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524
unit
5
Genetics
26-2 The Genetic Material
Section Objectives:
t Describe the experiments that showed that DNA is the
material.
r lisf the three chemical parts of a DNA nucleotide'
. Explainthe Watson-Crick model of DNA'
. Describe how DNA replicates in a living cell'
L
F
1
The Chemistry of the Gene
From the work of Sutton, Morgan, and many other researchers,,
was known by 1950 that chromosomes carry hereditary infor
tion. It was also certain that the information is present in dist
.lunits, called genes, arranged along the chromosomes like beads
a string. Still, no one knew what a gene was or how it wc
Without that knowledge, heredity and genetics could not be
understood. This understanding came in the ,1950s, when
chemical nature of the gene was discovered' The first clues to
chemical nature of the hereditary material were uncovered
earlier, however.
In 1869, Friedrich Miescher, a Swiss biochemist, i
material from the nuclei of fish sperm. He called the ma
nuclein (Noo klee un). other scientists showed that nuclein ra
made of carbon, hydrogen, oxygen, and nitrogen' It was also rich
phosphorus. When nuclein was shown to be acidic, its name
ctrangeo to nucleic acid. Later research found two kinds of nu
acid-deoxyribonucleic acid, or DNA, and ribonucleic acid,
RNA. DNA o..urr mainly in the nuclei of cells. RNA is found mair
in the cytoplasm.
In the 1920s, scientists found that chromosomes
DNA. It was already known that chromosomes contained prott
while the chemical structure of proteins was well understood,
structure of DNA was completely unknown' Although a few
tists suggested that DNA was the hereditary material, most
tists believed that only proteins were complex enough to
genetic information.
Protein vs. Nucleic Acids
It did not become clear until the 1950s that the hereditary
of the chromosomes was DNA' To understand how this came
you need to understand the experiments performed by
Griffith and several other researchers.
Griffith's Experiments In 1928, Frederick Griffith, an
bacteriologist, was trying to find a vaccine against pnet
Pneumonia is a disease caused by a kind of bacteria
pneumococcus (noo muh rAHr us). Griffith knew that there are
iypes of pneumococcus. See Figure 26-8. One type, called.Tyl
ii iurrounded by'an outer covering called a capsule' Type S
n_
F
-
surroul
la.
ia
Dead T
mice.
wi1
mice de
the dead
Remen
CAUS(
pnel
,or
w(
,Mi
Leod,
i
identi
In
o1
R bac
sul
thoug
conclr
Chapter
26
Modern
Genetics 525
a.
live
rrylJ.ir,E$(*6
c.
dead
Iype S
bacteria
tnouse
lives
blood of dead mouse
contains live Type S bacteria
live Ai
R<i
Type
bacteria
severe case of pneumonia. The other type, called Type R, is
not surrounded. by a capsule. Type R bacteria do not cause
cause a
pneumonia. If mice are injected with Type S bacteria, they develop
pneumonia and die. Mice injected with Type R bacteria show no ill
effects.
Dead Type S bacteria do
into mice.
not cause pneumonia when injected
In Griffith's key experiment, he mixed dead Type
S
bacteriawith tive Type R. When he inlected the mixture into mice.
the mice.developed pneumonia and died. Furthermore, the tissues
of the dead mice showed living Type S bacteria.
Remember that neither dead Type S nor live Type R bacteria
alone cause pneumonia. When brought together, however, they do
cause pneumonia
concluded
and iiving Type S bacteria appear. Griffith
that some factor from dead Type S bacteria could
change, or transform, Type R bacteria into-Type S. The changed
Dacteria were able to make capsules and to cause pneumonia in
rnice.
McCarty In 1944, Oswald
Avery, Colin
in New
Institute
and Maclyn McCarty of the Rockefeller
York identified
experiment
as
the transforming material in Griffith's
DNA. In
traits
in
inherited
new
other words, DNA produced the
rype R
DNA
is
the
that
bacteria. Although this was strong evidence
Senetic substance, many scientists remained unconvinced. They
thougrrt that protein must carry the hereditary information.
Itll
r,he conclusive
evidence supporting DNA was obtained by Alfred
nershey
and Martha Chase in 1952.
Macleod, and
lvery,
ivtacleod,
A
Figure 25-8
Griffith's Experiment. (A) Live Type S
bacteria will kill the mouse. (B) Live Type
R
bacteria are harmless. (C) Dead Type S bacteria
are harmless. (D) Dead Type S bacteria are
mixed with live Type R bacteria. (E) The
mixture kills the mouse, and live Type S are
present in the'mouse's tissues. Griffith
concluded that the Type R bacteria had been
transformed into Type S bacteria.
526
unit
5
Genetics
Hershey and Chase Alfred Hershey and Martha Chase made
use of viruses called bacteriophages to resolve the DNA t s. protein
argument. A bacteriophage, $ phage (FAY J) for short, is a virus that
Phage
a.
DNA tagged
protein coat
tagged with
with radioactive
.&
CI
@#
phosphorus
radioactive
;"'ffi
o.oll''"0..
I
I
cytoplasm
tested
radioactivity
"r.
ffi@
No Radiation
I
I
I
Radiation
Figure 26-9
The Hershey-Chase Experiment. (A)
Structure of one type of bacteriophage.
infects bacteria. This kind of virus is made of a DNA core surrounded by a protein coat. See Figure 26-9. A phage invades a
bacterium and makes hundreds of new phage particles once inside
the bacterial cell. The bacterial cell then breaks open, and the new
phage particles are let go. These can'attack other bacterial cells.
Hershey and Chase wanted to discover whether the whole phage
entered the bacterium or whether iust the DNA or the protein coat
entered. In hopes of answering this question, they tagged the
protein and the DNA of the phage particle with difterent radioactive
elements.
DNA contains phosphorus but no sulfur. Virus protein contains
sulfur but no phosphorus. Hershey and Chase tagged the phage
DNA with radioactive phosphorus. They tagged the protein coat
with radioactive sulfur. One group of bacteria was then exposed to
phages with radioactive DNA. Another group was exposed to
phages with radioactive protein. After large numbers of bacteria
had become infected with phages, the cytoplasm of the bacteria
was tested for radioactivity. The cells that had been infected by
phages with radioactive DNA showed a great deal of radioactivity,
The cells that had been infected by phages with radioactive protein
showed almost no radioactivity. This experiment proved that the
phage DNA enters the cells, while the phage protein stays outside
when phages i+fect bacteria.
If phage DNA alone can cause bacteria to make more phages, it
must be the DNA that carries the genetic instructions for making
phages. This experiment established DNA as the genetic material'
Ttre proUtem then became that of finding what DNA is made of and
how it works.
(B)
Bacteria infected by phages with proteln coats
are tagged with radioactive sulfur. The cell
contents do not become radioactive. (C)
Bacteria infected by phages with DNA are
tagged with radioactive phosphorus. The cell
contents become radioactive. Hershey and
Chase concluded that a phage infects a
bacterial cell by injectlng its DNA into the
bacterium. The protein coat remains outside.
Composition of
DNA
,
The first step in analyzing an unknown organic compound is to,fi{
out the chemical groups that form it. In the 1920s, P. A. LeySl
biochemist, carried out a chemical analysis of DNA. Levene fQ
that the DNA molecule is made up ol the following chemical gl
the s-carbon sugar deoxyribose (dee ahk see RY bohs); a
phate group; and four kinds of nitrogen-containing (nitrogt
bases. Two of the four bases, known as adenine and
kind of compound called a purine (rvoon een). The o
cytosine and thymine, are compounds called
uh deenz).
Levene found that there was one phosphate group
nitrogen-containing base for each sugar unit. He theretor
ed thit the basic unit of DNA is a sugar, a phosphate, and
four nitrogen-containing bases. He called this unit a
(uoo klee uh tyd). Since there are four difterent
lour different kinds of nucleotides. Many, many nuc
up a single DNA molecule.
Chapter 26
Modern
Genetics 527
of DNA
i-,.rt makeuP of DNAwas known' the::::::"t:?Y?:
ffxi};;Tfir#i'?[t+:"fi]i#it]i##,
fi]l#i:i;hllilt'::*r*it"':r
-ot
X#,T'-]i'i;1il;"*t
pi ece
,ff
i
n
r
orm ation tran sm
itdeoxyribose
Mauricewukin-s'y:l't}:::l:?,1^[::iBu::r:ffi1
i-'"v studies or DNA crvstars'
i"iJ[:#ffi:'1il;J,"J"
0.
-iay photo raol' in:y; :T:, :::;:n*f^t#l
the
tit<s;' A herix is lli
:fi'H,iJ::11iil"ffi "r " n"ri* 1'ui
g
rad
:in
phosPhate
"fi:;"Y*'ffi
Tnif;
'":f#:i
f#'d$;*:::llJfiparallel
ffiIfl
to each other' Pairs of
F-nhosphate groups
r
'"ni'g
ol a ladder. see Figure
the
[r,"
"ner
ffi1il"_rffi":ililg"n", ii'f" tuaa"'
fo'mt' the helix of
fr. twisting o, to"'ni'tn"
helix'
ilrr..'iirrt,
of
double
the DNA motecute \s a
Watson and Crick
work only if the
ro"'i"tiiitn"i' *oa"r couldwere
an adenine
tadder
made;;;;;f theconnected
to a cytosine'
or a g-uanine
connected to a thymine
For
molecule'
DNA
the.
;; a;; for
; model"agreed witn ait
DNA is always
1e
uol bases that
dioi
tn" urno""t of 1d3niy in
yy-*-1'?i119
of cytosine"Il also
"t
amount
;J';;-ile
sal
the
iine is alwaYs
couldvary'
bases along the chain
:ive
nple, it explained *nViame
as the amount
;ameasrnearlruuuL"'
:d1
lys
ained how, since the
;i';*lilur'n3a
"'Ot*t
be a code for genetic information'
of bases along one strand
"ti.i."rfh
it'"
In the double-helix *"O""f
every
"rder
the other strand' That is'
:rmines the matching;;;;;t
i1r$;;g111""""guanine(G)
o*er lli.rings are possible'
;t be ioined to cytosrne-iCj' tl"
is
of bases along one strand
pose, for example, tnai-tt"e traer
be
must
along the second strand
}TTAC. The matching'l'0"'
Each
be-comolementary'
to
IMTG. The two ,,'uni'-*"-'uii
according toth'e A-T *1-G-:
rnd is the complement;i;;H;;
a great
a""ui" rt"rix mo-del oI DNA was
Jil'J'iifi",il;$'.i:iffi
ie pairing rule. The
Crick' and Wilkins
:akthrough in the scient"
Had she lived'
1962'
"il"""trct'.W3tt?l'
in
eived the Nobel pri,IlJt ?t'it
*."f0 also have been a reciPient'
/ene
*"*
cal
D; a
*"o'---
t!
nine, arq
other
es
(pih
p and
e conclu
Of DNA
hol.an'exact copy of each
double-helix model also explains
tT: flT^**i::?
ur5 Lsu ut , re'y--- rn"
a*i's"t"iiJitition'
lmosome
maoe qur
ls made
mosome is
u*.' eak hydrogen bond.together
model are held
Pv
two strands ot
;;;-;i,h"
tn"t" f-""tf"t Ut"utt' and
I*pot"t
come
one of
rre copying begins,
rucleo'
DNA molecure
there
ides n
the
;ilfi;
of the ceil can
":li'::-lt:::,i::l
'xift"l""'J"f ;i,T:,""iffi;.iii"'l'iit'" nl:r.""
lastenontothe.o*pr#J*""t'uivoit"t-":.:Tl^":'-ijr1o"1t#,}
maxe a comprete compre-
ii:i::["1ffi".,J,:"1'ruil;i
tnev
l
Figure 26-10
Watson-Cri€k Model of DNA'
s28
Unit
5
,
Genetics
/
5
a
)
I
Y
nucleotide fastening onto
I
#X'strandorDNA
&
exposed base
of single
strand of DNA
E
adenine
Effi
guanine
A
I
I
wl
T--'l
phosphate
cytosine
free
TI
nucleotides
wi
s
deoxyribose
thymine
C
group
pt
mr
rh
si,
Figure 26-11
results in the formation of
DNA Replication' DNA replication
molecule'
DNA
original
the
like
two double'stranded
l9
an
;il.#;;..[
ty,
Th'
the old one' ln this way' two double'
mentary strand exactly like
like the original molecule are
stranded molecules
"*ucuy
double-stranded molecule contains
made. See Figure ZO-if-' facn
DNA'
tuutia and one new strand oI
;r;i"il
;;il
Where
ao.,
,"pii'"it;; tt""
rhrough experimentsi
;;;;;ii;i;
and end along the Dryl T'ttt'{:?
a9tlyi1ea that replication does
;;Gi;;,;"",,au""4"'"o.'iiii:l'Tili":r:l$"t*'?ll,ll,i,l
iii;o];I;.e,ongstrand,
,[#i#*f *;h;;;"
ll
[:-,,*:l*;?T;"':ffi
rapidty
molecule replicates
ttran
tn tiis way, a DNA
26-2 Section Review
and Chase use to show
1. What type of particles did Hershey
.
that DM
the genetic material?
nucleotide?
z. wiiZn ir'i.. chemical groups mlke yqa
molecule/
3. What is the shape of a DNA
held tosether?
4. How are the u.,t ptil il;';ff;"lecule
{orrowine
or DNA were to have the
sequence
rcccA;;;":'.y.d,ylql u" t'' base
;:'[iJlt|r..,1i:
sequence:
J:YY="--.
strand
(ordering)
the complementary stra nd?
Be