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1. Shouldn’t all the bacteria of the same species be clones?

Meet E. coli.

E. coli (Credit: National Institute of Allergies and Infectious Medicine)

E. coli is a bacterial species that lives in the colons of humans and all endothermic (“warm blooded”) vertebrates (specifically: mammals and birds). They’re about 2 micrometers long, and from 0.25 to one micrometer across (Wikipedia). E. coli are a normal and healthy part of what’s called our intestinal flora: you have about as many E. coli cells inside of you at any one moment as you have cells that are you (though that number of E. coli cells, 30 trillion, drops every time you have a bowel movement). Nat. Geo.

All bacteria, including E. coli, reproduce asexually through a process called binary fission, which you can follow through the diagram at right.

In “A,” you see a single bacterial cell with a cell wall (“1”), a membrane (“2”), cytoplasm (“3”), and a single circular chromosome (“4”). Bacteria also have additional pieces of DNA called “plasmids,” which are not shown at right (but which you’ll meet below).

When bacteria divide, they start by elongating, and by replicating their DNA, as shown in “B.” In “C,” you can see that the cell now has two chromosomes, and that a new cell wall is beginning to form (at “5”).

By step “D” the cell wall (now “6”) has divided the parent cell into two daughter cells, which split apart into the two daughter cells (shown at “E”).

This process clones the parent into two identical daughter cells. Yet, bacterial populations are genetically diverse. For example, E. coli consists of several strains, most of which are harmless. but some of which can cause bacterial irritation and diarrhea, which can lead to dehydration that can be fatal (especially in toddlers or people with compromised immunity). Where does this bacterial diversity come from? If bacteria multiply through cloning, why aren’t they all the same? Before reading further, think about what you know biology, and see if you can come up with (at least part of) the answer.

2. Five Causes of Bacterial Genetic Diversity

Bacteria genetically differ from one another for five reasons.

a. Mutation

Source: National Library of Medicine through Wikipedia

Bacterial genes are made of DNA. Despite DNA’s stability, mutations in DNA’s base sequence occasionally occur. And because bacteria reproduce so quickly (E. coli can double itself in as little as 20 minutes), even the smallest mutation rate will lead to large numbers of mutants within any E. coli population.

Note that in the example to the left, a single substitution of cytosine for adenine results in the amino acid proline replacing histidine (which could alter the tertiary interactions that give rise to the protein’s three dimensional conformation, potential changing the protein’s function as well).

b. Transformation

Bacterial transformation is a process that we’ve discussed before, because it played a major role in the identification of DNA as the genetic material in the 1930s and 40s, leading to the elucidation of DNA’s structure by Watson, Crick, Franklin, and Wilkins.

[qwiz qrecord_id=”sciencemusicvideosMeister1961-Bacterial Transformation Quiz”]

Source: Wikipedia

[q] See if you can explain what’s happening in the diagram below.

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[f]IERpYWdyYW1zICYjODIyMDsxJiM4MjIxOyBhbmQgJiM4MjIwOzImIzgyMjE7IHNob3cgdGhlIHR3byBzdHJhaW5zIG9mIHBuZXVtb25pYS1jYXVzaW5nIGJhY3RlcmlhIHRoYXQgd2VyZSB0aGUgc3ViamVjdCBvZiBGcmVkZXJpY2sgR3JpZmZpdGgmIzgyMTc7cyBleHBlcmltZW50IGRlbW9uc3RyYXRpbmcgYmFjdGVyaWFsIHRyYW5zZm9ybWF0aW9uLiBTdHJhaW4gJiM4MjIwOzEmIzgyMjE7IChjYWxsZWQgJiM4MjIwO3Ntb290aCYjODIyMTspIHBvc3Nlc3NlcyBhIGNhcHN1bGUsIGFuZCBraWxscyBtaWNlIHVwb24gaW5qZWN0aW9uLCB3aGlsZSBzdHJhaW4gJiM4MjIwOzImIzgyMjE7IGlzIGNhcHN1bGUtZnJlZSBhbmQgaGFybWxlc3Mu
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[q]Transformation can happen in two ways, each of which is below. See if you can explain to yourself (or your partner, if you’re working with one) what’s happening in each diagram. Then click the button to confirm your answer.

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[f]SW4gYm90aCBkaWFncmFtcywgJiM4MjIwO2ImIzgyMjE7IHJlcHJlc2VudHMgdGhlIGJhY3RlcmlhbCBjaHJvbW9zb21lLg==

Cg==Cg==Cg==Cg==
[Qq]The diagram to the left shows uptake of “naked” DNA. Cells absorb fragments of DNA (“a”) in their environment, and this DNA incorporates itself into the host cell chromosome (“c”).  The diagram to the right showns uptake of plasmid DNA. Plasmids (“a”) are small circles of DNA that are not a part of the main bacterial chromosome. Bacteria can exchange plasmids through a process called conjugation (discussed below) or plasmids can enter a bacterial cell. Once inside the cell, the plasmid DNA can be replicated and/or expressed.  

[/qwiz]

c. Bacterial Conjugation

Source: Wikipedia

Like one of the forms of transformation that we saw above, bacterial conjugation involves an exchange of plasmids. However, while bacterial transformation involves the accidental acquisition of foreign genetic material, conjugation is much more directed. Here’s how it works.

In step “1” we see two bacteria of the same species. Bacteria “I” has a plasmid, and this plasmid has genes for production of a structure called a pilus (at “c”). The second bacterium, at II, lacks both the plasmid (and the pilus).

Functionally, the pilus is like a retractable hook. When the pilus contacts the surface of another bacterium (as it does in step “2”), it grabs on. As it grabs on, the pilus retracts, pulling the two cells together.

In step 3, the enzyme DNA polymerase (at “e”) replicates the plasmid, Other enzymes (not shown) create a cytoplasmic bridge between the two cells. Through this bridge, a copy of the plasmid enters into cell II.

In step 4, you can see the result. Because cell II now has the plasmid, it expresses genes for the pilus.

Note that the plasmids behind the conjugation process can have a variety of genes. It’s thought that conjugation, in addition to mutation, is a key mechanism by which antibiotic resistance genes spread through bacterial population.

d. Transduction

In the previous module, we looked at transduction, the process by which accidents in viral replication result in moving DNA fragments from one bacterium to another. If you need to review, click here, then come on back to this module.

e. Transposition

Transposons, or transposable genetic elements, are DNA sequences that can move from one location in an organism’s genome to another. They were discovered by the Nobel-Prize winning geneticist Barbara McClintock (who had to wait about 40 years before her work was widely recognized and deemed Nobel Prize worthy).

Transposons occur in all living things, and as we’ll see later in our course, they make up 44% of the human genome Wikipedia (as well as the genomes of of most other mammals).

There are two ways to think about how transposons work.

The first is to imagine them working the way you work when you cut and paste text in a word processor. The transposon (1) codes for enzymes called transposases, represented by the Pacman scissors. The transposases cuts the transposon sequence out of its current location within a chromosome (2 and 3), and then insert the transposon back in somewhere else (4 and 5). This changes the sequence of genes on a chromosome, but it doesn’t increase the size of the genome: the number of nucleotides stays the same.

But transposons can also replicate themselves through a copy and paste mechanism. When you’re typing, and you copy and paste, you increase the number of characters in the document that you’re creating. The transposons that work through copy and paste are retrotransposons, and they work as follows.

In the image above, “1” shows a retrotransposon. In step 2, this transposon’s DNA is transcribed into RNA, which is shown in red. Next, reverse transcriptase (“3”) creates DNA from the RNA (steps “4” and “5.”)

Notice that the DNA of the original retrotransposon is still in its original location, which means that the DNA at “5” is a clone of “1.” In step 6, the new DNA is inserted somewhere else in the genome (“7”). The transposon, in this case, has both changed this organism’s genetic sequence, and increased its genome size.

Checking Understanding

That explains what you need to know about how genetic diversity is generated in bacteria. Take the quiz below to see how well you’ve mastered what you’ve read.

 

[qwiz qrecord_id=”sciencemusicvideosMeister1961-Bacterial Genetic Diversity (5 causes)” random = “true” style=”width: 650px !important; min-height: 400px !important;”]

[h] Genetic Diversity in Bacteria

[i]

[q] The process shown below represents [hangman]

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[q] The process shown below represents transformation with  [hangman] DNA.

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[q] The process shown below represents transformation with a  [hangman].

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[q] The process shown below represents  [hangman].

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[q] The structure shown at “c” is a  [hangman].

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[q] The enzyme shown at “e” is DNA [hangman].

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[q] The process shown below is [hangman].

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[q] The DNA elements shown at 1, 3, and 5 are [hangman].

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[q] The DNA elements shown at 1, 5, and 7 are [hangman].

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[q] The enzyme shown at 3 is reverse  [hangman].

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[q] The process shown below is  [hangman].

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[q] The result of the process below is two daughter cells that are genetic [hangman] of the parent cell.

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[q] The process below is called  [hangman].

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[q] Mutation, transduction, transformation, conjugation, and transposition are all processes that increase genetic [hangman] in bacterial populations.

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[q multiple_choice=”true”] The process below shows

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[c]IGNvbmp1Z2F0aW9u[Qq]

[f]IE5vLiBDb25qdWdhdGlvbiBpbnZvbHZlcyB0cmFuc2ZlciBvZiBhIHBsYXNtaWTCoCBieSBtZWFucyBvZiBhIHBpbHVzLg==[Qq]

[c]IHRyYW5zZHVjdGlvbg==[Qq]

[f]IE5vLiBUcmFuc2R1Y3Rpb24gaW52b2x2ZXMgdHJhbnNmZXIgb2YgYmFjdGVyaWFsIEROQcKgIHRoYXQgY29tZXMgYWJvdXQgdGhyb3VnaCBhY2NpZGVudHMgaW4gdGhlIHZpcmFsIHJlcGxpY2F0aW9uIHByb2Nlc3Mu[Qq]

[q multiple_choice=”true”] The process below shows

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[f]IE5vLiBUcmFuc3Bvc2l0aW9uIGludm9sdmVzIHRyYW5zcG9zb25zIG1vdmluZyBmcm9tIG9uZSBsb2NhdGlvbiBpbiB0aGUgZ2Vub21lIHRvIGFub3RoZXIgbG9jYXRpb24u[Qq]

[c]IHRyYW5zZm9ybWF0aW9u[Qq]

[f]IE5vLiBUaGlzIGRpYWdyYW0gc2hvd3MgaG93IEROQSBmcm9tIGhlYXQga2lsbGVkIHZpcnVsZW50IGJhY3RlcmlhIHdhcyBhYmxlIHRvIHRyYW5zZm9ybSBoYXJtbGVzcyBiYWN0ZXJpYSBpbnRvIGRlYWRseSBiYWN0ZXJpYS4=[Qq]

[c]IGNvbmp1Z2F0aW9u[Qq]

[f]IE5vLiBDb25qdWdhdGlvbiBpbnZvbHZlcyB0cmFuc2ZlciBvZiBhIHBsYXNtaWTCoCBieSBtZWFucyBvZiBhIHBpbHVzLg==[Qq]

[c]IHRyYW5zZHVjdGlvbg==[Qq]

[f]IE5vLiBUcmFuc2R1Y3Rpb24gaW52b2x2ZXMgdHJhbnNmZXIgb2YgYmFjdGVyaWFsIEROQcKgIHRoYXQgY29tZXMgYWJvdXQgdGhyb3VnaCBhY2NpZGVudHMgaW4gdGhlIHZpcmFsIHJlcGxpY2F0aW9uIHByb2Nlc3Mu[Qq]

[q multiple_choice=”true”] The process below shows

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[q multiple_choice=”true”] The process below shows

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[c]IHRyYW5zZHVjdGlvbg==[Qq]

[f]IE5vLiBUcmFuc2R1Y3Rpb24gaW52b2x2ZXMgdHJhbnNmZXIgb2YgYmFjdGVyaWFsIEROQcKgIHRoYXQgY29tZXMgYWJvdXQgdGhyb3VnaCBhY2NpZGVudHMgaW4gdGhlIHZpcmFsIHJlcGxpY2F0aW9uIHByb2Nlc3Mu[Qq]

[q multiple_choice=”true”] The process below shows

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[f]IE5vLiBUcmFuc3Bvc2l0aW9uIGludm9sdmVzIHRyYW5zcG9zb25zIG1vdmluZyBmcm9tIG9uZSBsb2NhdGlvbiBpbiB0aGUgZ2Vub21lIHRvIGFub3RoZXIgbG9jYXRpb24u[Qq]

[c]IHRyYW5zZm9ybWF0aW9u[Qq]

[f]IE5vLiBUaGlzIGRpYWdyYW0gc2hvd3MgaG93IEROQSBmcm9tIGhlYXQga2lsbGVkIHZpcnVsZW50IGJhY3RlcmlhIHdhcyBhYmxlIHRvIHRyYW5zZm9ybSBoYXJtbGVzcyBiYWN0ZXJpYSBpbnRvIGRlYWRseSBiYWN0ZXJpYS4=[Qq]

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[q] In the diagram below, which letter or number indicates fragments of DNA that are floating in the environment?

[textentry single_char=”true”]

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[f]IEV4Y2VsbGVudCEgTGV0dGVyICYjODIyMDthJiM4MjIxOyByZXByZXNlbnRzIEROQSBmcmFnbWVudHMgdGhhdCBhcmUgZmxvYXRpbmcgaW4gdGhlIGVudmlyb25tZW50Lg==[Qq]

[c]IEVudGVyIHdvcmQ=[Qq]

[f]IFNvcnJ5LCB0aGF0JiM4MjE3O3Mgbm90IGNvcnJlY3Qu[Qq]

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[q] In the diagram below, which letter or number indicates the pre-transformation bacterial chromosome?

[textentry single_char=”true”]

[c]IG I=[Qq]

[f]IEV4Y2VsbGVudCEgTGV0dGVyICYjODIyMDtiJiM4MjIxOyByZXByZXNlbnRzIHRoZSBiYWN0ZXJpYWwgY2hyb21vc29tZSBiZWZvcmUgdHJhbnNmb3JtYXRpb24u[Qq]

[c]IEVudGVyIHdvcmQ=[Qq]

[f]IFNvcnJ5LCB0aGF0JiM4MjE3O3Mgbm90IGNvcnJlY3Qu[Qq]

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[q] In the diagram below, which letter indicates the a transformed chromosome (one that has taken up foreign DNA)?

[textentry single_char=”true”]

[c]IGM7 Mw==[Qq]

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[f]IE5vLg==[Qq]

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[q] In the diagram below, which letter or number indicates a plasmid?

[textentry single_char=”true”]

[c]IG E=[Qq]

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[q] In the diagram below, which letter or number indicates a plasmid?

[textentry single_char=”true”]

[c]IG I=[Qq]

[f]IE5pY2UhIExldHRlciAmIzgyMjA7YiYjODIyMTsgcmVwcmVzZW50cyBhIHBsYXNtaWQu[Qq]

[c]IEVudGVyIHdvcmQ=[Qq]

[f]IFNvcnJ5LCB0aGF0JiM4MjE3O3Mgbm90IGNvcnJlY3Qu[Qq]

[c]ICo=[Qq]

[f]IE5vLCBidXQgaGVyZSYjODIxNztzIGEgaGludDogQSBwbGFzbWlkIGlzIGEgY2lyY2xlIG9mIGV4dHJhLWNocm9tb3NvbWFsIEROQSBmb3VuZCBvdXRzaWRlIHRoZSBtYWluIGJhY3RlcmlhbCBjaHJvbW9zb21lLg==[Qq]

[q] In the diagram below, which letter or number indicates the main bacterial chromosome?

[textentry single_char=”true”]

[c]IG E=[Qq]

[f]IE5pY2UhIExldHRlciAmIzgyMjA7YSYjODIyMTsgcmVwcmVzZW50cyB0aGUgbWFpbiBiYWN0ZXJpYWwgY2hyb21vc29tZS4=[Qq]

[c]IEVudGVyIHdvcmQ=[Qq]

[f]IFNvcnJ5LCB0aGF0JiM4MjE3O3Mgbm90IGNvcnJlY3Qu[Qq]

[c]ICo=[Qq]

[f]IE5vLCBUaGUgYmFjdGVyaWFsIGNocm9tb3NvbWUgaXMgYSBsb25nIGxvb3Agb2YgRE5BLCBtdWNoIGJpZ2dlciB0aGFuIHRoZSBwbGFzbWlkICh3aGljaCBpcyBzaG93IGF0ICYjODIyMDtiJiM4MjIxOyk=[Qq]

[q] In the diagram below, which letter or number indicates a pilus?

[textentry single_char=”true”]

[c]IG M=[Qq]

[f]IE5pY2UhIExldHRlciAmIzgyMjA7YyYjODIyMTsgcmVwcmVzZW50cyBhIHBpbHVzLg==[Qq]

[c]IEVudGVyIHdvcmQ=[Qq]

[f]IFNvcnJ5LCB0aGF0JiM4MjE3O3Mgbm90IGNvcnJlY3Qu[Qq]

[c]ICo=[Qq]

[f]IE5vLCBUaGUgcGlsdXMgaXMgYW4gZXh0ZW5zaW9uIGZyb20gdGhlIGNlbGwgdGhhdCBncmFicyBvbnRvIG90aGVyIGNlbGxzLg==[Qq]

[q] In the diagram below, which letter or number indicates DNA polymerase?

[textentry single_char=”true”]

[c]IG U=[Qq]

[f]IE5pY2UhIExldHRlciAmIzgyMjA7ZSYjODIyMTsgcmVwcmVzZW50cyBETkEgcG9seW1lcmFzZS4=[Qq]

[c]IEVudGVyIHdvcmQ=[Qq]

[f]IFNvcnJ5LCB0aGF0JiM4MjE3O3Mgbm90IGNvcnJlY3Qu[Qq]

[c]ICo=[Qq]

[f]IE5vLCBUcnkgdG8gZmluZCBzb21ldGhpbmcgdGhlIGNvdWxkIHJlcHJlc2VudCBhbiBlbnp5bWUgdGhhdCBpcyByZXBsaWNhdGluZyB0aGUgcGxhc21pZC4=[Qq]

[q] In the diagram below, which number represents a transposon that has moved to a new position in the genome?

[textentry single_char=”true”]

[c]ID U=[Qq]

[f]IE5pY2UhIExldHRlciAmIzgyMjA7NSYjODIyMTsgcmVwcmVzZW50cyBhIHRyYW5zcG9zb24gdGhhdCBoYXMgbW92ZWQgdG8gYSBuZXcgcG9zaXRpb24gaW4gdGhlIGdlbm9tZS4=[Qq]

[c]IEVudGVyIHdvcmQ=[Qq]

[f]IFNvcnJ5LCB0aGF0JiM4MjE3O3Mgbm90IGNvcnJlY3Qu[Qq]

[c]ICo=[Qq]

[f]IE5vLiBJZiAmIzgyMjA7MSYjODIyMTsgaXMgdGhlIG9yaWdpbmFsIHRyYW5zcG9zb24sIHdoYXQmIzgyMTc7cyB0aGUgb25seSB0aGluZyB0aGF0IGNvdWxkIHJlcHJlc2VudCB0aGlzIHRyYW5zcG9zb24gaW4gYSBuZXcgcG9zaXRpb24gaW4gdGhlIGdlbm9tZT8=[Qq]

[q] In the diagram below, which number represents the process of transposon insertion?

[textentry single_char=”true”]

[c]ID Q=[Qq]

[f]IE5pY2UhIExldHRlciAmIzgyMjA7NCYjODIyMTsgcmVwcmVzZW50cyB0aGUgcHJvY2VzcyBvZiB0cmFuc3Bvc29uIGluc2VydGlvbi4=[Qq]

[c]IEVudGVyIHdvcmQ=[Qq]

[f]IFNvcnJ5LCB0aGF0JiM4MjE3O3Mgbm90IGNvcnJlY3Qu[Qq]

[c]ICo=[Qq]

[f]IE5vLiBJZiAmIzgyMjA7MyYjODIyMTsgaXMgdGhlIHRyYW5zcG9zb24gYXdheSBmcm9tIGl0cyBvcmlnaW5hbCBwb3NpdGlvbiwgYW5kICYjODIyMDs1JiM4MjIxOyByZXByZXNlbnRzIHRoZSB0cmFuc3Bvc29uIGluIGl0cyBuZXcgcG9zaXRpb24sIHdoYXQmIzgyMTc7cyB0aGUgb25seSB0aGluZyB0aGF0IGNvdWxkIHJlcHJlc2VudCB0aGUgcHJvY2VzcyBvZiBpbnNlcnRpb24/[Qq]

[q] In the diagram below, which number represents reverse transcriptase?

[textentry single_char=”true”]

[c]ID M=[Qq]

[f]IE5pY2UhIExldHRlciAmIzgyMjA7MyYjODIyMTsgcmVwcmVzZW50cyByZXZlcnNlIHRyYW5zY3JpcHRhc2Uu[Qq]

[c]IEVudGVyIHdvcmQ=[Qq]

[f]IFNvcnJ5LCB0aGF0JiM4MjE3O3Mgbm90IGNvcnJlY3Qu[Qq]

[c]ICo=[Qq]

[f]IE5vLiBBbiBSTkEgdHJhbnNjcmlwdCBvZiB0aGUgcmV0cm90cmFuc3Bvc29uIEROQSBpcyBzaG93biBpbiByZWQsIHJpZ2h0IGJlbG93IG51bWJlciAmIzgyMjA7Mi4mIzgyMjE7IFdoaWNoIG51bWJlciBvbiB0aGUgZGlhZ3JhbSByZXByZXNlbnRzIGFuIGVuenltZSB0aGF0IGlzIGNvbnZlcnRpbmcgdGhpcyB0cmFuc3Bvc29uIFJOQSBpbnRvIHRyYW5zcG9zb24gRE5BPw==[Qq]

[q] In the diagram below, which letter represents a phage that has picked up bacterial DNA, instead of viral DNA?

[textentry single_char=”true”]

[c]IG Y=[Qq]

[f]IE5pY2UhIExldHRlciAmIzgyMjA7ZiYjODIyMTsgcmVwcmVzZW50cyBhIHBoYWdlIHRoYXQgaGFzIHBpY2tlZCB1cCBETkEgZnJvbSB0aGUgYmFjdGVyaXVtIGl0IGp1c3QgYXR0YWNrZWQsIGluc3RlYWQgb2YgdmlyYWwgRE5BLg==[Qq]

[c]IEVudGVyIHdvcmQ=[Qq]

[f]IFNvcnJ5LCB0aGF0JiM4MjE3O3Mgbm90IGNvcnJlY3Qu[Qq]

[c]ICo=[Qq]

[f]IE5vLiBTdGFydCBieSBub3RpbmcgdGhlIGNvbG9ycyBvZiB0aGUgdmlyYWwgRE5BIGFuZCBiYWN0ZXJpYWwgRE5BIGluIGZpZ3VyZSAxLiBUaGVuIGNhcmVmdWxseSBleGFtaW5lIGZpZ3VyZXMgMiBhbmQgMywgYW5kIGlkZW50aWZ5IHRoZSB2aXJ1cyB0aGF0IGhhcyBwaWNrZWQgdXAgdGhlIHdyb25nIEROQS4=[Qq]

[/qwiz]

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