Chapter 9: Molecular structure of DNA and RNA
§9.1 Identification of DNA as the genetic material
Genetic material must meet four criteria:
1. Information → Genetic material must contain the information necessary to construct an entire
organism. It must provide the blueprints for determining the inherited traits of an organism.
2. Transmission → During reproduction, the genetic material must be passed from parents to
offspring.
3. Replication →During cell divsion it must be copied.
4. Variation → The genetic material must vary in ways that can account for the known phenotypic
differences within each species.
Griffth’s experiment indicated that genetic material can transform 一 Streptococcus
Certain strains of S. pneumoniae secrete a polysaccharide capsule, whereas other strains do not.
When streaked onto petri plates containing a solid growth medium, capsule-secreting strains produce
colonies with a smooth morphology, whereas those strains unable to secrete a capsule produce colonies
with a rough appearance.
Different forms of S. pneumoniae also vary in their virulence (ability to cause disease). When
smooth strains of S. pneumoniae infect a mouse, the capsule allows the bacteria to escape and attack the
mouse’s immune system. The bacteria can grow and kill the mouse. The nonencapsulated (rough)
bacteria are destroyed by the animal’s immune system.
Griffith conducted experiments that involved the injection of live and/or heat killed bacteria into
mice. He obeserved whether or not the bacteria caused lethal infection. He worked with two strains of S.
pneumoniae → type S (smooth) and type R (rough).
Living type S bacteria Living type R bacteria Heat-killed type S Living type R and
injected in mouse injected in mouse injected in mouse heat-killed type S
Mouse died Mouse survived Mouse survived Mouse died
Type S bacteria were No living bacteria No living bacteria Type S bacteria were
isolated from the dead isolated from the dead isolated from the dead isolated from the dead
mouse mouse mouse mouse
Because living type R bacteria could not proliferate and kill the mouse, the interpretation of the results
is that something from the dead type S bacteria was transforming the type R bacteria into type S
bacteria. Griffith called this transformation, and the unidentified substance causing this to occur the
transformation principle.
The transformed bacteria acquired the information to make a capsule. Among different strains
variation exists in the ability to create a capsule and to cause mortality in mice. The genetic material that
is necessary to create a capsule must be replicated so it can be transmitted from mother to daughter-
cells during cell division. These observations are consistent with the idea that the formation of a capsule
is governed by the bacteria’s genetic material, meeting the four criteria. The experiment showed that
some genetic material from dead bacteria had been transferred to living bacteria and provided them
with a new trait.
Avery, Macleod and McCarty showed that DNA is the substance that transforms bacteria
To seperate the components DNA, RNA, proteins and carbohydrates and to determine if any of them
was the genetic material, the researchers used established biochemical purification procedures and
prepared extracts from type S bacterial strains that contained each type of these molecules. After many
attempts with different types of extracts, they discovered that one of the extracts, the one that
§9.1 Identification of DNA as the genetic material
Genetic material must meet four criteria:
1. Information → Genetic material must contain the information necessary to construct an entire
organism. It must provide the blueprints for determining the inherited traits of an organism.
2. Transmission → During reproduction, the genetic material must be passed from parents to
offspring.
3. Replication →During cell divsion it must be copied.
4. Variation → The genetic material must vary in ways that can account for the known phenotypic
differences within each species.
Griffth’s experiment indicated that genetic material can transform 一 Streptococcus
Certain strains of S. pneumoniae secrete a polysaccharide capsule, whereas other strains do not.
When streaked onto petri plates containing a solid growth medium, capsule-secreting strains produce
colonies with a smooth morphology, whereas those strains unable to secrete a capsule produce colonies
with a rough appearance.
Different forms of S. pneumoniae also vary in their virulence (ability to cause disease). When
smooth strains of S. pneumoniae infect a mouse, the capsule allows the bacteria to escape and attack the
mouse’s immune system. The bacteria can grow and kill the mouse. The nonencapsulated (rough)
bacteria are destroyed by the animal’s immune system.
Griffith conducted experiments that involved the injection of live and/or heat killed bacteria into
mice. He obeserved whether or not the bacteria caused lethal infection. He worked with two strains of S.
pneumoniae → type S (smooth) and type R (rough).
Living type S bacteria Living type R bacteria Heat-killed type S Living type R and
injected in mouse injected in mouse injected in mouse heat-killed type S
Mouse died Mouse survived Mouse survived Mouse died
Type S bacteria were No living bacteria No living bacteria Type S bacteria were
isolated from the dead isolated from the dead isolated from the dead isolated from the dead
mouse mouse mouse mouse
Because living type R bacteria could not proliferate and kill the mouse, the interpretation of the results
is that something from the dead type S bacteria was transforming the type R bacteria into type S
bacteria. Griffith called this transformation, and the unidentified substance causing this to occur the
transformation principle.
The transformed bacteria acquired the information to make a capsule. Among different strains
variation exists in the ability to create a capsule and to cause mortality in mice. The genetic material that
is necessary to create a capsule must be replicated so it can be transmitted from mother to daughter-
cells during cell division. These observations are consistent with the idea that the formation of a capsule
is governed by the bacteria’s genetic material, meeting the four criteria. The experiment showed that
some genetic material from dead bacteria had been transferred to living bacteria and provided them
with a new trait.
Avery, Macleod and McCarty showed that DNA is the substance that transforms bacteria
To seperate the components DNA, RNA, proteins and carbohydrates and to determine if any of them
was the genetic material, the researchers used established biochemical purification procedures and
prepared extracts from type S bacterial strains that contained each type of these molecules. After many
attempts with different types of extracts, they discovered that one of the extracts, the one that
