Download 7.3 Translation (HL ONLY)

DNA, RNA, Protein Synthesis &
GENES
RNA and Protein Synthesis
Essential Idea
Information transferred from DNA to mRNA
is translated into an amino acid sequence
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Understandings
• Initiation of translation involves assembly of the components
that carry out the process.
• Synthesis of the polypeptide involves a repeated cycle of
events.
• Disassembly of the components follows termination of
translation.
• Free ribosomes synthesize proteins for use primarily within
the cell.
• Bound ribosomes synthesize proteins primarily for secretion
or for use in lysosomes.
• Translation can occur immediately after transcription in
prokaryotes due to the absence of a nuclear membrane.
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Translation
Messenger RNA is
transcribed in the nucleus,
and then enters the cytoplasm
where it attaches to a
ribosome.
Nucleus
mRNA
IB Assessment Statement
Explain that each tRNA molecule is recognized by
a tRNA-activating enzyme that binds a specific
amino acid to the tRNA, using ATP for energy.
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A tRNA molecule consists of a single RNA
strand that is only about 80 nucleotides
long
A
C
C
Flattened into one plane to reveal its base
pairing, a tRNA molecule looks like a
cloverleaf
3
Amino acid
attachment site
5
Hydrogen
bonds
Anticodon
Two-dimensional structure
Amino acid
attachment site
5
3
Hydrogen
bonds
3
Anticodon
Three-dimensional structure
5
Anticodon
Symbol used in this book
Because of hydrogen
bonds, tRNA actually
twists and folds into a
three-dimensional
molecule
tRNA is roughly L-shaped
Similar to DNA, t RNA has
a 3’ end and a 5’ end.
3
Amino acid
attachment site
5
Hydrogen
bonds
Anticodon
Two-dimensional structure
Amino acid
attachment site
5
3
Hydrogen
bonds
3
Anticodon
Three-dimensional structure
5
Anticodon
Symbol used in this book
The 3’ end forms a site
where the amino acid
attaches.
ATP is needed for the
attachment of the amino
acid to the tRNA 3’ end.
LE 17-14b
Amino acid
attachment site
5
3
Hydrogen
bonds
3
Anticodon
Three-dimensional structure
5
Anticodon
Symbol used in this book
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Activation of tRNA
a) Amino acid which is
specific to each tRNA.
(b) CCA base sequence to
which the amino acid is
attached by the
'Activating Enzyme'.
(c) Complementary base
pairing sequence.
Helical in shape.
(d) 8 free bases nonpairing giving one loop of
RNA.
Activation of tRNA
• (e) 7 free bases non-pairing
giving second loop of RNA.
• (f) Small open loop of RNA
which is variable in shape
between different tRNA.
• (g) Anti-codon (3 bases)
which binds to the mRNA
codon (3 bases) this is
specific to the amino acids
being carried. The anticodon is complementary to
the sense DNA
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Copyright Pearson Prentice Hall
tRNA activation
http://www.phschool.com/science/biology_
place/biocoach/translation/addaa.html
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IB ASSESSMENT STATEMENT
Outline the structure of ribosomes, including protein
and RNA composition, large and small subunits,
three tRNA binding sites and mRNA binding sites.
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Ribosome structure
Proteins and Ribosomal RNA
combine in the structure
Large sub-unit and a small subunit
Large sub-unit has three
binding sites for tRNA
molecules ( E, P and A site).
Small sub-unit has a binding
site for mRNA
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Ribosome Function
Ribosomes are enzymes.
The catalyse the translation of
mRNA into a polypeptide.
Their substrate is mRNA.
Each ribosome can catalyse
the transcription of different
mRNA.
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Copyright Pearson Prentice Hall
Accurate translation requires two steps:
1. First step: a correct match between a tRNA and
an amino acid, done by the enzyme aminoacyltRNA synthetase
2. Second step: a correct match between the tRNA
anticodon and an mRNA codon
LE 17-15
Amino acid
Aminoacyl-tRNA
synthetase (enzyme)
Pyrophosphate
Phosphates
tRNA
AMP
Aminoacyl tRNA
(an “activated
amino acid”)
Translation
The ribosome binds new tRNA molecules and amino
acids as it moves along the mRNA.
Phenylalanine
Methionine
Ribosome
mRNA
Start codon
tRNA
Lysine
Ribosomes
Ribosomes facilitate specific coupling of tRNA
anticodons with mRNA codons in protein synthesis
The two ribosomal subunits (large and small) are
made of proteins and ribosomal RNA (rRNA)
LE 17-16a
tRNA
molecules
Growing
polypeptide
Exit tunnel
Large
subunit
E P
A
Small
subunit
5
3
mRNA
Computer model of functioning ribosome
A ribosome has three binding sites for tRNA:
1. The P site holds the tRNA that carries the
growing polypeptide chain
2. The A site holds the tRNA that carries the next
amino acid to be added to the chain
3. The E site is the exit site, where discharged
tRNAs leave the ribosome
LE 17-16b
P site (Peptidyl-tRNA
binding site)
A site (AminoacyltRNA binding site)
E site
(Exit site)
E
P
A
mRNA
binding site
Schematic model showing binding sites
Large
subunit
Small
subunit
LE 17-16c
Amino end
Growing polypeptide
Next amino acid
to be added to
polypeptide chain
E
tRNA
mRNA
5
3
Codons
Schematic model with mRNA and tRNA
Translation occurs in a 5’→ 3’ direction.
During translation, the ribosome moves along the
mRNA towards the 3’ end. The start codon is
nearer to the 5’ end.
Amino end
Growing polypeptide
Next amino acid
to be added to
polypeptide chain
E
tRNA
mRNA
5
3
Codons
Schematic model with mRNA and tRNA
Translation
Protein Synthesis
Lysine
tRNA
5’
3’
Translation direction
mRNA
Ribosome
Protein Synthesis (building a polypeptide):
•
Ribosomes are the site of polypeptides
synthesis.
• This involves linking amino acids together
through condensation (dehydration) reactions.
Building a Polypeptide
The three stages of translation (building a
polypeptide) on the ribosomes:
1. Initiation
2. Elongation
3. Termination
7.4.3 Stages of translation.
• Initiation: In which the ribosome, tRNA and
mRNA come together to begin the translation of
the mRNA.
• Elongation: tRNA molecules attach to the mRNA
based on the codon-anticodon recognition. Amino
acids are brought together and polymerised into
the primary structure of the polypeptide.
• Termination: mRNA and the ribosomes detach
from one another. The polypeptide is released
and the tRNA return to be charged with more
amino acid.
Initiation:
– The initiation stage of translation brings together
mRNA and a tRNA with the first amino acid, and
the two ribosomal subunits
First, a small ribosomal subunit binds with mRNA
and a special initiator tRNA
Large
ribosomal
subunit
P site
Initiator tRNA
GTP GDP
E
mRNA
5
3
5
A
3
Start codon
mRNA binding site
Small
ribosomal
subunit
Translation initiation complex
Initiation:
– Then the small subunit moves along the mRNA
until it reaches the start codon (AUG)
Proteins called initiation factors bring in the large
subunit so the initiator tRNA occupies the P site
Large
ribosomal
subunit
P site
Initiator tRNA
GTP GDP
E
A
mRNA
5
3
5
3
Start codon
mRNA binding site
Small
ribosomal
subunit
Translation initiation complex
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Elongation of the Polypeptide Chain
• During the elongation stage, amino acids are
added one by one to the preceding amino acid
• A peptide bond is formed between the amino
acids.
Animation: Translation
Translation
The process continues until the ribosome
reaches a stop codon.
Polypeptide
Ribosome
tRNA
mRNA
Termination occurs when a stop codon in
the mRNA reaches the A site of the
ribosome
The polypeptide, and the translation
assembly then comes apart and is released
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Release
factor
Free
polypeptide
5
3
3
3
5
5
Stop codon
(UAG, UAA, or UGA)
When a ribosome reaches a stop
codon on mRNA, the A site of the
ribosome accepts a protein called
a release factor instead of tRNA.
The release factor hydrolyzes the
bond between the tRNA in the
P site and the last amino acid of the
polypeptide chain. The polypeptide
is thus freed from the ribosome.
The two ribosomal subunits
and the other components
of the assembly dissociate.
IB Assessment Statement:
State that translation occurs in a 5’→ 3’ direction.
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Translation Direction
Translation of the mRNA takes place from the 5' free
end to the free 3' end.
Ribosomes move along the mRNA in this direction.
The genetic code is translated from the 5' free end to
the 3' free end.
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IB Assessment Statement
Draw and label a diagram showing the structure of a
peptide bond between two amino acids.
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Peptide Bond formation
1. During translation
amino acids are joined
together to form
polypeptides.
2. The specific sequence
of amino acids is called
the primary structure.
3. Between each amino
acid a peptide bond
forms to join them
together.
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Peptide Bond formation
4. In this example the amino acids
are both Alanine in which the R
group is a single hydrogen.
5. The carboxyl acid end on the first
amino acid is orientated to the
amino group of the second
amino acid.
6. The -OH group and -H are
removed to form water
(condensation reaction).
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Peptide Bond formation
7. The bond forms between the terminal
carbon on the first amino acid and the
nitrogen on the second amino acid.
8. The backbone of the molecule has the
sequence N-C-C-N-C-C
9. Polypeptides maintain this sequence
no matter how long the chain.
10. The R groups project from the
backbone.
11. As the amino acids are added in
translation the polypeptide folds up
into it specific shape.
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Click here for translation animation
http://www.stolaf.edu/people/giannini/flashanim
at/molgenetics/translation.swf
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The Roles of RNA and DNA
The Roles of RNA and DNA
The cell uses the DNA “master plan” to prepare
RNA “blueprints.” The DNA stays in the
nucleus.
The RNA molecules go to the protein building
sites in the cytoplasm—the ribosomes.
Genes and Proteins
Genes and Proteins
Genes contain instructions for assembling
proteins.
Many proteins are enzymes, which catalyze and
regulate chemical reactions.
Proteins are each specifically designed to build or
operate a component of a living cell.
Genes and Proteins
Codon
The sequence of
bases in DNA is used
as a template for
mRNA.
The codons of mRNA
specify the sequence
of amino acids in a
protein.
Codon Codon
Single strand of DNA
Codon Codon Codon
mRNA
Alanine Arginine Leucine
Amino acids within
a polypeptide
IB ASSESSMENT STATEMENT
Explain the process of translation, including
ribosomes, polysomes, start codons and stop
codons.
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INITIATION OF
TRANSLATION (more
detailed)
The tRNA charged with
Methionine has the anticodon UAC. This is
complementary to the
start codon (mRNA) of
AUG.
The small sub unit of the
ribosome associates with
the Methionine tRNA.
The small unit of the
ribosome moves over the
START codon.
The large unit of the
ribosome moves over the
mRNA.
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INITIATION OF TRANSLATION
(more detailed)
There are three binding sites for
tRNA on the large sub unit.
A-(Amino acid) is the position
which the new tRNA codonanticodon binds making sure that
the correct amino acid is in
position.
P-(Polypeptide) is the position in
which the amino acid on the
tRNA adds to the polypeptide.
E-( Exit) is the position the tRNA
(without amino acid) locates and
is the released from the
ribosome to become reactivated.
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INITIATION OF TRANSLATION
(more detailed)
The START codon (AUG) occupies
the P site.
The A site is free for the
complementary tRNA to bind.
Specificity is maintained by the
codon-anticodon binding which is
a major feature of the ribosome
function.
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Elongation OF TRANSLATION
(more detailed)
In this sequence the A site has the
codon CCG.
The tRNA anticodon GGC which
carried Proline hydrogen bonds
with the codon bases.
The codon -anticodon binding has
placed the two amino acids
methionine and proline beside
each other.
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Elongation OF TRANSLATION
(more detailed)
(a)The bond between the tRNA and
methionine is broken.
This releases free energy.
(b)The free energy is used to form
the peptide bond between
methionine and proline.
The large sub-unit then moves to
three bases (one codon) towards
the 3' end of the mRNA.
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Elongation OF TRANSLATION
(more detailed)
Both units of the ribosome are now
located over the second and
third codons
(a) The tRNA for methionine is on
the E site and is released from
the ribosome. It will beceom
recharged with methionine in the
cell cytoplasm.
The tRNA for proline is in the P site.
(b) The A site for the next tRNA is
free and holds the codon base
sequence GCU.
The tRNA for Alanine has the anticodon CGA which is
complementary to the codon on
the A site.
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Elongation OF TRANSLATION
(more detailed)
The anticodon tRNA for Alanine
complementary base pairs with
the A site codon.
The ribosome checks that this is the
correct tRNA and therefore
amino acid.
The bond between the tRNA and
Proline is broken.
Free energy is released.
A peptide bond is formed between
Proline and Alanine.
The peptide chain will be folding
and shaping.
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Termination OF TRANSLATION (more
detailed)
The end of the codon sequence in mRNA
has been reached.
The ribosome encounters a termination
sequence signaling the end of translation.
The ribosome moves the alanine tRNA to
the P site.
The polypeptide is released from the
translation process.
The ribosome has no new codons read.
The two sub units move and separate.
The protein will now be further modified in
either the endoplasmic reticulum, golgi or
secreted in a vesicle.
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IB Assessment Statement
State that free ribosomes synthesize proteins for use
primarily within the cell, and that bound ribosomes
synthesize proteins primarily for secretion or for
lysosomes.
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Free vs. Bound Ribosomes
Free ribosomes in the cytoplasm
are associated with the synthesis
of proteins for internal use in the
cell.
Ribosomes which are attached to
the wall of the endoplasmic
reticulum are associated with
proteins which will ne placed into
vesicles and secreted form the
cell.
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Copyright Pearson Prentice Hall
Animations about Transcription/ Translation
https://www.youtube.com/watch?v=NJxobgkPEAo
http://www.medschool.lsuhsc.edu/biochemistry/images/protein%20synthesi
s_1.swf
http://bio3400.nicerweb.com/med/Vid/Klug8e/ch14/14_2_3a_00i.swf
http://ihome.cuhk.edu.hk/~z045513/virtuallab/animation/Translation.swf
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END OF SECTION