INTRODUCTION
The basic requirements/ main properties of Genetic Material:
a) It must be stable;
b) It should be able to store biologically useful information in the coded form for the control of
biological functions of the cells;
c) It must be capable of being expressed when needed;
d) It must be capable of accurate replication;
e) It must be transmitted from parent to progeny without change.
Experiments that suggested that genes are made of DNA
a) The Avery–MacLeod–McCarty experiment/ The transforming
principle
Avery and his colleagues demonstrated that the transforming
principle is DNA using Streptococcus pneumoniae. When
harmless (R strain) bacteria were injected into mice, the mice
survived. However, when these harmless bacteria were mixed
with a cell extract from virulent (S strain) bacteria, the mice
died.
This showed that a substance in the extract transformed the
harmless bacteria into virulent ones, which were later recovered
from the dead mice.
To identify this substance, the extract was treated with different
enzymes. Protease (which destroys proteins) and ribonuclease
(which destroys RNA) did not stop transformation. In contrast,
deoxyribonuclease (DNase) destroyed the transforming ability.
This proved that DNA is the transforming principle and the
genetic material.
1
, b) The Hershey–Chase experiment
The Hershey–Chase experiment (1952) used T2
bacteriophages, which are viruses made of DNA inside a
protein coat. These phages infect bacteria by attaching to the
bacterial surface and injecting their genetic material into the cell.
To determine whether DNA or protein is the genetic material,
Hershey and Chase labeled:
• Phage DNA with radioactive phosphorus (³²P), because
DNA contains phosphorus
• Phage protein with radioactive sulfur (³⁵S), because
proteins contain sulfur but DNA does not
After the phages infected bacteria, the mixture was shaken to
remove the empty phage coats from the bacterial surface. The
culture was then centrifuged, separating:
• A pellet containing bacteria and injected genetic material
• A supernatant containing empty phage protein coats
They found that the bacterial pellet contained most of the ³²P (DNA) but very little ³⁵S (protein). This
showed that DNA entered the bacterial cells, not protein.
2
The basic requirements/ main properties of Genetic Material:
a) It must be stable;
b) It should be able to store biologically useful information in the coded form for the control of
biological functions of the cells;
c) It must be capable of being expressed when needed;
d) It must be capable of accurate replication;
e) It must be transmitted from parent to progeny without change.
Experiments that suggested that genes are made of DNA
a) The Avery–MacLeod–McCarty experiment/ The transforming
principle
Avery and his colleagues demonstrated that the transforming
principle is DNA using Streptococcus pneumoniae. When
harmless (R strain) bacteria were injected into mice, the mice
survived. However, when these harmless bacteria were mixed
with a cell extract from virulent (S strain) bacteria, the mice
died.
This showed that a substance in the extract transformed the
harmless bacteria into virulent ones, which were later recovered
from the dead mice.
To identify this substance, the extract was treated with different
enzymes. Protease (which destroys proteins) and ribonuclease
(which destroys RNA) did not stop transformation. In contrast,
deoxyribonuclease (DNase) destroyed the transforming ability.
This proved that DNA is the transforming principle and the
genetic material.
1
, b) The Hershey–Chase experiment
The Hershey–Chase experiment (1952) used T2
bacteriophages, which are viruses made of DNA inside a
protein coat. These phages infect bacteria by attaching to the
bacterial surface and injecting their genetic material into the cell.
To determine whether DNA or protein is the genetic material,
Hershey and Chase labeled:
• Phage DNA with radioactive phosphorus (³²P), because
DNA contains phosphorus
• Phage protein with radioactive sulfur (³⁵S), because
proteins contain sulfur but DNA does not
After the phages infected bacteria, the mixture was shaken to
remove the empty phage coats from the bacterial surface. The
culture was then centrifuged, separating:
• A pellet containing bacteria and injected genetic material
• A supernatant containing empty phage protein coats
They found that the bacterial pellet contained most of the ³²P (DNA) but very little ³⁵S (protein). This
showed that DNA entered the bacterial cells, not protein.
2
