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Nucleic acids as genetic
information carriers
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DNA as the Source of Genetic Information
Although DNA was discovered in 1869 b !riedrich "iescheras a new# acidic# $hos$horus containing substance made u$ ofver large molecules that he named %nuclein its biologicalrole was not recogni'ed(
In 1889 )ichard Altmann introduced the term %nucleic acid&( * 19++ the $urine and $rimidine bases were ,nown(
-went ears later# the two ,inds of nucleic acids# )NA andDNA# were distinguished(
An incidental but $recise observation .19/80 and relevantinvestigations .190 indicated that DNA could be the carrier ofgenetic information(
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Discovery of transformation
-he idea that genetic material is nucleic acid
had its roots in the discover of transformation
in 19/8(
-he $henomenon was first observed in 19/8
b !red Griffith# an 2nglish $hsician whose
s$ecial interest was the bacterium that causes
$neumonia# Streptococcus pneumonia.
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Transformation
Griffith had succeeded in isolating several differentstrains of S. pneumonia .t$e I# II# III# and so forth0(
In the virulent .disease causing0 forms of a strain#each bacterium is surrounded b a $olsaccharide
coat# which ma,es the bacterial colon a$$ear smoothwhen grown on an agar $late3 these forms are referredto as S# for smooth(
Griffith found that these virulent forms occasionall
mutated to non4virulent forms# which lac, a$olsaccharide coat and $roduce a rough a$$earingcolon on an agar $late3 these forms are referred to as)# for rough(
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Transformation
Griffith finall concluded that the t$e II) bacteria
had somehow been transformed, ac5uiring the genetic
virulence of the dead t$e IIIS bacteria(
-his transformation had $roduced a $ermanent#genetic change in the bacteria3 though Griffith didnt
understand the nature of transformation# he theori'ed
that some substance in the $olsaccharide coat of the
dead bacteria might be res$onsible(
7e called this substance the transforming principle.
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-ransforming =rinci$le
=roteins# fats# and ribonucleic acids .)NA0 were allsimilarl shown not to be the transforming agent(
-he mi
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-he 7ershe4hase 2
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-he 7ershe4hase 2/0# biologists did notunderstand e+@ nucleicacid# that a $hage infects a cell b first attaching tothe cell wall# and that $rogen $hages are ultimatel$roduced within the cell(
*ecause the $rogen carried the same traits as theinfecting $hage# genetic material from the infecting$hage must be transmitted to the $rogen# but howthis occurs was un,nown
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-he 7ershe4hase 2S0 and the DNA with radioactive$hos$horus .A/=0(
?hen bacteria were infected with the labeled bacterio$hage#
onl
A/
= .DNA0 entered the cells# and not the
A>
S .ca$sular$rotein0(
-he subse5uent formation of new# com$lete $hage $articlesin the cell $roved that DNA was the e
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-ransfection 2
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)NA As Genetic "aterial
In their wor,# )NA and coat $roteins werese$arated and isolated from -"Fand a second viralstrain# 7olmes ribgrass .7)0(
-hen# mi
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)NA As Genetic "aterial
Also in 19>6# Alfred Gierer and Gerhard
Schramm demonstrated that )NA isolated
from -"F is sufficient to infect tobacco
$lants and direct the $roduction of new -"F$articles# confirming that )NA carries genetic
instructions(
