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Protein Synthesis
Replication
 A single DNA strand can serve as a template, or pattern, for
a new strand.
 This is the principle of semi-conservation
Replication copies genetic information.
 Suppose everyone took off their shoes and placed their left
shoe in a line, and the right was tossed into a pile.
 You could easily match the right shoe with the left.
 Similarly, a new strand of DNA can be synthesized when the
other strand is a template to guide the process.
 The order of the bases is preserved, and DNA can be accurately replicated
over and over again.
Proteins carry out the process of
replication.
 Enzymes and other proteins do the actual work of
replication.
 An enzyme unzips the DNA
 An enzyme holds the DNA apart during replication
 DNA polymerase bonds new nucleotides to the “opened” DNA
strand.
The Replication Process
1. Enzymes begin to unzip the double helix at numerous places along the
chromosome.
2. Free-floating nucleotides pair, one by one, with the bases on the
template strand as they are exposed.
 DNA polymerase bonds the nucleotides together.
3. Two identical molecules of DNA result
 Each new molecule has one strand from the original molecule and one new
strand.
 Semi-conservative
Replication is fast and accurate.
 About 50 nucleotides are added every second to a new strand
of DNA.
 The process takes just a few hours.
 DNA polymerase can detect errors in the strand and correct
them.
 Errors are limited to about one error per 1 billion nucleotides.
Central Dogma
 Central Dogma
 Information flows from DNA to RNA to proteins
 Replication copies DNA
 Transcription converts a DNA message into an intermediate molecule
called RNA
 Translation interprets an RNA message into a string of amino acids, called
a polypeptide.
 A single polypeptide or many polypeptides working together make a
protein.
Transcription
 When DNA converts into an intermediate molecule called
RNA.
 RNA is necessary to make proteins.
Transcription
 Replication and transcription occur in the nucleus.
 Translation will occur in the cytoplasm
 Transcription is the process of copying a sequence of DNA to
produce a complimentary strand of RNA
 It is catalyzed by RNA polymerase.
 Bonds nucleotides together in a chain to make a new RNA molecule.
Transcription
1. RNA polymerases recognize the
transcription “start” site of a gene.
 RNA polymerase begins to unwind a
segment of DNA.
2. RNA polymerase, using one strand of
DNA as a template, strings together a
complimentary strand of RNA
nucleotides.
 The growing RNA strand hangs freely as it
is transcribed, and the DNA strand zips
back together.
3. Once the entire gene has been
transcribed, the RNA strand detaches
completely from the DNA.
RNA Carries DNA’s Instructions
 RNA acts as an
intermediate link
between DNA in
the nucleus and
protein synthesis
in the cytoplasm.
 RNA is like a
temporary copy
of DNA that is
used and then
destroyed
Transcription makes three types of RNA
 Messenger RNA (mRNA)
 Intermediate message that is
translated into a protein.
 Ribosomal RNA (rRNA)
 Forms part of ribosomes, a
cell’s protein factory.
 Transfer RNA (tRNA)
 Brings amino acids from the
cytoplasm to a ribosome to
help make a growing
protein.
The transcription process is cool!
 Transcription enables a cell to adjust to changing demands.
 Say you are trapped by an ANGRY bear! Your body needs
adrenaline (a hormone/protein) to run FAST!
 Transcription allows your body to make a lot of adrenaline very quickly

Translation
 Translation is the process that converts, or translates, an mRNA
message into a polypeptide.
 One or more polypeptides make a protein.
 RNA uses adenine, uracil, guanine, cytosine to code for 20 amino acids
Triplet Code
 In the genetic code, all “words” are made up of three letters
(nucleotides).
 A codon is a three-nucleotide sequence that codes for an amino
acid.
Amino Acids
 Amino acids are bonded by
peptide bonds to create
polypeptides and then proteins.
 Many amino acids are coded for by
more than one codon.
 For example: Leucine is
represented by six codons: CUU,
CUC, CUA, CUG, UUA, and
UUG
 In most cases, codons that represent
the same amino acid share the same
first two nucleotides.
 Having many codons represent a
single amino acid makes DNA
more tolerant of point mutations.
Start and Stop Codons
 Start Codon
 Signals the start of translation
 Is the amino acid methionine (AUG)
 Translation always begins with AUG…
 Stop Codon
 Signals the end of the amino acid chain
 Coded for by UAA, UAG, or UGA
The Reading Frame
 For the mRNA code to be translated correctly, codons must be
read in the right order.
 This order is called the reading frame
 Changing the reading frame completely changes the resulting
protein.
 Therefore, it is very important for the mRNA to have a clear START and STOP
codon.
Common Language
 The genetic code is shared by
almost all organisms (even
viruses)
 It is considered to be a universal
code
 That means that the codon UUU
codes for phenylalanine in an
armadillo, a cactus, a yeast, or a
human.
 This suggests that all organisms arose
from a common ancestor.
 This also means that scientists can
insert a gene from one organism into
another organism to make a
functional protein
Ribosomes
 To translate codons into a physical amino acid, a ribosome
and tRNA are used.
 Ribosomes are a combination of rRNA and proteins.
 They catalyze the reaction that forms bonds between amino acids.
 They are the site of protein synthesis.
Ribosomes
 Ribosomes have a large and small subunit that fit together
and pull the mRNA strand through.
 The small subunit holds onto the mRNA strand.
 The large subunit holds onto the growing protein.
Transfer RNA (tRNA)
 tRNA acts as a sort of adaptor
between mRNA and amino
acids.
 It carries free floating amino
acids from the cytoplasm to the
ribosome.
 The tRNA is shaped like a plus
sign
 One end of the plus sign is
attached to a specific amino
acid.
 The bottom has an anticodon
that recognizes a specific
codon on the mRNA strand
o Ex. The anticodon CCC
pairs with the mRNA
codon GGG.
Amino Acids are linked to become a protein
1. The ribosome attaches to an mRNA molecule and exposes
one codon.
2. The exposed codon attracts a complimentary tRNA molecule
bearing an amino acid.
 The tRNA anticodon pairs with the mRNA codon.
Amino Acids are linked to become a protein
3. The ribosome helps form a
peptide bond between the
two amino acids.
 The ribosome then breaks the bond
between the tRNA molecule and its
amino acid.
4. The ribosome pulls the mRNA
strand the length of one
codon.
 The tRNA disengages from the
mRNA (leaving its amino acid bonded
with the mRNA) to pick up a new
amino acid.
5. The ribosome continues to
move down the mRNA strand,
attaching new amino acids to
the growing protein, until it
reaches a stop codon.
How to make a protein…
Ribosomes
Large
subunit
P
Site
A
Site
mRNA
Small subunit
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A U G
C U A C U U C G
How to make a protein…
aa1
aa2
2-tRNA
1-tRNA
anticodon
hydrogen
bonds
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U A C
A U G
codon
G A U
C U A C U U C G A
mRNA
How to make a protein…
peptide bond
aa1
aa3
aa2
3-tRNA
1-tRNA
anticodon
hydrogen
bonds
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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
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
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
30
Ribosomes move over one codon
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
32
Ribosomes move over one codon
aa4
aa5
Termination
aa199
aa3 primary
structure
aa2 of a protein
aa200
aa1
200-tRNA
A C U
mRNA
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terminator
or stop
codon
C A U G U U U A G
Transcription to Translation
End Product –The Protein!
 The end product of protein synthesis is a
primary structure of a protein
 A sequence of amino acids bonded together by
peptide bonds
aa2
35
aa1
aa3
aa4
aa5
aa199
aa200
Gene Expression and Regulation
 Depending on an organisms needs, a gene can make a lot of
protein, a little protein, or none at all.