Fact that it also finally laid to rest the question

fact that it also finally laid to rest the question about
the chemical nature of the genetic material, first asked
some 75 years earlier, went unnoticed.
FIGURE 2 The Hershey–Chase experiment showed that the genes of
T2 bacteriophage, a type of virus that infects E. coli , are made of DNA.
Hershey and Chase knew that T2 bacteriophage was made of DNA and
protein. We now know that the DNA contained a protein “head,” which
is attached to a “body” and “legs,” also made of protein. Hershey and
Chase added some T2 bacteriophages to a culture of E. coli bacteria, and
then waited a few minutes to allow the viruses to inject their genes into
the cells. They then agitated the culture in a blender in order to detach
the empty virus particles from the surfaces of the bacteria. The culture
was then centrifuged, which collects the bacteria plus virus genes as a
pellet at the bottom of the tube, but leaves the empty virus particles in
suspension. Hershey and Chase found that the bacterial pellet contained
most of the viral DNA, but only 20% of the viral protein. Hershey and
Chase also allowed some of the bacteria to complete their infection
cycles. The new viruses that were produced inherited 50% of the DNA
from the original T2 bacteriophages, but only 1% of the protein.
centrifuge
continue
infection
cycle
virus attached
to bacteria
pellet
of bacteria
virus now
detached
new viruses contain 50%
of the original virus DNA
but only 1% of the protein
wait a few minutes
for infection to
occur, then agitate
pellet contains 70% of
the virus DNA, but only
20% of the virus protein
protein
DNA
RESEARCH BRIEFING 2.2

26 CHAPTER 2: DNA
most quickly and the longer ones the slowest. By the time they get to the
end of the capillary they have formed bands, each band containing the
molecules that are one nucleotide longer than those in the preceding band
(
Figure 2.20B
).
Each of the four dideoxynucleotides emits a fluorescent signal of a different
wavelength. A fluorescence detector is therefore used to identify which of
the four dideoxynucleotides is present in each band. By identifying whether
the dideoxynucleotide is an A, a C, a G, or a T, the detector is able to build up
the DNA sequence of the cloned fragment nucleotide by nucleotide (
Figure
2.21 ). The sequence can be printed out for examination by the operator, or
entered directly into a storage device for future analysis.
C A CC G C A T C G A AA TT AA C TT C C AAA G TT AA G C TT GG
ddA
ddT
ddT
ddC
ddG
ddNTPs – each emits a
different fluorescent signal
sequencing reactions,
fractionation of products
fluorescent bands
move past the detector
ddA
ddA
ddG
ddC
ddC
ddG
imaging system
detector
(A)
10 20 30
(B)
FIGURE 2.20 Capillary gel electrophoresis.
(A) A capillary gel. (B) The principle behind
separation of DNA molecules by capillary
gel electrophoresis. During electrophoresis,
the DNA molecules become separated
according to their lengths, the shortest ones
moving most quickly through the gel. The
molecules therefore form a series of bands
in the capillary tube.