Wikipedia.
In simple terms:
Function of the RNA primer: DNA polymerases need a double-stranded DNA region to which they can attach in order to begin copying the rest of the DNA strand. In order to provide this double-stranded attachment site, RNA primers are added by primase, an RNA polymerase which does not require such an attachment site itself.
When DNA replication starts, one primer is needed at the start of the leading strand. The lagging strand will need new primers regularly, and they mark the start of the stretches known as Okazaki fragments.
Why is RNA used instead of DNA?: Adding primers is an error-prone process, and DNA replication must copy the original strand with as high fidelity as possible. Using RNA for primers and then copying the original using DNA means that there is "memory" of which bits of the new strand were originally primers and hence more likely to contain errors (any RNA in the copy must have been primers). This allows removing them, in order to then replace them with proper high-fidelity DNA copies. When this happens, there is double-stranded DNA around the sites where primers were located, allowing DNA polymerases to attach there and fill in the gaps.
Aside from this, I would guess using RNA for priming may be more energy-efficient as RNA is more reactive than DNA.
protected by Chris♦ May 8 '15 at 11:35
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The 6 Most Gigantic Everything in the History of War
In the words of Mahatma Gandhi, "War makes people act like dicks." We don't want to glorify war or otherwise portray it as something other than terrible. Yet, war makes people think big, and sometimes you have to sit back and be amazed by what humans can accomplish when they really, really want to kill each other.
#6. The Big Gun: The Schwerer Gustav
The Gustav is the biggest gun ever fired in combat, period.
"Heavy Gustav," as the Germans called it, was 150 feet-long and weighed 2.7 million pounds (or, as much as 750 full-sized sedans). If you're having trouble grasping the scale of this monster, let's look at the shells it fired:
That's not a toy tank sitting in this thing's shadow. The shells were 11 feet-long, and nearly three feet wide. They weighed 14,000 pounds. The Gustav could hurl them 23 miles. It took a half hour just to load it.
If you're wondering why the war didn't end the moment they rolled this terror onto the battlefield, you have to realize how laughably impractical the thing was. It took 250 men to get it assembled and ready to fire, then another 2,500 to lay twin rails for the damn thing just so they could move it towards Russia, the only country on Earth large enough for Heavy Gustav to hit.
It was basically a weaponized suspension bridge.
The Germans actually tried to fit Gustav's 800 millimeter gun onto a tank, the aptly-named Landkreuzer P. 1500 Monster (yes, Monster). Fortunately, it never left the drawing board of a man who we suspect had a very small penis.
#5. The Big Battle: The Battle of Kursk
The Battle of Kursk was Germany's ill-fated blitzkrieg against the USSR in 1943, the Third Reich's Hail Mary play following the epic ass-kicking they had just suffered at Stalingrad.
Also known as "The Battle of That Building Where Sergei's Mom Used to Live."
How serious were they about the Battle of Kursk? Look what they brought:
Almost 800,000 men. About 3,000 tanks. More than 2,000 planes. Nearly 10,000 cannons and mortars.
It would be one of the largest attack forces ever assembled in human history. What could possibly stop it? How about the single most powerful defense force ever assembled? The Soviets caught wind of the force the Nazis were massing, and reinforced the region with this:
Nearly 2,000,000 men. More than 5,000 tanks. More than 25,000 big guns and nearly 3,000 aircraft.
The Soviet defensive line alone was more than 150 miles long, and 95 miles deep. They dug 3,000 miles of trenches, and planted a million landmines. Once it was show time in July 1943, you had the best the Germans had versus the best the Soviets had in the largest toe-to-toe battle in human history.
Who won? Well, the Soviets became the first military to ever stop a blitzkrieg. But holy shit it wasn't easy. By the time the Nazis decided they'd had enough, over 6,000 destroyed German and Soviet tanks sat burning on the battlefield. As many as 5,000 aircraft lay scattered in smoking hunks of twisted metal.
More than one million uniformed corpses littered the ground.
To put it in perspective: that's more losses than every war in American history up to that point put together. And remember, although large enough to be passed off as its own freaking war, the Battle of Kursk was just one battle in the larger Eastern Front of WWII.
"I may be out of line here guys, but this almost seems like too much war."
Keep in mind, about 80 percent of the losses were on the Soviet side. It didn't matter. While the Germans never recovered from their bloodbath at Kursk, the Red Army needed only one new wave of draft-slips to get back up to their old numbers.
"They will choke on our dead!"
#4. The Big Explosion: The Messines Mines
The Messines were 19 enormous mines that were detonated during WWI in the largest planned explosion in history until the atomic bomb came along. Back then, there was only one surefire way to blow a hole in the planet large enough to create a haunted lake: combine lots of state-sponsored TNT with a maniac who thinks big.
TNT: It's Not Just For Coyotes Anymore
General Herbert Plumer of the British Army was just such a maniac. He was trying to win the Battle of Messines at the time and, knowing that there are few problems that cannot be solved by a big enough explosion, he spent 18 months planning the single deadliest non-nuclear blast in history.
"Surely this will draw attention from my droopy hat."
The operation called for 21 massive mines--and by massive, we're talking more than 50,000 pounds of explosives per mine. Six hundred tons total. All of this would be stashed inside almost five miles of tunnels they dug right under where the Germans were standing.
When they finally punched the big red button, the Earth-shaking blast was enough to kill 10,000 Germans, win the battle and tear the planet 19 new assholes which ranged from 105- to 260-feet across.
Wait, did we say 19? What happened to the other two bombs?
They didn't go off. Then they forgot about them.
The two "mislaid" mines were helpfully left buried in the Belgian countryside by the British army until one exploded on June 17, 1955 after being struck by lightning and killing a cow (researchers have confirmed this to be the single unluckiest cow in the history of existence). As for the other, it's been narrowed to an unspecified location in present-day Belgium that top ranking British officials refer to as "we have no goddamned idea."

subset: commoncrawl


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From what I understand the L-plates and the camera specific anti-twist plates are only made for the Arca Swiss quick release system. Am I right? Are there no anti-twist plates or L-plates that are compatible with the Manfrotto system?
Manfrotto has L-plates: http://www.manfrotto.com/l-bracket-q2 (a few varieties).
Someone else has already mentioned the Manfrotto L-plates. Therefore a little more general advice:
Read this before you start investing significant money in camera support. Afterwards try to play around a bit with ReallyRightStuff, Kirk or similar quick release equipment. After that you probably don't want to touch most of your Manfrotto stuff again. The dovetail precision equipment is just so much better, functionally and stylistically.
Most high-quality camera support will outlive many camera body iterations (though probably not the camera specific L-plates), and tends to be a good long-term investment. If you sell the stuff again at some point it will still be worth quite a lot.
Personally I like the endless combination possibilities of ReallyRightStuff equipment, and its even inter-brand compatibility. As an example, here is a picture of an extra L-plate used to point the camera downwards without tilting the ballhead by 90°:
Bottom up this consists of tripod, ballhead, quick release plate, L-plate, two back-on-back mini quick release plates to connect to the camera L-plate, camera. Very sturdy, very adjustable, and a joy to set up... If I don't do improvised product shots like this, basically the same equipment serves me well for nature, landscape and panorama photos.


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"I spent my entire youth idolizing Nancy Drew. I'm pushing forty now, but some fantasies die hard."
Besides her beloved collection of vintage designer clothing, there is nothing Cece Caruso cherishes more than her childhood memories of Nancy Drew. Her near obsession with the fictional teenage sleuth led her to become a professional biographer of classic mystery writers. And now that she's working on a book about Nancy's pseudonymous creator, "Carolyn Keene, " Cece's in heaven.
At the L. A. home of another rabid Drew-ophile, Cece finds a treasure trove of useful memorabilia, including one unique and somewhat shocking collectible. Later she finds a dead body -- and a puzzle that would sorely test the skills of her spunky girlhood heroine. Now she'll have to channel her former idol and unmask a murderer, and the killer may be coming for Cece next.
About the Author
Susan Kandel is a former art critic for the Los Angeles Times. She has taught at New York University and UCLA, and served as editor of the international journal artext. She lives in West Hollywood, California, with her husband, two daughters, and dog.
Praise For…
"Fact-filled and funny; a worthy successor to Cece's bright debut."
-Kirkus Reviews
"The action moves along smartly."
"A funny, original, well-plotted murder mystery."
-Entertainment Weekly
"There's lots of fun Nancy Drew lore in this airy entertainment."
-New Orleans Times-Picayune
"Kandel offers up a race against time, a nail-biting chase and some unexpected twists and turns."
-Denver Rocky Mountain News
-Sacramento Bee
"A cool, refreshingly tart summer read."
"Full of comedy, intelligence and knowledge, this is a mystery you want to share."
-Trenton NJ Times
"Not a Girl Detective...is as sparkling as a spilled packet of Sweet 'n Low."
-Washington Post Book World
"Entertaining quirks...Kandel also takes care to provide some intellectual stimulus for Cece."
-Marilyn Stasio, New York Times Book Review
"Page-turning...The story is gloriously nostalgic."