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DNA
It’s in our Genes!
DNA-What is it?
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DNA stands for deoxyribonucleic acid
It is a nucleic acid that contains our
genetic/hereditary information (located in our
nucleus)
DNA’s main role is the long-term storage of
information
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Similar to blueprints or a recipe because it
contains instructions needed to construct cells
The DNA segments that contain the genetic
information are called genes
DNA’s makeup
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DNA consists of two long polymers of simple
units called nucleotides
DNA polymers can be enormous molecules
containing millions of nucleotides
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Nucleotides are made up of a 5 carbon sugar (in
this case deoxyribose), a phosphate group and
one of four nitrogenous bases (Adenine,
Cytosine, Thymine and Guanine)
DNA’s structure
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In living organisms, DNA does not usually
exist as a single molecule, but instead as a
tightly-associated pair of molecules
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These two long strands entwine like vines, in the
shape of a double helix (or a twisted ladder)
The nucleotide repeats containing both the
segment of the backbone of the molecule,
which holds the chain together, and a base,
which interacts with the other DNA strand in
the helix.
The Rest of the Story
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The backbone is made up of deoxyribose (the
sugar) and phosphate groups of each
nucleotide
The bases form the rungs of the ladder and
are connected by hydrogen bonds
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H bonds are very weak
The nucleotides can be joined together in any
order
The Bases
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Purines
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Bases adenine and
guanine
Double ring structure
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Pyrimidines
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Bases cytosine and
thymine
Single ring structures
Chargaff
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This is a rule/principle that applies to DNA’s
nucleotides
The percentages of G & C and A & T are
almost equal in any sample of DNA
For example: in humans we are made up of
about 30.9 % A and 29.4 % T (almost equal!)
and about 19.9 % G with 19.8% C
X-Ray Evidence
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Rosalind Franklin began to study DNA using
a technique called X-ray diffraction
Her work did not reveal the structure of DNA
but did show that there are two strands which
are twisted around each other
The Double Helix
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Francis Crick and James Watson were trying
to understand the structure of DNA at the
same time Franklin was
They built 3-D models in an attempt to figure
it out
Using Franklin’s pictures they were about to
construct the model of the double helix
Complementary Pairing
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Also called base pairing
The following bases ALWAYS pair up with
one another in DNA:
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A with T
G with C
Each pair forms one of the rungs of the
ladder
The Code
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DNA molecules carry the code for all the
genes of an organism
Genes are pieces of the DNA molecule that
code for specific proteins
The process of making genes into proteins is
called protein synthesis-which occurs
OUTSIDE of the nucleus on the ribosome
Replication
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Before protein synthesis occurs DNA has to
replicate
DNA must replicate itself and does so in the
following steps:
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The double strands of the DNA unzip to single strands
New DNA strands are assembled from the freefloating nucleotides in the cell’s nucleus
An enzyme known as DNA polymerase collects the
bases and matches them to their complement along
the single strand of DNA
When its complete, there will be two new, identical
DNA double helixes
BREAK! WORK ON DNA
WORKSHEET! YOU HAVE 15
MINUTES TO COMPLETE IT!
Steps of Protein Synthesis
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The DNA code of the gene segment must be
copied in the nucleus of the cell
The code must be carried from the nucleus
into the cytoplasm and then to the ribosome
The protein is then assembled from the code
and released
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These steps are carried about by RNA
(Ribonucleic acid)
Carrying the Code Out
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RNA is a molecule that is used to translate
the code from a DNA molecule into a protein
It is very similar to DNA except: it is single
stranded, it’s sugar is ribose and instead of
thymine as a base, it uses Uracil (so A pairs
with U in RNA only!)
There are three types of RNA: messenger,
ribosomal and transfer (ALL are involved in
protein synthesis)
RNA Types
Transcription
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Step one of protein synthesis is the
manufacturing of messenger RNA (mRNA)
This making of the mRNA is called
transcription
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Transcription begins when a region of the DNA
unwinds and separates (this separated segment
is a gene)
This unwound segment serves as a template for
the soon to be mRNA strand
mRNA
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The mRNA strand is assembled from
individual RNA nucleotides that are present
in the nucleus
RNA polymerase (an enzyme) picks up these
nucleotides and matches them to their DNA
complement from the template that has just
been unzipped
At this point, the mRNA separates and
leaves the nucleus-moving into the
cytoplasm and settling on a ribosome-this is
where translation begins
Translation
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Ribosomes are made up of ribosomal RNA
(rRNA)
On these ribosomes mRNA is decoded
(translated) and a corresponding polypeptide
(amino acids) is formed
When we “decode” a chain of nucleotides we
“read” it in a three base code called a codon
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For example: our mRNA sequence could be
AUGACAGAUUAG
The corresponding codon would be AUG ACA GAU
UAG
Codons
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The three nucleotide codon has the specific
function of corresponding to a particular
amino acid
How does this work?
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The mRNA is bound to the surface of the
ribosome at the first nucleotide segment (called a
start codon)
The cytoplasm in which the rest floats contains
amino acids and a third kind of RNA, transfer
RNA (tRNA)
tRNA
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This molecule contains a three part
nucleotide segment called an anticodon (this
is the exact match of one mRNA codon)
The anticodon corresponds exactly to one of
the 20 kinds of amino acids
Once the tRNA binds the amino acid it
travels to the ribosome surface and there the
three tRNA bases pair with their three
complementary mRNA bases
Finishing Up Translation
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The amino acid that is bound to the tRNA is
then added to the growing polypeptide chain
at the surface of the ribosome
The ribosome facilitates this process by
moving along the mRNA chain until it
reaches a stop codon (a three nucleotide
segment that tells the ribosome that the
translation is complete)
The ribosome then releases the newlyformed polypeptide chain which moves out
into the cell as a fully functioning protein
Proteins
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There are many proteins within every cell
They provide structure but do not act as
energy sources
Proteins make up enzymes
 Enzymes help carry out reactions in the cell
– In addition, they transport other molecules and are
part of our bodies’ defense against diseases
Activity
Look at the following mRNA chain
AUG ACA GAU UAG
1. How many codons does it contain?
2. AUG stands for which nucleotide bases?
3. If you hadn’t been told, how could you determine
if this was a DNA or a RNA segment?
4. AUG is a common start codon and also codes
for the amino acid methionine. Which codon
above is the stop codon?
5. If this strand was complete, how many amino
acids would the protein contain?
Review-You should now be able to
answer the following questions
*Protein synthesis begins with the
manufacturing of a molecule of?
mRNA
rRNA tRNA
Nucelotide
*What are ribosomes made up of?
mRNA rRNA tRNA Protein
*Proteins are made up of polypeptide
chains. Polypeptide chains are composed
of?
mRNA rRNA tRNA Amino Acids
More Review
*What does tRNA carry?
The mRNA to the ribosome
The nucleotide bases to the mRNA
An amino acid to the ribosome
An amino acid to the cytoplasm
*Which of the following is the last step in protein
synthesis?
tRNA bonds to an amino acid in the cytoplasm
The stop codon binds to the ribosome and the
polypeptide is released
DNA unravels to expose a gene segment
mRNA bonds to tRNA
Last But Not Least
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Which sugars are found in DNA and RNA?
What are proteins made up of?
What role does DNA play in protein synthesis?
Make sure you can define the following:
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DNA
RNA
Gene
Anticodon
tRNA
rRNA
*Codon
*Amino Acid
*Enzyme
*Nucleotide
*Protein Synthesis
*Start codon
*Translation
*Transcription
*Complement
*Base
*mRNA
*Stop codon