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PROTEIN
SYNTHESIS
1
DNA AND
GENES
DNA
■ DNA contains genes,
sequences of nucleotide
bases
■ Genes have different
alleles.
■ These genes code for
polypeptides (proteins)
Genes & Proteins
 Proteins are made of amino
acids linked together by
covalent bonds called peptide
bonds
 Dehydration synthesis or
condensation reactions form the
bonds
 20 different amino acids exist
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Amino Acid Structure
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Polypeptides
■ Amino acid chains
are called
polypeptides
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DNA Begins the Process
■ DNA is found inside the
nucleus
■ Proteins, however, are made in
the cytosol of cells by
organelles called ribosomes
■ Ribosomes may be free in the
cytosol or attached to the
surface of rough ER
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Starting with DNA
■ DNA ‘s code must be copied and
taken to the cytosol
■ In the cytosol, 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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RNA
Roles of RNA and DNA
■ DNA is the MASTER PLAN
■ RNA is the copy of the
Master Plan
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RNA Differs from DNA
■ RNA has a sugar ribose
DNA has a sugar deoxyribose
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Other Differences
■
RNA contains the base
uracil (U)
DNA has thymine (T)
■
RNA molecule is singlestranded
DNA is double-stranded
DNA
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Structure of RNA
13
.
Three Types of RNA
■ Messenger RNA (mRNA) copies
DNA’s code & carries the genetic
information to the ribosomes
■ Ribosomal RNA (rRNA), along with
protein, makes up the ribosomes
■ Transfer RNA (tRNA) transfers
amino acids to the ribosomes
where proteins are synthesized
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Messenger RNA
■ Long Straight chain of
Nucleotides
■ Made in the Nucleus
■ Copies DNA & leaves
through nuclear pores
■ Contains the Nitrogen
Bases A, G, C, U
( no T )
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Messenger RNA (mRNA)
■ Carries the information for a specific
protein
■ Made up of 500 to 1000 nucleotides
long
■ Sequence of 3 bases called codon
■ AUG – methionine or start codon
■ UAA, UAG, or UGA – stop codons
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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 in the nucleolus
■ Associates with proteins
to form ribosomes
■ Site of protein Synthesis
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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
UGA
ACU
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The Genetic Code
■ A codon designates an amino
acid
■ An amino acid may have more
than one codon
■ There are 20 amino acids, but
64 possible codons
■ Some codons tell the ribosome
to stop translating
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The Genetic Code
•Use the
code by
reading from
the center to
the outside
•Example:
AUG codes
for
Methionine
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Name the Amino Acids
■ GGG?
■ UCA?
■ CAU?
■ GCA?
■ AAA?
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TRANSCRIPTION
AND
TRANSLATION
Pathway to Making a Protein
DNA
mRNA
tRNA (ribosomes)
Protein
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Protein Synthesis
 The production or synthesis of
polypeptide chains (proteins)
 Two phases:
Transcription & Translation
 mRNA must be processed before
it leaves the nucleus of eukaryotic
cells
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DNA  RNA  Protein
Eukaryotic
Cell
Nuclear
membrane
DNA
Transcription
Pre-mRNA
RNA Processing
mRNA
Ribosome
Translation
Protein
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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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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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Transcription
■ Promoters are regions on DNA
that show where RNA Polymerase
must bind to begin the
Transcription of RNA
■ Called the TATA box
■ Specific base sequences act as
signals to stop
■ Called the termination signal
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RNA Polymerase
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mRNA Processing
■ After the DNA is transcribed
into RNA, editing must be done
to the nucleotide chain to make
the RNA functional
■ Introns, non-functional segments
of DNA are snipped out of the
chain
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mRNA Editing
■ Exons, segments of RNA that code
for proteins, are then rejoined by the
enzyme ligase
■ A guanine triphosphate cap is added
to the 5” end of the newly copied
mRNA
■ A poly A tail is added to the 3’ end
of the RNA
■ The newly processed mRNA can then
leave the nucleus
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Result of Transcription
CAP
New Transcript
Tail
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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
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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Transcription
Translation
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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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Step 1- Initiation
■ mRNA transcript start
codon AUG 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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Step 2 - Elongation
■ As ribosome moves, two tRNA
with their amino acids move into
site A and P of the ribosome
■ Peptide bonds join the amino acids
Initiation
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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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
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
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
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
aa200
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Messenger RNA (mRNA)
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
Primary structure of a protein
aa1
aa2
aa3
aa4
aa5
aa6
peptide bonds
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