Download AVERY and DNA

the information within us
THE HISTORY OF DNA DISCOVERY
Every since Mendel first stumbled across the basis of genetics, people have been
fascinated about how genes pass traits from generation to generation. But up until
the mid 1900’s, how those genes did their job was a mystery.
To truly understand genetics, biologists first had to discover the chemical nature of
the gene. If the structures that carry the genetic information could be identified, it
might be possible to know how they work.
FREDERICK GRIFFITH
In 1928, English scientist FREDERICK GRIFFITH set out to try and find a reason
why certain bacteria cause pneumonia. Although he did start with this goal, what
happened next would change how the world looks at genetics.
Griffith knew that there was two strands of bacteria that could cause pneumonia;
one called Type S, which is surrounded by an outer covering called a capsule and one
called Type R, which is not surrounded by this outer covering. Both grew very well
in the lab, but as it turned out, only Type S caused sickness. Each bacteria strain
grew differently, easily showing one from the other. One strain grew smooth, Type
S, which caused the disease and another grew roughly, Type R, which did not.
Griffith took samples of both strains of bacteria and injected lab mice. Mice injected
with the smooth strain caught the disease and died. On the other hand, mice injected
with the rough strain lived. This caused Griffith to think that the smooth strain
contained a poison.
Griffith thought that if he heated the smooth cells, it would kill off the poison. He
did so and then injected mice with the heat-treated cells. The mice did not
die…showing that the cause was not a poison released by the cell. It had to be
something else.
What Griffith did next was he mixed the heat killed cells with the living rough, safe
cells and injected more mice.
Both cells, by themselves should have done nothing to the mice. But instead this
time, the mice died. What happened?
When Griffith performed an autopsy on the mice, he found that the lungs of the mice
were filled with disease causing bacteria. Somehow, the disease passed from the
smooth cells to the rough cells, causing the disease.
Griffith coined what happened here TRANSFORMATION.
He concluded that something was in the bacteria that could transfer genes with
information from one cell to another.
AVERY and DNA
In 1944, Canadian scientist OSWALD AVERY decided to recreate Griffith’s work.
Avery put the disease causing cells through special enzymes that killed off proteins,
lipids, carbohydrates and even certain nucleic acids. But transformation still
occurred.
This ruled out that any of these may have caused the transformation and it had to be
something else.
Finally, Avery put enzymes that destroyed the DNA of the cell and transformation
did not happen this time.
From this, it was concluded that DNA is the nucleic acid that stores and transmits
the genetic information from generation to generation.
THE HERSHEY-CHASE EXPERIMENT
By 1952, many scientists still did not believe the results and demanded more proof.
Two American scientists, ALFRED HERSHEY and MARTHA CHASE began
studying VIRUSES, non-living particles smaller than a cell that can infect living
organisms.
A kind of virus that infects and kills bacteria is called a
BACTERIOPHAGE. A virus is built from a DNA or RNA core
coated with protein. The virus lands on a cell and injects it’s DNA
into a cell. The DNA replicates inside the cell, creating new
viruses, eventually busting open the cell, spilling new viruses into
the body.
Now that we known what a virus is, we can
start to understand what happened next in
the progression of the discovery of the
importance of DNA.
Hershey and Chase reasoned that if they could
determine which part of the virus, the DNA or protein,
entered a cell after infection, they could determine what
genes were made up of…either DNA or protein.
Hershey and Chase bred certain viruses with certain radioactive isotopes attached to
them, a phosphorus isotope to the DNA and a sulfur isotope to the protein of the
virus.
If sulfur was found inside the bacteria, they would know
genes were made of protein, and it would be made out
of DNA if phosphorus was found. When all was said
and done, Hershey and Chase found nothing but
phosphorus inside…showing that genes were made up
of DNA, not proteins.
The structure of DNA
You would think that scientists would be happy with just
knowing that genes were made out of DNA….but
no…they needed to know more. They wanted to know
what DNA was made out of.
They wanted to figure out three main things…
1. How information is passed from generation to
generation.
2. How that information is actually put to work as
inheritable traits.
3. How these genes are easily copied.
DNA is a long molecule made up of units called
NUCLEOTIDES. Each nucleotide is made up of three
basic parts:
1. 5-CARBON SUGAR CALLED
DEOXYRIBOSE
2. PHOSPHATE GROUP
3. NITROGEN BASE
Of those nitrogen bases, there are
two categories, each with two
types…
1.
2.
PURINES (Two rings)
a. ADENINE
b. GUANINE
PYRIMIDINES (1 ring)
a. CYTOSINE
b. THYMINE
The backbone of DNA is composed of the sugar
and phosphate groups, while the nitrogen bases fill
the insides like rungs on a ladder.
American biochemist, Erwin Chargaff was
interested in the structure of DNA and especially
with the nitrogen groups which make up the rungs.
In his studies, Chargaff found that there was a relationship between the percentages
of the bases. No matter the organism, it seemed that Adenine and Thymine always
matched up and that Guanine and Cytosine matched.
This was interesting, but Chargaff had no idea what this would mean.
Later, British scientist Rosalind Franklin used a technique called X-Ray diffraction
to gain information about the structure of DNA.
The X-rays showed that the DNA made crosses, showing a possible helix shape.
This also showed that there are two backbones and the bases are found as the rungs.
At the same time as Franklin’s experiments, scientists James Watson and Francis
Crick were trying to build a model of DNA with wire and cardboard. It was when
Watson and Crick saw Franklin’s results, the shape of DNA became official…the
DOUBLE HELIX.
Watson and Crick proclaimed that weak hydrogen bonds hold together the nitrogen
bases, causing the double helix design and finally realized the BASE PAIRING of
the nitrogen pairs….
A=T
G=C
In DNA there has to be one adenine for every thymine and one guanine for every
cytosine.