Download Protein Synthesis Notes

Transcription, Translation &
Protein Synthesis
Do you remember what proteins
are made of ?
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Hundreds of Amino Acids link
together to make one Protein
􀂄 There are 20 types of amino acids, some
we can make, and some we can’t
􀂄 There are infinite combinations of amino
acids
􀂄 Can be hundreds or thousands monomers
Long
These long chains are called polypeptide
chains
Protein Synthesis
 Protein synthesis is the process in
which a cell makes protein based on the
message contained within its DNA.
 However:
– DNA is only found in the nucleus
– Proteins are only made outside the
nucleus – in the cytoplasm.
 Houston, we have a problem.
Protein Synthesis
 How do the many different messages
within the DNA molecule get to the
many ribosomes outside the nucleus?
 A molecular cousin of DNA – RNA – is
used to carry these messages.
Ribonucleic Acids (RNA)


The job of RNA (ribonucleic acid) is to carry
messages from the DNA (in the nucleus) to
the ribosomes (in the cytoplasm).
There are three types of RNA:
1. mRNA – carries a message from the DNA to the
cytoplasm
2. tRNA – transports amino acids to the mRNA to
make a protein
3. rRNA – make up ribosomes, which make
protein.
Ribonucleic Acids (RNA)
 RNA is almost exactly like DNA, except:
– Contains a ribose sugar, instead of a
deoxyribose sugar (hence the name…)
– Contains uracil instead of thymine.
– RNA is single-stranded, not doublestranded (usually…)
Ribonucleic Acids (RNA)
Protein Synthesis
 Occurs in TWO steps:
1. Transcription – the genetic information
from a strand of DNA is copied into a
strand of mRNA
2. Translation – the mRNA, with the help of
the ribosome, forms a chain of amino
acids (eventually forming a protein) based
on the information contained on the mRNA.
The Central Dogma
 This order of events is called the
central dogma of molecular biology:
DNA
RNA
P
R
O
T
E
I
N
Step One: Transcription
DNA unzips: enzymes split apart base
pairs and unwind the DNA double helix.
2. Bases pair up: Free nucleotides in the cell
find their complementary bases along the
new strands with the help of RNA
polymerase. What will be different??
3. New backbone formed: The sugarphosphate backbone is assembled to
complete the RNA strand, and separates
from the DNA strand.
1.
Step One: Transcription
 Watch this simplified animation:
http://www.stolaf.edu/people/giannini/flash
animat/molgenetics/transcription.swf
 Watch the more complex animation!
 http://www-
class.unl.edu/biochem/gp2/m_biology/a
nimation/gene/gene_a2.html
Step One: Transcription
 Try it! What RNA strand will be made
from the following DNA sequence?
TACGCATGACTAGCAAGTCTAACT
Step One: Transcription
 Try it! What RNA strand will be made
from the following DNA sequence?
TACGCATGACTAGCAAGTCTAACT
AUGCGUACUGAUCGUUCAGAUUGA
Step 1½: RNA Editing


An mRNA molecule has to be “edited”
in order to be useful. There’s a lot of
unnecessary information that needs to
be removed.
An mRNA sequence that does NOT
code for protein is called an intron. A
sequence that is useful in making a
protein is called an exon.
Step 1½: RNA Editing
DNA
transcription
pre-RNA (in nucleus)
exon 1
intron
RNA editing
exon 2
intron
intron
intron
RNA (in cytoplasm)
exon 1
exon 2
exon 3
exon 3
Step Two: Translation “to decode or
decipher the meaning of”


Now that our mRNA molecule has
been made, it’s time for its message to
be made into a protein sequence.
How does the mRNA sequence
translate into an amino acid
sequence?
Step Two: Translation
 Problem:
– There are 20 different amino acids.
– There are 4 RNA bases.
A T C G
phe
ile
leu
val
met
pro
ser
ala
thr
his
tyr
asn
gln
asp
lys
cys
glu
arg
trp
gly
Step Two: Translation
 Watch this simplified animation:
http://www.stolaf.edu/people/giannini/flash
animat/molgenetics/translation.swf
 Watch the more complex animation!
 http://www-
class.unl.edu/biochem/gp2/m_biology/a
nimation/gene/gene_a3.html
This is a molecule of messenger RNA.
It was made in the nucleus by
transcription from a DNA molecule.
codon
A U G G G C U U AAA G C A G U G C A C G U U
mRNA molecule
A ribosome on the rough endoplasmic
reticulum attaches to the mRNA
molecule.
ribosome
A U G G G C U U AAA G C A G U G C A C G U U
Amino acid
tRNA molecule
A transfer RNA molecule arrives.
It brings an amino acid to the first three
bases (codon) on the mRNA.
anticodon
The three unpaired bases (anticodon)
on the tRNA link up with the codon.
UAC
A U G G G C U U AAA G C A G U G C A C G U U
Another tRNA molecule comes into
place, bringing a second amino acid.
Its anticodon links up with the second
codon on the mRNA.
UAC
A U G G G C U U AAA G C A G U G C A C G U U
Peptide bond
A peptide bond forms between the
two amino acids.
A U G G G C U U AAA G C A G U G C A C G U U
The first tRNA molecule releases its amino
acid and moves off into the cytoplasm.
A U G G G C U U AAA G C A G U G C A C G U U
The ribosome moves along the mRNA to
the next codon.
A U G G G C U U AAA G C A G U G C A C G U U
Another tRNA molecule brings
the next amino acid into place.
A U G G G C U U AAA G C A G U G C A C G U U
A peptide bond joins the second
and third amino acids to form a
polypeptide chain.
A U G G G C U U AAA G C A G U G C A C G U U
The process continues.
The polypeptide chain gets longer.
This continues until a termination
(stop) codon is reached.
The polypeptide is then complete.
A U G G G C U U AAA G C A G U G C A C G U U
Step Two: Translation
2. Since each 3-letter combination
“codes” for an amino acid, you need to
figure out what amino acid matches up
with each codon:
AUG|CGU|ACU|GAU|CGU|UCA|GAU|UGA
met
?
The Genetic Code
The Genetic Code
Step Two: Translation
2. Since each 3-letter combination
“codes” for an amino acid, you need to
figure out what amino acid matches up
with each codon:
AUG|CGU|ACU|GAU|CGU|UCA|GAU|UGA
met
arg
thr
asp
arg
ser
asp
???
The Genetic Code
Step Two: Translation
2. Since each 3-letter combination
“codes” for an amino acid, you need to
figure out what amino acid matches up
with each codon:
AUG|CGU|ACU|GAU|CGU|UCA|GAU|UGA
met
met
thr
asp
arg
ser
asp
STOP
RECAP:
DNA is transcribed
into mRNA in the
nucleus.
2. The mRNA leaves the
nucleus and enters the
cytoplasm.
3. The protein is
translated from the
mRNA sequence
using tRNA and amino
acids.
1.