protein synthesis-translation
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Protein Synthesis-Translation
Warm-up #61. How is RNA different than DNA?2. Name the 3 steps of transcription.3. What is the end product of transcription?4. 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?
mRNA
From nucleus to cytoplasm
DNA
transcription
nucleus cytoplasm
translation
trait
aa
aa
aa
aa
aa
aa
aa
aaaa
aa
protein
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
• 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
The mRNA code
shows the “dictionary” for the mRNA codons and their corresponding amino acids (know how to use)
1. DNA: AGC-GTG-CCA2. 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-CCA2. mRNA: UCG-CAC-GGU3. amino acid chain (protein) = Serine- Histadine- Glycine
· Transfer RNA (tRNA) is the interpreter between the 2 forms of information: base sequence in mRNA and amino acid sequence in polypeptides
tRNA Structure
Amino acid location (CCA)
3 exposed bases
-an enzyme links a specific amino acid from the cytosol/cytoplasm to each tRNA molecule on the 3’ end (CCA) using ATP for energy
LE 17-14b
Hydrogenbonds
Amino acidattachment site5
3
3 5
Anticodon
Symbol that is widely used
Anticodon
Three-dimensional structure
LE 17-15Amino acid Aminoacyl-tRNA
synthetase (enzyme)
Pyrophosphate
Phosphates
tRNA
AMP
Aminoacyl tRNA(an “activatedamino 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)
Amino acid
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-tRNAbinding site)
E site (Exit site)
mRNAbinding site
A site (Aminoacyl-tRNA binding site)
Largesubunit
Smallsubunit
Schematic model showing binding sites
E P A
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
Met
GTPInitiator tRNA
mRNA
53
mRNA binding site
Smallribosomalsubunit
Start codon (AUG)
P site
5 3
Translation initiation complex
E A
Largeribosomalsubunit
GDP
Met
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
Ribosome ready fornext aminoacyl tRNA
mRNA
5
Amino endof polypeptide
E
Psite
Asite
3
2
2 GDP
E
P A
GTP
GTP
GDP
E
P A
E
P A
Fueled by GTP= Guanosine Triphosphate
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)
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