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How does DNA
instruct cells to
make
PROTEINS?
1
Part I
DNA, Genes,
and Proteins
DNA and genes
• Some stretches of DNA are
called genes.
• Genes are stretches of
nucleotide bases (DNA)
that code for proteins.
• Proteins are used to build
cells and do much of the
work inside cells.
Genes and Proteins
• Each gene code is copied in the
nucleus and taken to the
cytoplasm.
• Here the code is deciphered and
converted into a string of amino
acids (a protein)
• Each different protein has its
own gene, somewhere on the
chromosome, that codes for it.
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Part II
COPYING
THE GENE
DNA cannot leave the nucleus of eukaryotic cells...
but proteins are made outside of the nucleus by
organelles called ribosomes
human cheek
cell
Elodea leaf cell
mitochondria
chloroplasts
vacuole
nucleus
(DNA here)
(DNA here)
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Think of ribosomes as factories
that make proteins
ribosomes
(proteins
made here)
(proteins
made here)
nucleus
(DNA here)
(DNA here)
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DNA and ribosomes are at different locations
in a prokaryotic cell.
ribosomes
(proteins made
here)
DNA
Q. Ribosomes make protein but are
not in the same location as DNA in
a cell. How can proteins be made
according to the DNA information
when they are in different places? 8
A. Take a copy of the
Gene to the ribosome.
• mRNA transfers a copy of the
gene on the DNA in the nucleus
to the ribosomes.
• Ribosomes build proteins
according to the mRNA
information received.
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mRNA: the messenger
RNA is how the body gets
information from the
nucleus (DNA) to the
place where
protein gets made
(ribosomes)
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Information flow from
DNA to trait
DNA
Stored in
nucleus
protein
Made by
ribosomes
outside of
nucleus
Observed
trait
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Information flow from
DNA to trait
DNA
Stored in
nucleus
messenger
RNA
protein
Made by
ribosomes
outside of
nucleus
Observed
trait
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DNA information  mRNA
information
DNA
messenger
RNA
Transcription is the process
used to convert DNA information
into mRNA information.
Note: DNA does not become RNA; the
information in DNA is copied as RNA
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Part III
RNA
(Ribonucleic acid)
and Transcription
What is RNA, anyways?
How is it different than
DNA?
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Differences between DNA
and RNA
DNA
Double strand
RNA
Single strand
Deoxyribose sugar
Ribose sugar
Contains thymine
(and A, G, & C)
Contains uracil
(and A, G, & C)
Very large molecule
Small molecule
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Different Sugars
DNA
RNA
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Can you spot the difference?
Different Bases
Can you spot the difference?
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RNA IS COPIED FROM DNA
DNA
(double stranded, kept “safe” in
nucleus)
Genes are
Copied
RNA
(single stranded - mobile)
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The Transcription process
• Promoters are a specific set of
bases on DNA that show where a
gene begins.
• For transcription to occur, the
enzyme RNA polymerase binds to
DNA at the promoter and
separates the DNA strands
• RNA Polymerase then uses one
strand of DNA as a template to
assemble nucleotides into a copy
of the gene (mRNA)
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The Transcription Process
• Terminators! Are a specific set
of bases to show where the
gene ends.
• RNA polymerase stops copying
the gene here, moves off to find
another gene, the transcript is
released and the DNA “zips”
back up.
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Transcription of RNA from a
template strand of DNA
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Transcription
DNA zips back
together
DNA unzips
DNA
ACTTTACGGCAT
ACTTTACGGCAT
TGAAATGCCGTA
RNA copy
made
ACUUUACGGCAU
TGAAATGCCGTA
RNA
ACTTTACGGCAT
TGAAATGCCGTA
ACUUUACGGCAU
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If the DNA sequence is this:
TACGAGTTACATAAA
ATGCTCAATGTATTT
What is the sequence of
the mRNA?
(Use the bottom strand as
the template for mRNA)
UACGAGUUACAUAAA
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Animation of
Transcription
• http://www.fed.cuhk.edu.hk/~jo
hnson/teaching/genetics/animati
ons/transcription.htm
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Part IV
Decoding the
mRNA:
What is the code?
The Genetic Code
“The Problem”
• Somehow we need to read the
order of nucleotides on mRNA and
have that tell us the order of amino
acids within each protein
• As there are 20 amino acids and
only 4 different bases each
nucleotide on its own cant specify
the position of a different amino
acid
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The genetic code
“The solution”
• If a word can only be a single letter long
how many words can there be in the
English language?
• If we can have two letters form a word how
many words can we make now? (aa, ab, ac,
ba, bb, bc, etc.)
• If two nucleotides can code for an amino
acid how many amino acids can we code
for?
• There are 64 possible ways to combine
three nucleotides (43). More than enough to
code for 20 amino acids.
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The Codon
• A codon is a set of three
nucleotides on mRNA and
designates an amino acid
• There are 20 amino acids, but
64 possible codons
• So each amino acid may have
more than one codon that
codes for it.
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A Codon Chart
•Decode by
reading the
first then
second
then third
base.
•Example:
AUG codes
for
Methionine
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Part IV
Turning mRNA
into protein:
Translation
Introducing….
Another RNA molecule; the final
player in our story…
tRNA
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Transfer RNA (tRNA)
• An RNA molecule with attachment
site at one end for an amino acid.
• The opposite end has three
nucleotide bases called the
anticodon.
• If there are 64 possible codons
how many different tRNA
molecules do you think there are?
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Transfer RNA
amino acid
attachment site
Amino acid
U A C
anticodon
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Codons and Anticodons
The 3 bases of an
anticodon are
complementary
to the 3 bases of
a codon
Amino Acid
tRNA
anticodon
UGA
GCAAUCACUACGGCA
codon
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Translation
• Translation is the process
of of decoding the mRNA
into a protein.
• Ribosomes read mRNA
three bases or 1 codon at a
time and construct the
proteins
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1. A Ribosome binds
to mRNA
mRNA
A U G
C U A C U U C G
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2. The Ribosome helps the
correct tRNA bind to mRNA
aa1
aa2
2-tRNA
1-tRNA
anticodon
hydrogen
bonds
U A C
A U G
codon
G A U
C U A C U U C G A
mRNA
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3.The Ribosome then helps the next
correct tRNA bind to mRNA and a
peptide bond forms
aa1
aa3
aa2
peptide bond
3-tRNA
1-tRNA
U A C
A U G
2-tRNA
G A A
G A U
C U A C U U C G A
mRNA
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aa1
4. All Change !!
aa3
aa2
1-tRNA
3-tRNA
U A C
(leaves)
2-tRNA
A U G
G A A
G A U
C U A C U U C G A
mRNA
Ribosomes move over one codon
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5. Etc. Etc. !!
aa1
aa2
aa4
aa3
4-tRNA
2-tRNA
A U G
3-tRNA
G C U
G A U G A A
C U A C U U C G A A C U
mRNA
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aa1
peptide bonds
aa4
aa2
aa3
2-tRNA
4-tRNA
G A U
(leaves)
3-tRNA
A U G
G C U
G A A
C U A C U U C G A A C U
mRNA
Ribosomes move over one codon
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aa1
peptide bonds
aa2
aa3
3-tRNA
aa4
4-tRNA
G A A G C U
G C U A C U U C G A A C U
mRNA
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aa5
aa1
aa2
aa3
aa4
5-tRNA
U G A
3-tRNA
G A A
4-tRNA
G C U
G C U A C U U C G A A C U
mRNA
Ribosomes move over one codon
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aa4
aa5
aa199
aa3 primary
structure
aa2 of a protein
aa200
aa1
200-tRNA
A C U
terminator
or stop
codon
C A U G U U U A G
mRNA
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End Product –The Protein!
• The end products of protein
synthesis is a primary structure
of a protein
• A sequence of amino acid
bonded together by peptide
bonds
aa2
aa1
aa3
aa4
aa5
aa199
aa200
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A Gene (DNA)
start
codon
mRNA
A U G G G C U C C A U C G G C G C A U A A
codon 1
protein methionine
codon 2
codon 3
glycine
serine
codon 4
isoleucine
codon 5
codon 6
glycine
alanine
codon 7
stop
codon
A Protein
aa1
aa2
aa3
peptide bonds
aa4
aa5
aa6
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THE END!!!
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