How did scientists
discover that genes are
made of DNA?
By the early 1900s, studies of dividing
cells provided strong evidence that genes
are part of chromosomes.
Soon, biologist found that eukaryotic chromosomes
are composed only of protein and DNA, so one of
these must be the molecule of hereditary.
Transformed bacteria revealed the link
between genes and DNA
In the late 1920s, Frederick Griffith was
trying to make a vaccine to prevent
bacterial pneumonia, a major cause of
death at that time.
Injecting antibacterial vaccines
weakened but living strain into an
animal may stimulate immunity
against the disease- causing strains.
He experimented with two
strains of the bacterium
Streptococcus pneumonaie.
R- strain does not cause
pneumonia. The mouse remains
healthy.
S- strain causes pneumonia. Most mouse
contracts pneumonia and dies.
Heat- killed S- strain does not caused
pneumonia. Most mouse remains
healthy.
A substance from heat-killed- S-
strain can transform the harmless
R-strain into deadly S-strain.
Mouse contracts pneumonia and
dies.
Griffith hypothesized that some substances in the
heat-killed S-strain changed the living, harmless R-strain
bacteria into deadly S-strain, a process he called
trasformation.
He never discovered an effective pneumonia vaccine so this experiment was a
failure. However, his experiment marked a turning point in our understanding of
genetics.
The Transformation Molecule is DNA
in 1944 Oswaldo Avery, Colin MacLeod, and
Maclyn McCarty discovered that the
trasformating molecule in DNA. The insolated
DNA form S-starch bacteria, mixed it with live
R-strain bacteria, and produce live S-strain
starin bacteria.
The protein destroying enzymes did nor prevent
transformation. However, treating DNA- destroying
enzymes. Therefore, they concluded that tranformation
must be caused by DNA, and not by traces of protein
contaminating the DNA.
DNA is the genetic material in many
organisms. DNA is the hereditary molecule
of certain viruses.