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BIO 1140 Chapter Notes -Adenine, Polysaccharide, Covalent Bond

Course Code
BIO 1140
Doug Johnson

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Question Set #6
12.1 Establishing DNA as the Hereditary
First half of the 20th century, many scientists
believed that proteins were the most likely
candidate for the hereditary molecules since
they appeared to offer greater opportunities
for information than nucleic acids
Proteins contain 20 types of amino acids
Frederick Griffith (1928) used 2 strains of
Smooth strain, S, has
polysaccharide capsule surrounding
each cell and forms colonies that
appear smooth and glossy when
grown on a culture plate
The S was injected into mice
causing pneumonia and it killed the
mice in a day or 2
Rough strain, R, does not have the
polysaccharide capsule and forms
colonies with a nonshiny, rough
R was injected and the mice lived
Capsule was responsible for the
virulence (risk of infection) of the S
If S bacteria was killed by heating
before injecting them into the mice
(remained healthy)
But if he injected living R bacteria
along with the heat-killed S, many of
the mice died
In some way, living R bacteria had
acquired the ability to make the
polysaccharide capsule from the
dead S bacteria  changed to S cells
Transformed bacteria were altered
permanently; smooth, infective trait
was stably inherited by descendants
of transformed bacteria
Conversion of R to S 
Agent responsible  transforming
Proteins and nucleic acids are built of
various combinations of different amino
acids and nucleotides (gives then a
complexity of structure that makes them
likely candidates for carrying information
needed for transformation)
Oswald Avery (1940s) performed
experiment designed to identify chemical
nature of transforming principle that can
change R to S
Attempted to reproduce
transformation using bacteria
growing in culture tubes
Used heat to kill virulent S and then
treated macromolecules extracted
from cells with enzymes that break
down each of 3 main candidate
molecules (protein, DNA and other
nucleic acids, RNA)
When DNA was destroyed  no
transformation occurred
1944, transforming principle was
DNA; believed that the genetic
material was protein
Unlikely to be DNA, since it has 4
different components
Protein with 20 different components
seemed a far superior medium for
coding information
Alfred D. Hershey (1952) removed any
remaining doubts that DNA is the hereditary
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