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PROTEIN
SYNTHESIS
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1
DNA
• DNA contains genes,
sequences of nucleotide
bases
• These Genes code for
polypeptides (proteins)
• Proteins are used to build
cells and do much of the
work inside cells
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Amino Acid Structure
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Polypeptides
• Amino acid
chains are
called
polypeptides
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Starting with DNA
• DNA ‘s code must be copied
and taken to the cytosol
• In the cytoplasm, this code
must be read so amino acids
can be assembled to make
polypeptides (proteins)
• This process is called
PROTEIN SYNTHESIS
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5
Pathway to Making a
Protein
DNA
mRNA
Read by ribosomes
Protein
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DNA  RNA  Protein
Nuclear
membrane
DNA
Transcription
Eukaryotic
Cell
Pre-mRNA
RNA Processing
mRNA
Ribosome
Translation
Protein
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RNA
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RNA Differs from DNA
1. RNA
DNA
2. RNA
DNA
3. RNA
DNA
has a sugar ribose
has a sugar deoxyribose
contains the base uracil (U)
has thymine (T)
molecule is single-stranded
is double-stranded
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Structure of RNA
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.
Three Types of RNA
• Ribosomal RNA (rRNA), along
with protein, makes up the
ribosomes
• Messenger RNA (mRNA) copies
DNA’s code & carries the
genetic information to the
ribosomes
• Transfer RNA (tRNA) transfers
amino acids to the ribosomes
where proteins are synthesized
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Ribosomal RNA (rRNA)
• rRNA is a single strand 100 to 3000
nucleotides long
• Globular in shape
• Made inside the nucleus of a cell and
exported to cytoplasm
• Associates with
proteins to form
ribosomes
• Site of protein
Synthesis
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Messenger RNA
• Long Straight chain of
Nucleotides
• Copies DNA and carries
the information for a
specific protein
• Made up of 500 to 1000+
nucleotides long, organized
into 3-base codons
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Remember the
Complementary Bases
On DNA:
A-T
C-G
On RNA:
A-U
C-G
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Genetic Code
 DNA contains a triplet code
 Every three bases on DNA stands
for ONE amino acid
 Each three-letter unit on mRNA is
called a codon
 Most amino acids have more than
one codon!
 20 amino acids: 64 different triplets
 ALL organisms use the SAME code
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The Genetic Code
•Example:
AUG codes
for
Methionine
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Transfer RNA (tRNA)
• Clover-leaf shape
• Single stranded molecule with
attachment site at one end
for an amino acid
• Opposite end has three
nucleotide bases called the
anticodon
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Transfer RNA
amino acid
attachment site
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
• Example: Codon ACU
Anticodon UGA
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UGA
ACU
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Two Parts of Protein
Synthesis
 Transcription makes an RNA
molecule complementary to a
portion of DNA
 Translation occurs when the
sequence of bases of mRNA
DIRECTS the sequence of amino
acids in a polypeptide
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Transcription
Translation
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Transcription
• The process of copying
the sequence of one
strand of DNA, the
template strand
• mRNA copies the template
strand
• Requires the enzyme RNA
Polymerase
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Template Strand
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Question:
 What would be the
complementary RNA strand
for the following DNA
sequence?
DNA 5’-GCGTATG-3’
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Answer:
• DNA 5’-GCGTATG-3’
• RNA 3’-CGCAUAC-5’
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Transcription
• During transcription, RNA
polymerase binds to DNA and
separates the DNA strands
• RNA Polymerase then uses
one strand of DNA as a
template to assemble
nucleotides into RNA
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RNA Polymerase
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mRNA Transcript
•mRNA leaves the nucleus
through its pores and goes to
the ribosomes
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Translation
• Translation is the process
of decoding the mRNA
into a polypeptide chain
• Ribosomes read mRNA
three bases or 1 codon at
a time and construct the
proteins
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30
Ribosomes
• Made of a large and small
subunit
• Composed of rRNA (40%)
and proteins (60%)
• Have two sites for tRNA
attachment --- P and A
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Translation
• Three steps:
1. initiation: mRNA binds to
ribosome at start codon (AUG)
2. elongation: amino acids linked
3. termination: stop codon
(UAG, UAA, or UGA) signals
ribosome to release mRNA
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Step 1- Initiation
• mRNA transcript
attaches to the
small ribosomal
subunit
• Small subunit
attaches to large
ribosomal subunit
mRNA transcript
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Ribosomes
Large
subunit
P
Site
A
Site
mRNA
Small
subunit
A U G
C U A C U U C G
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Initiation
aa2
aa1
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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Step 2 - Elongation
• As ribosome moves, tRNA with their
amino acids move into the ribosome
• Peptide bonds join the amino acids
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Elongation
peptide bond
aa1
aa3
aa2
3-tRNA
1-tRNA
anticodon
hydrogen
bonds
U A C
A U G
codon
2-tRNA
G A A
G A U
C U A C U U C G A
mRNA
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aa1
peptide bond
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
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Ribosomes move over one codon
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aa1
peptide bonds
aa4
aa2
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
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Ribosomes move over one codon
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aa1
peptide bonds
aa5
aa2
aa3
aa4
5-tRNA
U G A
3-tRNA
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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peptide bonds
aa1
aa5
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
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Ribosomes move over one codon
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aa4
aa5
Termination
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
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aa200
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