DNA, DNA Synthesis, and
Protein Synthesis
Chapter 12 Notes
AKA
Molecular Genetics
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1900's: Scientists knew that 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, British bacteriologist Fredrick Griffith experiment
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What is the the genetic material behind inheritance?
Griffith injected two different strains of bacteria
(Streptococcus pneumoniae) into mice.
One strain caused infection (pathogenic/virulent) and one did
not.
 He called the virulent strain the smooth or S strain.
 He called the non-virulent strain the rough or R strain.
• Isolated the “transforming factor” in the S-strain
• Showed it was DNA.
• His results were not widely accepted…not conclusive enough.
• So scientist kept looking for clearer results; Protein or DNA?
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Alfred Hershey, bacteriologist
Martha Chase geneticist
Hershey & Chase Experiment  Provided conclusive evidence
that DNA was in fact the transforming factor.
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bacteriophage (a virus that attacks bacteria).
Made of the two key components  protein and DNA
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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 it
infected the bacteria (E. coli).
Once the virus infected the bacteria with its genetic material, they
monitored which radioactive material was inherited by the bacteria.
This would identify the genetic material as proteins or DNA.
 Provided conclusive evidence that DNA was in fact
the transforming factor.
Hypothesis 1:
Hypothesis 2:
Scientists were now confident that they had discovered what the
genetic material was, but questions remained:
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What is the structure of DNA?
How does DNA communicate information?
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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BUT…How do those nucleotides fit together in DNA?
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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 the DNA there are four
different nitrogenous bases:
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Adenine
Guanine
Cytosine
Thymine
Side note: Uracil (In RNA, replaces Thymine)
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In 1951, Rosalind
Franklin used X-rays
(crystallography) to
photograph DNA.
The DNA molecule was
in the shape of a
twisted ladder known
as a double helix.
Photo 51
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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.
Why did its structure matter?
Why was everyone so anxious
to find out!?
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DNA is like a twisted
ladder made up of
alternating strands of
deoxyribose (sugar)
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 DNA strands.
The two strands DNA are referred to as antiparrellel, meaning
they run parallel to eachother, but in opposite directions.
This orientation is important to understand because it explains how
DNA replicates.
One end of the DNA strand is referred to as the 5' (five-prime)
end, and the other end is referred to as the 3' (three-prime) end.
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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 protein 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.
*Nucleic Acids
are the largest
molecules in
our bodies
• "It has not escaped our notice that the
specific pairing we have postulated
immediately suggests a possible copying
mechanism for the genetic material."
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The way 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.
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Semiconservative Replication occurs in three stages:
1.
2.
3.
unwinding
base pairing
Joining
1 During unwinding, an enzyme called DNA helicase unwinds or
unzips the DNA double helix.
2 After the strands unwind, another enzyme called DNA
polymerase, adds nucleotides to the new strand in
complementary base pairs.
3 Joining is more complex on the lagging strand than the
leading strand… DNA Ligase joins the Okazaki fragments
• DNA polymerase, adds nucleotides to the growing (new) strand
in complementary base pairs.
5’
DNA Polymerase adds
complimentary
nucleotides to the 3’ prime
end of the growing (new)
strand
3’
5’
3’
5’
3’
5’
3’
5’
3’
5’
3’
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Because the strands are antiparallel, one of the strands can be
replicated continuously from one end to the other. This section
that is replicated continuously is called the leading strand.
The other strand, called the lagging strand, has to be replicated
in reverse order in sections of nucleotides. These sections of
nucleotides are called Okazaki fragments.
DNA polymerases add nucleotides to the 3' end of a growing strand
DNA polymerases add nucleotides to the 3' end of a growing dna strand
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The Okazaki fragments are then glued together by another
enzyme called DNA ligase
• ANIMATION… GET IT?
• CRASH COURSE: DNA Replication (Short)
Whole Video
• Animation PLUS Quiz
• Central Dogma
• Dogma is a principle or set of principles laid down by an authority as
incontrovertibly true.
• Central Dogma of Biology
•DNA  mRNA  Protein
• ….proteins allow cells to function
• Genotype determines proteins form/fcn which determines phenotype
• T & T = Transcription and Translation
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DNA contains a code that is transcribed into another nucleic acid
called RNA (ribonucleic acid).
RNA is the photocopy of DNA that directs synthesis of proteins.
This is process is known as the Central Dogma of biology.
DNA is transcribed by Messenger RNA (mRNA).
Messenger RNA carries information to the ribosomes.
Ribosomes (Ribosomal RNA - rRNA) and Transfer RNA (tRNA)
translate the code to make the proteins.
This is how genes are expressed as traits. = Molecular Genetics
HOMEWORK!!! = 12.3 Read/Notes
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RNA is similar to DNA. 3 differences are:
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RNA contains the sugar ribose instead of deoxyribose.
RNA uses the nitrogen base Uracil in place of Thymine.
RNA is single-stranded while DNA is double-stranded.
There are three main types of RNA that play a role
in protein synthesis They are:
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Messenger RNA (mRNA)
Ribosomal RNA (rRNA)
Transfer RNA (tRNA).
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The job or role of mRNA is transcription.
Transcription is the process of copying the DNA code.
This is the role of messenger RNA (mRNA).
Messenger RNA enters the nucleus, a small portion of the DNA
strand is copied. Then the messenger RNA leaves the nucleus
after copying down a part of the code to make a protein.
Always REMEMBER the factory
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After the DNA is unwound in the nucleus, an enzyme comes along to
assist in base pairing, called RNA polymerase.
RNA polymerase assists mRNA in recording what information is
found on a portion of the DNA strand.
Messenger RNA transcribes the code in complementary base pairs,
similar to the way DNA bases are paired during replication
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except when the base pair Adenine is paired, Adenine pairs with
Uracil instead of Thymine. (AU)
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After the mRNA is transcribed, mRNA can leave the nucleus
through nuclear pores and enter into the cytoplasm to find
transfer RNA (tRNA) and ribosomal RNA (rRNA).
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After a mRNA finds a ribosome, the code is read and translated
by interpreters called transfer RNA (tRNA).
tRNA interprets the code on the mRNA by reading its bases in
groups of three called Codons.
Transfer RNA molecules each have their own Anticodon that
only matches with a specific codon.
Translation Animation. (
Codon – mRNA or DNA
Anticodon - tRNA
Ribosome
• Complete, detailed T&T Animation
• Crash Course: Transcription and Translation.
• Teacher’s Pet: Transcription and Translation
• Teaches how to use the RNA Codon Chart to find the amino acid
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The DNA code is
read as a three-base
code system.
Each codon matches
with a specific
anticodon and a
specific amino acid.
By joining multiple
amino acids together,
proteins can be
assembled.
• Only 20 amino acids.
5’
3’
5’
3’
5’
• What molecule is 1?
What molecule is 2?
Instructions:
• 1. Pass out blank bingo cards
• A simple exercise to help • 2. Students should fill out
students learn how to use each of the blanks with an
amino acid from the codon
a codon table to
translate mRNA into its
chart.
associated amino acids. • 3. Teacher calls out 3 bases
(A, T, G, C)
• 4. Students find the amino
acid that is associated with
the codon and mark the
square (use bingo chips or
pennies or other
miscellaneous items)