Download bio ch 8 - Saint Joseph High School

Nucleic Acids and
Protein Synthesis
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Biologists call the program of the cell the
‘genetic code’
Genetic refers to anything related to heredity
The genetic code is the way in which cells store
the program that they seem to pass from one
generation to the next.
DNA is the molecule that carries this genetic
code.
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Fredrick Griffith’s Pneumonia/Mice
Experiments- 1928
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Was working with two different strains of bacteria
that cause pneumonia -one that was disease causing
(we’ll call it strain D- for disease) and one that didn’t
(we call that strain N for normal or not disease
causing.
Strain D – culture with smooth edges
Strain N- culture with ragged edges
 What’s a culture you ask- look see
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D strain
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N strain
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Injected mice – they died
Heat killed the strain- injected mice- they lived
Injected mice – they lived
Mixed heat killed D with N strain and then
injected mice.
What do you think happened next?
If the disease-causing D strain had been heat
killed, it should not have been a problem for the
mouse- so what had happened? And to make
matters worse, when he grew out cultures from
the dead mice, there was smooth disease-causing
colonies instead of the rough non-disease
causing ones he had started out with.
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Griffith theorized that
when the heat- killed D
strain and N strain were
mixed together, some
factor had transferred
from the heat killed
strain to the live ones
thus transforming the N
strain into an active D
strain. But what was
this transforming
factor?
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A new group in 1944
at the Rockefeller
Institute in New York
City decided to repeat
Griffith’s work and
see if they could
figure it out.
Who?
Oswald Avery
 Maclyn McCarty
 Colin MacLeod
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Handsome
fellas, huh?
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They made an extract,
or juice from the heatkilled bacteria and tried
to destroy whatever it
was that was causing
the disease by using a
bunch of different
enzymes.
It kept killing mice until
they got to the DNA
destroying enzyme.
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They tested it one at a
time to see which type
of enzyme would
destroy the ‘factor’- one
that destroyed proteins,
or lipids, or
carbohydrates, or
ribonucleic acid or
deoxyribonucleic acid.
It kept killing mice until
they got to the DNA
destroying enzyme.
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This time the mice
lived.
So DNA was the
transforming factor!
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DNA (deoxyribonucleic acid) is the nucleic acid
that stores and transmits the genetic
information from one generation to another.
DNA carries the genetic code.
That
figures!
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But scientists are very skeptical so a bunch of
other scientists had to prove it again with other
experiments before it was fully believed.
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Who? Alfred Hershey & Martha Chase (no he had
nothing to do with chocolate)
When? 1952
Where? Some lab in America
What? did a bunch of experiments with viruses
called bacteriophages to show that is the DNA in
the virus that was transmitted into the bacteria.
( a virus with the name of a bacteria in it- why would
someone call it that?
 Why? Well, because the word means bacteria-killer and
this type of virus kills only bacteria.
 Oh.
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They put radioactive tags on different parts of
the virus and the only part that entered a
bacteria was the DNA, so that’s the only thing
that was involved in killing the cell.
So, finally, the Hershey-Chase experiments
added to the Avery & Co. work showed
conclusively that DNA was the molecule that
carried the genetic code.
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So the next thing to do was figure out what
DNA looked like and how it worked.
The scientists could figure out the chemical
involved- that part was easy, but how those
chemicals were put together and how it
worked was another thing.
So let’s look at what they knew.
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DNA is a polymer formed from units called
nucleotides.
Each nucleotide is a molecule made up of three
basic parts:
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A 5-carbon sugar called deoxyribose
A phosphate group
A nitrogenous base
There are four different nitrogenous bases –
Adenine and Guanine are double-ring bases
that belong to a chemical group called purines,
Thymine and cytosine are single ring bases that
belong to a group of chemical compounds
called pyrimidines
A and T are always paired together while G
and C are always paired together
Purines
Pyrimidines
Adenine
A
Thymine
T
Guanine
G
Cytosine
C
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By the 1950’s scientists new what chemicals
made up DNA but not what it looked like or
how it worked
A British scientist, Rosalind Franklin and her
associate Maurice Wilkens were able to
successfully x-ray a strand of DNA.
It wasn’t really conclusive in itself but it
provided just the information that Watson and
Crick needed.
Who are
they?
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With information from
Franklin, two young
scientists, Francis Crick and
James Watson figured out
how DNA looked and
worked.
It was a double helix
structure and Franklin’s xray showed them that it
twisted.
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Looks like a twisted ladder
Sugar & phosphate forms the hand-rails
Base pairs form the rungs
Very specific base pairing –
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A-T
C-G
Held together by weak hydrogen bonds that
can easily be separated
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Watson, Crick, Wilkens won a Nobel prize in
1962.
Franklin had died in 1958 and never got credit
for her accomplishment.
Watson and Crick are still working in GeneticsWatson headed up the Human Genome Project
that recently decoded all 3 billion ‘rungs’ of
human code.
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Before a cell divides it must make an exact copy of
all of its DNA so that each new cell has its own
copy.
This duplication process is called DNA replication.
DNA replication or DNA synthesis is carried out
by a series of enzymes.
The enzymes separate or ‘unzip’ the two strands of
the double helix, insert the appropriate nucleotide
matching the bases and produce covalent sugarphosphate bonds to join it all together.
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Ribonucleic acid (RNA) is the nucleic acid that
acts as a messenger between DNA and the
ribosomes and carries out the process by which
proteins are made from amino acids.
There are three different types of RNA
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mRNA – messenger RNA
tRNA- transfer RNA
rRNA- ribosomal RNA
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Similar to DNA but
with these differences
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Only single stranded
Contains ribose sugar
instead of
deoxyribose
Uses the base Uracil
instead of the base
Thymine
 A-U instead of A-T
1.
2.
3.
4.
5.
The DNA in the nucleus carries the code for a
particular protein somewhere in its 3 billion ‘steps’ of
code.
An enzyme (RNA polymerase, if you want to know)
knows where the code is and when we want to make
that protein, will go to the part on the DNA and
‘unzip’ it
mRNA starts to form from loose RNA nucleotides
hanging out in the nucleus. The RNA will line up as
the compliment to the DNA (A-U; T-A;C-G;G-C) until
it makes a strand that represents the protein.
The single sided strand of mRNA will then leave the
nucleus and go out into the cytoplasm where it finds a
ribosome.
The DNA joins back up (zips up)
6. The mRNA now attaches to a ribosome that
‘reads’ the code alerting another type of RNA
(tRNA) as to what the code says.
7. The tRNA has three special bases (anti-codon)
that will match up with three bases on the mRNA
(codon)
8. When it comes across a particular codon, it is
like saying, “go and get such and such amino
acid and bring it here” which the tRNA does.
8. The amino acids are connected together and
the protein is released.
9. The mRNA falls apart into separate
nucleotides and go back into the nucleus to await
the next call for a protein.
10. And voila- protein!