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Chapter 12 Notes, DNA, RNA,
and Protein Synthesis
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By the early 1900's, scientists knew that genes and
chromosomes were responsible for traits being inherited from
parents to offspring.
However, the key component of the chromosomes that actually
contained the genetic information remained a mystery.
Chemical analysis of chromosomes told them that the genetic
material had to be either proteins or nucleic acids (DNA), but
they didn't know which one was responsible for carrying the
genetic information.
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In 1928, a British bacteriologist by the name of Fredrick Griffith
performed an experiment to try to discover what the genetic
material was.
Griffith injected two different strains of a bacteria
(Streptococcus pneumoniae) into mice.
One strain was covered in a sugar coat and one was not.
The strain that had the sugar coat he called the smooth or S
strain.
The strain that lacked the sugar coat he called the rough or R
strain.
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The smooth strain was the virulent (disease causing) strain. The
rough strain was not.
This was the result of his experiments
Mice + smooth (virulent) strain = dead mice
Mice + rough (nonvirulent) strain = live mice
Mice + smooth (virulent) strain after the smooth strain had been
killed with heat = live mice
Mice + rough (nonvirulent) strain + heat killed smooth (virulent)
strain = dead mice
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Griffith concluded that a disease causing factor was
transforming the rough (nonvirulent) strain into the smooth
(virulent) strain of bacteria
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In 1952, a bacteriologist by the name of Alfred Hershey, and a
geneticist by the name of Martha Chase provided conclusive
evidence that DNA was in fact the transforming factor.
Their experiment involved a special type of virus called a
bacteriophage.
A bacteriophage is a virus that attacks bacteria.
The bacteriophage was ideal for this experiment because it was
made of the two key components (protein and DNA) which were
thought to be possible molecules responsible for inheritance.
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Hershey and Chase used a technique called radioactive
labeling to trace both the protein and the DNA of the
bacteriophage after infecting a bacteria.
Once the virus infected the bacteria with its genetic material,
they monitored which radioactive material was inherited by the
bacteria.
This would tell them if the genetic material was in the proteins or
the DNA.
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Scientists were now confident that they had discovered what the
genetic material was, but questions about the structure of DNA
and how DNA communicated information remained.
What they discovered is that DNA is made up of nucleotides.
A nucleotide is a sugar molecule, a phosphate molecule, and a
nitrogenous base.
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In the DNA there are
four different
nitrogenous bases
Adenine
Guanine
Cytosine
Thymine
Uracil (In RNA,
replaces Thymine)
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In the 1950s, Erwin Chargaff discovered that in every organism
the amount of guanine and cytosine, and the amount of adenine
and thymine was nearly equal. This is called Chargaff's rule.
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In 1951, Rosalind
Franklin used X-rays to
photograph DNA.
Photo 51 showed that
the DNA molecule was
in the shape of a
twisted ladder known
as a double helix.
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James Watson and
Francis Crick used data
from Chargaff and
Franklin's photo to build
the first accurate model
of DNA.
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DNA is like a twisted
ladder made up of
alternating strands of
deoxyribose and
phosphate.
The rails of the ladder
are joined by the
bases. (adenine,
guanine, cytosine, and
thymine)
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Each nitrogen base
pairs up with another
base in what is known
as complementary
base pairing.
Purine bases pair with
pyrimidine bases.
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Adenine and Guanine
are called purines.
Cytosine and Thymine
are called
pyrimidines.
Adenine always pairs
with Thymine.
Guanine always pairs
with Cytosine.
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Another important feature of the DNA structure is the orientation
of the strands.
The two strands DNA are antiparrellel, meaning they run
parallel but in opposite directions.
This orientation is important to understand because it determines
how DNA replicates.
One end is referred to as the 5' (five-prime) and the other is
referred to as the 3' (three-prime).
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We will discuss the
importance of this
orientation later
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Just one strand of DNA in one chromosome can be up to 245
million base pairs long!
And remember humans have 46 chromosomes
It has been estimated that if all the DNA from just one cell of a
human's body was unwound, it would stretch about 6 ft long!
That means the DNA in one cell is about 100,000 times longer
than the cell itself!
And amazingly, it all fits into the nucleus, which only takes up
about 10% of the cell's volume!
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So how does all that information fit into a cell?
DNA coils tightly around small balls of proteins called histones.
Histones and phosphates from the DNA combine together to
form nucleosomes.
Nucleosomes combine together to form chromatin fibers, and the
chromatin fibers combine together to form the chromosomes.
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When Watson and Crick created their model of the DNA
double helix, they also proposed a possible way that DNA
might get replicated.
The way they proposed DNA gets replicated is called
semiconservative replication.
In semiconservative replication, one of the strands always gets
copied and the other strand is a copy from the original parent
or template strand.
The process is similar to how sourdough bread is made. In order
to make it you need a starter batch (original template).
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Semiconservative Replication occurs in three stages: unwinding,
base pairing, and joining
During unwinding, an enzyme called DNA helicase unwinds or
unzips the DNA double helix.
After the strands are unwound, another enzyme called DNA
polymerase, adds nucleotides to the new strand in
complementary base pairs.
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Because the strands are antiparallel, one of the strands can be
continously replicated and is therefore called the leading
strand.
The other strand, called the lagging strand, has to be replicated
in reverse order in sections of nucleotides called Okazaki
fragments.
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The Okazaki fragments are then glued together by another
enzyme called DNA ligase
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It is now known that DNA contains a code that is transcribed and
translated by another substance called RNA (ribonucleic acid).
RNA guides the synthesis of proteins.
This is what is known as the Central Dogma.
DNA is transcribed by Messenger RNA.
Messenger RNA carries the information.
Ribosomal and Transfer RNA translate the code to make
proteins.
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RNA is similar to DNA. It is a nucleic acid. RNA contains the
sugar ribose instead of deoxyribose. Instead of using Thymine
as one of its base pairs, Thymine is replaced by Uracil.
Another major difference between RNA and DNA, is that RNA is
single stranded.
There are three main types of RNA that play a role in protein
synthesis They are Messenger RNA (mRNA for short),
Ribosomal RNA (rRNA) and Transfer RNA (tRNA).
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The job or role of mRNA is transcription.
To transcribe means to copy or rewrite something.
Transcription, as it applies to DNA means to copy or rewrite the
DNA code.
This is the role of messenger RNA (mRNA).
The messenger RNA enters the nucleus, the DNA strand is
unzipped and copied. Then the messenger RNA leaves the
nucleus with the code.
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After the DNA is unzipped in the nucleus, an enzyme comes
along called RNA polymerase.
RNA polymerase assists mRNA in recording what information is
found on a portion of the DNA strand, called the template
strand.
Messenger RNA records the code in complementary base pairs,
similar to the way DNA bases are paired during replication
except when the base pair Adenine is encountered, Adenine
pairs with Uracil instead of Thymine.
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After the mRNA is transcribed, mRNA can then leave the nucleus
through nuclear pores and enter into the cytoplasm to find tRNA
and rRNA.
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After a mRNA finds a rRNA, the code is read and translated by
interpreters called transfer RNA.
Transfer RNA (tRNA) interprets the code by reading the bases in
groups of three called Codons.
Transfer RNA molecules each have their own Anticodon that
only matches with a specific codon.
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The DNA code is
made up of a threebase code system.
Each codon matches
with a specific
anticodon and a
specific amino acid.
By joining multiple
amino acids together,
proteins can be
assembled.