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DNA and RNA
Chapter 12, Page 287
What is DNA? How was it discovered?
How do genes work? What are they
made of? Are they single molecules?
Polymers?
Deoxyribonucleic acid (DNA)
• First discovered in 1869 by Johann Friedrich
Miescher (1844-1895), a young Swiss chemist
studying in Germany.
• Collected pus (white blood cells), which have very
large nuclei.
Read more:
http://www.answers.com/topic/history-of-dnastructure-and-function#ixzz1HpeKEEv8
• From these, he purified a new compound,
which he termed “nuclein." Miescher showed
that nuclein was a large molecule, acidic, and
rich in phosphorus.
• One of his students renamed the compound
“nucleic acid”.
• In 1885 the German biologist Oskar Hertwig
(1849-1922) suggested that nucleic acid might
be the hereditary material, based on its
presence in the nucleus and the growing
belief that the nucleus was the center of
heredity.
Griffith and Transformation
In 1928, Frederick Griffith, British scientist,
was trying to figure out how certain bacteria
caused pneumonia.
-He isolated 2 different strains of bacteria
from pneumonia in mice (one which caused
pneumonia; the other harmless).
- The disease causing strain grew “smooth”
colonies while the harmless one has rough
edges.
-When Griffith injected mice with bacteria
from smooth colonies, they died.
-When he injected them with the rough-edged
bacteria, they did not get sick at all.
-He thought that the disease-causing bacteria
was releasing a poison. He heated a sample to
kill the bacteria and injected the mice. The
mice survived.
Transformation
• Next, Griffith mixed the heat-killed formerly
disease-causing bacteria with the live
harmless ones and injected the mixture into
the mice.
• The mice developed pneumonia and many of
them died.
• Somehow the heat-killed bacteria had passed
their disease-causing ability to the harmless
strain.
• Griffith named this process transformation
because one strain of bacteria (the harmless
one) had been transformed by the other
(disease-causing).
• The transformation was passed onto the
offspring of the bacteria.
• He knew that it happened but he didn’t try to
explain why.
Avery and DNA (p. 288)
• 1944
• Biologists at Rockefeller
Institute in New York led by
Canadian, Oswald Avery
• Repeated Griffith’s work to
try to discover which
molecule in the heat-killed
bacteria was responsible
for transformation.
• Made an extract from the juice of the heatkilled bacteria.
• Using enzymes, his team destroyed first, the
proteins, then, the lipids, next, the
carbohydrates, one by one, in the extract.
• After they destroyed each one, they injected
the extract into the mice. They still died of
pneumonia.
• Transformation had still occurred.
• Finally, they destroyed the DNA and injected
the extract into the mice. The mice survived.
• DNA was the transforming factor.
• Avery and the other scientists discovered
that DNA stores and transmits the genetic
information from one generation of an
organism to the next.
Avery’s Experiment
Hershey Chase Experiments
• 2 Americans
• 1952
• Trying to confirm that
DNA contained
hereditary material
• Worked with
bacteriophages
(Remember virus with
DNA or RNA core
surrounded by protein)
• They knew that bacteriophage were injecting
genetic information into the bacterium,
reproducing and eventually destroying the
bacterium.
• There was debate on whether it was the DNA
core or the protein coat that actually entered
the bacterium.
Radioactive Markers
• To determine if the DNA or the protein
entered the cell to infect it, they grew viruses
in cultures containing radioactive isotopes of
phosphorous and sulfur.
• Proteins contain almost no phosphorous and
DNA, no sulfur.
If the sulfur was found in the bacteria after
the virus “infected” it, the protein would have
infected the bacteria.
If the phosphorus was found, the DNA would
have infected it.
The genetic material that infected the bacteria
was DNA.
The Structure of DNA
(deoxyribonucleic acid)
DNA is a long polymer made up of monomers
called nucleotides.
• Each nucleotide is made
up of three basic
components:
– a 5-carbon sugar called
deoxyribose
– a phosphate group
– a nitrogenous base
(nitrogen base)
• There are four kinds of
nitrogen-containing
bases in DNA:
guanine
purines
adenine
2 rings
cytosine
pyrimidines
thymine
1 ring
• The backbone of a DNA
chain is formed by the
sugar and phosphate
groups of each nucleotide.
• The nitrogenous bases stick
out sideways from the
chain.
• The nucleotides can be
joined in any order (as long
as T pairs with A; C with G)
so any sequence is possible.
• DNA Structure
Chargaff’s Rules
Rosalin Franklin
•
•
•
•
Franklin
British
1950’s
Purified DNA, stretched out
the fibers in a thin glass
tube, X-rayed them and
recorded the patterns on
film.
Franklin and X-Ray Evidence
• X shape pattern shows
DNA twisted around
each other, a shape
known as the helix
• Angle of the X suggests
2 strands
• Nitrogen bases are near
the center of the
molecule
What do you mean, you can’t see it!!
The Double Helix
• Crick – British physicist
• Watson – American
biologist
• 1953 in England
• Using Franklin’s x-rays,
identified:
- the double helix in
which 2 strands are
wound around each
other.
Watson and Crick
- that hydrogen bonds could form
only between certain
nitrogenous bases (A & T, C & G)
and provide just enough force to
keep them together. They called
this principle base pairing.
• Hydrogen bonds are weak, but enough of
them can hold it all together.
• G and C form 3 hydrogen bonds between
them.
T and A form 2.
Prime
• The exposed
phosphate end of a
nucleotide is called
the 5’ end (said 5
prime). An exposed
deoxyribose is the 3’
end (stated 3 prime).
• Notice how one strand
of DNA begins with a 5’
and ends with a 3’ and
the complementary
stand begins with a 3’
and ends with a 5’.
• Each strand of the DNA
double helix “runs in
the opposite” direction,
antiparallel.
Chromosomes
• In each human diploid cell, there are almost 2
meters of DNA.
• Proteins called histones coil the DNA tightly to
make sure that it fits into the nucleus.
See page 229 in your text.
– DNA-wrapped histones (balls of protein)
form nucleosomes.
- Nucleosomes pack together to form a thick
fiber which is coiled and looped,
chromatin.
• During most of the cell cycle, these fibers are
dispersed throughout the nucleus so that they
are hard to see.
• At the beginning of mitosis, the fibers are
drawn together and become more visible.
• Then they coil tightly into chromosomes
• https://www.youtube.com/watch?v=5UoKYGK
xxMI
DNA Replication
DNA Replication
• Before a cell divides, it duplicates or copies its
DNA in a process called replication.
What happens during replication?
• In most prokaryotes, DNA replication begins at
a single point and continues until the whole
chromosome is replicated.
• In eukaryotic chromosomes, DNA replication
occurs at hundreds of places. The sites where
separation and replication occur are called
replication forks.
Enzymes
• DNA replication is carried out by a host of
enzymes.
• Enzymes are biological catalysts or assistants.
They consist of various types of proteins that
work to drive the chemical reaction required
for a specific action. Enzymes can either
launch a reaction or speed it up.
How does replication take place?
Unzipping of DNA
• Helicase is an
enzyme capable
of breaking the
hydrogen bonds
in the DNA
double helix. It
unzips the
strands.
• Helicase usually begins
at a bond between
adenine and thymine.
These bonds are easier
to split because there
are only 2 hydrogen
bonds. Cytosine and
Guanine have 3.
• Once the DNA
strands are
separated, DNA
polymerase reads
the sequence and
brings the correct
free-floating
nucleotides to join
to each strand.
DNA Polymerase
• The enzyme DNA
polymerase joins
the nucleotides to
the strand.
• In this way, two
strands are made
from the one
original strand.
• DNA is read in a 3’ to 5’
direction.
• The new strand is
complementary to the
“template”. The new
strand is therefore
being synthesized in a
5’ to 3’ direction.
• One branch or prong is
called the leading
strand. The polymerase
reads it in a continual 3’
to 5’ direction.
• The other prong is
called the lagging
strand. It is directionally
opposite from the
leading strand
• This means that the
polymerase has to
attach nucleotides in a
direction leading away
from the replication
fork.
• Therefore the
nucleotides must be
synthesized in short
strands.
• These short
strands are
called Okazaki
fragments.
• The Okazaki
fragments are
then joined by
another
enzyme called
DNA ligase
In each cell, 1000 nucleotides are synthesized
per second.
• Enzymes are constantly moving up and down
the DNA molecule---unzipping, bringing in the
correct bases, checking for errors and cutting
them out and replacing them with correct
bases. This checking for errors and fixing them
is called proofreading and is done by DNA
polymerase.
DNA Replication
• crash course