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HIV/REVERSE/TRANSCRIPTION/AND/AZT/ INTRO D UCTION/ The$human$immunodeficiency$virus$(HIV)$infects$and$destroys$cells$of$the$im mu ne$system,$weakening$a$persons$ability$ to$fight$other$infec (BioInteractive)

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The HIV/REVERSE/TRANSCRIPTION/AND/AZT/ INTRO D UCTION/ The$human$immunodeficiency$virus$(HIV)$infects$and$destroys$cells$of$the$im mu ne$system,$weakening$a$persons$ability$ to$fight$other$infec (BioInter form is 5 pages long and contains:

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Student Handout
Hands-on Activity
HIV Reverse Transcription and AZT
////January/30,/2017/
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HIV/REVERSE/TRANSCRIPTION/AND/AZT/
INTRO D UCTION/
The$human$immunodeficiency$virus$(HIV)$infects$and$destroys$cells$of$the$im mu ne$system,$weakening$a$person’s$ability$
to$fight$other$infec tio n s $an d $d ise a se s .$A cq u ire d $im munodeficiency$syndrome$(AID S)$occurs$when$a$person’s$immune$
system$is$severely$compromised.$W ith o u t$tre at m e n t,$most$people$with$AIDS$die.$Fortunately,$researchers$have$
developed$drugs$to$treat$HIV$infection.$$
A$combination$therapy, $or$drug$cocktail,$of$several$drugs$at$one$time$makes$an$HIV$infection$a$treatable$chronic$disease$
rather$than$a$term in al$illnes s.$The$drug$azidothym idine$(AZT)$is $one$of$the$drugs$commonly$used$in$t h is$d ru g $co c kt ail.$
AZT$targets$a$critical$step$in$the$HIV$replication$cycle:$reverse$transcription.$$
HIV$has$a$single-stranded$RNA/genome.$During$rever se $tran s crip tio n$an$HIV$enzyme$converts$the$HIV$RNA$into$DNA$so$
that$it$can$be$inserted ,$or$integ rated ,$into$th e$ho st$cell$gen om e .$Rev erse $transc ription $is$diffe re n t$fro m$usual$
transcription,$wh ere $DN A$is$tran scribe d$to$R NA .$In$t h is$a c tivit y,$y o u $w ill$le a rn $h o w$reverse$transcription$and$AZT$work.$
First,$however,$let’s$review$the$structures$of$DNA $an d$R NA .$
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DNA/and/RNA/Review/
You$probably$know$that$the$instructions$for$building$all$the$proteins$that$make$up$an$animal$or$plant$cell$are$stored$in$
DNA.$DNA$is$a$nucleic$acid.$RNA$is$also$a$nucleic$acid.$The$genome$of$HIV$is$made$of$single-stranded$RNA.$
Nucleic$acids$(DNA$and$RNA)$are$made$up$of$a$sequence$of$nucleotides.$All$nucleo tid e s$h av e $a$co mmon$stru ctu re :$a$
phosphate$group$linked$to$a$sugar$m olecule$(ribose$in$RNA$and$deoxyribose$in$DNA,$Figure/1)$that$is$in$turn$linked$to$
one$of$four$nitrogenous$bases/(Figure/2).$The$base s$adenine,$guanine,$an d$cytosine$are$found$in$both$DNA$and$RNA;$
thymine$is$found$on ly$in $D N A,$and$uracil$is$found$o n ly$in $R N A .$T h e $b as e s$a re $o fte n $ab b re v iat ed $A ,$G ,$C ,$T ,$an d $U ,$
respectively.$$
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Figure/1./The/sugars/of/nucleic/acids.$All$nucleic$acids$contain$a$five-carbon$sugar$molecule.$In$RNA$the$sugar$molecule$is$ribose $an d$in$D N A $it$is$
deoxyribose$(the$second$carbon$has$a$hydroxyl$group$and$a$hydrogen$in$ribose$and$two $h yd ro ge n s$in $d eo x yrib o s e).$B y$c o nv e nt ion ,$th e $ca rb on $
atoms$of$the$sugar$are$numbered$with$primes.$
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Student Handout
Hands-on Activity
HIV Reverse Transcription and AZT
////January/30,/2017/
//////////////////////////////////Page/2/of/5/
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Figure/2./Nucleosides/and/nucleotides—what’s/the/difference?/A$sugar$molecule$bound$to$a$base$is$called$a$nucleoside.$The$nucleosides$are$
thymidine,$adenosin e,$gua no sine ,$cytidine ,$and $uridin e.$When$the$nucleoside$is$bound$to$a$phosphate$group,$it$is$called$a$nucle o tid e.$$
$
During$RNA$or$DNA$synthesis,$the$hydroxy l$g r o u p $on$the$3ʹ$carbon$of$the$su gar$of$one$nucleotide$forms$a$bond$with$the$
phosphate$of$another$nucleotide.$In$this$way,$o n e $n u cle o t id e$is $jo in e d $to $a n o ther$to$form $a $chain$that$len g t h en s$in$th e $5ʹ$
(phospha te)$to $3ʹ$(hyd roxy l)$directio n$(Figure/3).$$
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$
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Figure/3./Two/illustrations/of/DNA/nucleotide/chains.$The$left$side$of$the$figure$shows$the$chemical$structure$of$a$DNA$sequence$that$is$three$
nucleotides$long.$The$5’$end$has$a$free$phosphat e$gro u p $(att ac he d $to$th e $5’$ca rbo n $o f$th e$rib os e),$and$the$3’$end$has$a$free$hydroxyl$(OH)$group.$
When$a$new$nucleotide$is$attached,$its$phosphate$group$would$form$a$bond$with$the$3’$hydroxyl$group,$so$the$sequ ence$grows$in$th e$5’$to$3’$
direction.$The$right$side$of$the$figure$sho w s$a $sim p lifie d$w a y$o f$illustrating$the$same$sequ en ce.$Alth o u gh ,$stric tly$s pe ak in g,$th e$lett ers$A ,$G ,$C,$T,$an d $
U$stand$for$bases,$they$are$also$often$used$in$diagrams$to$represent$the$whole$nucleotides$containing$these$bases.$By$convention,$DNA$sequences$
are$written$in$the$5’$to$3’$direction.$
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Student Handout
Hands-on Activity
HIV Reverse Transcription and AZT
////January/30,/2017/
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AZT$is$structurally$very$similar$to$the$nucleoside$thymidine$(Figure/4).$In$a$cell,$AZT$can$becom e$link ed $to$a$ph o sph a te$
group$to$form$a$nucleotide$that$can$be$incorporated$in$a$growing$nucleic$acid$chain.$
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Figure/4./AZT/is/structurally/similar/to/thymidine .$Thymidine$is$a$naturally$occurring$nucleoside.$AZT$is$a$drug.$$
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HIV$contains$an$enzyme$called$reverse$transcriptase$that$transcribes$RN A$into $DN A .$When$HIV$infects$a$cell,$the$reverse$
transcriptase$tra nsc ribes$th e$viral$RN A $geno me$into$a$complementary$DNA$strand.$The$enzyme$then$catalyzes$the$
synthesis$of$a$second$DNA$strand,$complementary$to$the$first.$The$double-stranded$viral$DNA$is$then$inserted$into$a$host$
cell’s$chromosome.$The$host$cell’s$RNA$polymerase$transcribes$the$viral$DNA$in to $R N A $molecules$th a t$c an $s er ve $b o th $to $
provide$instructions$for$the$synthesis$of$viral$proteins$and$as$genomes$for$new$viruses.$
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Let’s$examine$this$process!$
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Student Handout
Hands-on Activity
HIV Reverse Transcription and AZT
////January/30,/2017/
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BACKGROUND/QUESTIONS/
1.$Draw$a$sequence$of$10/RNA/nucleotides$to$represent$a $segm e nt$of$th e$HIV$genome.$You$can$just$use$the$shorthand$
version$with$a$string$of$letters.$Make$sure$you$use$each$of$the$four$RNA$nucleotides$at$least$once.$$
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2.$The$first$step$in$reverse$transcription$is$the$synthesis$of$a$complementary$DNA$strand.$Draw$a$diagram$of$the$RNA$
segment$from$step$1$paired$with$DNA$nucleotide$bases.$
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3.$The$se co n d $ste p $is$th e$s yn th es is$o f$a$D N A $st r an d$complementary$to$the$first$to$produce$double-stranded$viral$DNA.$
Draw$a$diagram$of$a$double-stranded$DNA$m olecule.$$
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4.$Compare$your$drawing$of$the$RNA/DN A$n ucle otide $ba se$pa irs$(in$Qu estio n$2)$to $the$DNA$nucleotide$base$pairs$(in$
Question$3),$and$in$each$drawing,$circle$the$nucleotide$base$pairs$that$are$different.$
5.$The$double-stranded$DNA $is$incorporated$into$th e$ho st$ce ll$DNA .$What$is$the$function$of$DNA$in$cells?$
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ACTIVITY$
Your$teacher$will$provide$an$actual$sequence$of$HIV$RNA$(written$as$a$series$of$lette rs)$an d$a$set$of$DNA/nucleotides.$
Using$the$nucleotides,$synthesize$a$DNA$strand$complem entary$to$the$viral$RNA$tem plate.$The$nucleotides$should$be$
assembled$as$follows$(Figure/5).$
$
Figure/5.$Example$of$how$the$growing$DNA$chain$is$assembled.$Each$nucleotide$cutout$has$a$tab$that$sticks$out$which$represents$the$5’$phosphate$
group$that$attaches$to$a$sticky$square$that$represents$the$3’$OH$group.$$
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Student Handout
Hands-on Activity
HIV Reverse Transcription and AZT
////January/30,/2017/
//////////////////////////////////Page/5/of/5/
6.$Record$the$sequence$of$the$new ly$synth esize d$D NA $stran d$be low $ an d$then$take$apart$your$DNA$sequence.$
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7.$Now$replace$one$of$your$T$nucleotides$with$AZT.$AZT$is$similar$in$mo lec u la r$st ru ct ur e$t o $th ymidine$but $w ith $o n e $
importa n t$d iff er en c e .$Ex amine$Figure$3 $sh ow ing$the$chem ical$structures$of$thymidine$and $of$AZT.$What$are$the$
differences$between$the$two$structures?$
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8.$In$cells$where$AZT$is$present,$AZT$can$be$incorporated$in$a$growing$DNA$chain$in$place$of$a$thymidine$base.$ Predict$
what$might$happen$if$AZT$is$incorpo ra te d .$
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9.$Repeat$step$6$above$in$a$cell$where$AZT$is$present.$Write$down$the$sequence$of$the$DNA$you$constructed.$$
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10.$How$is$this$sequence$different$from$the$sequence$in$step$6?$
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11.$Using$the$information$above,$explain$how$AZT$affects$the$process$of$reverse$transcription.$$
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HIVREVERSETRANSCRIPTIONANDAZT INTRO D UCTION The$human$immunodeficiency$virus$(HIV)$infects$and$destroys$cells$of$the$im mu ne$system,$weakening$a$persons$ability$ to$fight$other$infec (BioInteractive)

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