Transcript Making Proteins

Protein Synthesis-Translation
Warm-up #6
1.
2.
3.
4.
How is RNA different than DNA?
Name the 3 steps of transcription.
What is the end product of transcription?
What is the difference between the sense and
antisense DNA strands?
5. What is the role of RNA polymerase?
6. Describe the initiation step in transcription.
7. What are the “extra” things that happens to the
mRNA in eukaryotic cells BEFORE it leaves
the nucleus?
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From nucleus to cytoplasm
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transcription
DNA
mRNA
protein
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translation
trait
nucleus
cytoplasm
Things you will need to know in
order to understand translation:
1. mRNA codon chart
2. Redundancy of the code
3. Structure of tRNA
4. Ribosome structure
Redundant but not Ambiguous
• Codons GAA and GAG both specify
glutamic acid (redundancy)
• Neither of them specifies any other amino
acid (no ambiguity)
mRNA Codons:
· codon = group of 3 mRNA nucleotides
**1 codon codes for 1 amino acid
· During translation, proteins are
synthesized according to the genetic
message of sequential codons along the
mRNA
The mRNA code
• For ALL life!
– strongest support for a
common origin for all
life
• Code has
duplicates
– several codons for
each amino acid
– mutation insurance!
 Start codon


AUG
methionine
 Stop codons

UGA, UAA, UAG
shows the “dictionary” for the mRNA codons
and their corresponding amino acids (know
how to use)
1. DNA: AGC-GTG-CCA
2. mRNA:
3. amino acid chain (protein):
______________________
shows the “dictionary” for the mRNA codons
and their corresponding amino acids (know
how to use)
1. DNA: AGC-GTG-CCA
2. mRNA: UCG-CAC-GGU
3. amino acid chain (protein) =
Serine- Histadine- Glycine
tRNA Structure
· Transfer RNA (tRNA) is the interpreter
between the 2 forms of information: base
sequence in mRNA and amino acid sequence in
polypeptides
-an enzyme links a specific amino acid from the
cytosol/cytoplasm to each tRNA molecule on the
3’ end (CCA) using ATP for energy
Amino acid
location (CCA)
3 exposed bases
LE 17-14b
Amino acid
attachment site
5
3
Hydrogen
bonds
3
Anticodon
Three-dimensional structure
5
Anticodon
Symbol that is widely used
LE 17-15
Amino acid
Aminoacyl-tRNA
synthetase (enzyme)
Pyrophosphate
Phosphates
tRNA
AMP
Aminoacyl tRNA
(an “activated
amino acid”)
Amino acid
· Molecules of tRNA are specific
for only 1 amino acid
-one end of tRNA attaches to
a specific amino acid
-the other end Hydrogen bonds
to mRNA codon by base pairing
(anticodon = a sequence of 3
bases on tRNA)
Ribosome Structure
• large ribosomal subunit (50s)
• small ribosomal subunit (30s)
– each subunit = ribosome/protein complex
• Consists of an mRNA binding site plus
3 tRNA binding sites
E
P
A
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
Process of Translation
• Translation includes 4 processes:
initiation, elongation, translocation, and
termination
1. Initiation of Translation
• mRNA binds to small ribosomal
subunit
• Initiator tRNA brings1st amino
acid, Methionine (Met) to the
initiation site on mRNA (start
codon AUG= And U Go)
• the Large ribosomal subunit
binds
– Met is in the “P” site
– “A” site is available for the
next tRNA
LE 17-17
Large
ribosomal
subunit
P site
Initiator tRNA
GTP
GDP
E
A
mRNA
5
3
Start codon
(AUG)
mRNA binding site
Small
ribosomal
subunit
5
3
Translation initiation complex
2. Elongation
• The next tRNA anticodon
complementary base pairs with the
mRNA codon in the “A” site aligning the
appropriate amino acid next to “Met.”
• Ribosome forms a peptide bond between
“Met” and the 2nd amino acid and it
passes the elongating polypetide chain
to the tRNA in the “A” site
3. Translocation
• Ribosome moves (translocates-change
location) the tRNA in the “A” site to the “P”
site
• The empty tRNA moves to the “E” site
where it is released
• mRNA moves along/through the ribosome
to expose the next mRNA codon to the “A”
site
• the protein chain is growing longer
LE 17-18
Fueled by GTP=
Guanosine Triphosphate
Amino end
of polypeptide
E
3
mRNA
Ribosome ready for
next aminoacyl tRNA
P A
site site
5
2
GTP
2 GDP
E
E
P
A
P
GDP
GTP
E
P
A
A
4. Termination
• When a “STOP” codon
(UGA, UAA, UAG) is
reached on the mRNA,
there isn’t a
complementary tRNA so….
– UGA = U Go Away
– UAA= U Are Awesome
– UAG= U Are Great
4. Termination
• A “release factor” binds to the “A”
site
– Causes the addition of a water
molecule to the polypeptide which
hydrolyzes the completed
polypeptide from the tRNA
• Ribsomal subunits dissociate
(break apart from the mRNA)
• Newly formed protein goes to
rough ER for modification, then
to golgi for packaging then
released from cell (exocytosis)