Translation Protein Biosynthesis. Central Dogma DNA RNA protein transcription translation.
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Transcript of Translation Protein Biosynthesis. Central Dogma DNA RNA protein transcription translation.
Translation
Protein Biosynthesis
Central Dogma
DNA
RNA
protein
transcription
translation
mRNAs are exported for translation
Through nuclear pore complex Recognizes and transports ONLY
completed mRNA
Translation
Process by which the sequence of nucleotides in an mRNA directs the incorporation of amino acids into a protein
Necessary Components for Translation mRNA tRNAs covalently linked to amino acids Ribosome
Three phases of Translation Initiation Elongation Termination
The mRNA strand is “read” and amino acids are linked together to make a protein by the ribosome
mRNA
Carries the genetic information from the chromosomes to the ribosomes
How is the language of nucleic acid sequences translated into the amino acid language of proteins?
mRNA is decoded in sets of three nucleotides = codons
Genetic Code
Each codon specifies either an amino acid or stop signal to translation
There are only 20 amino acids and there are 64 possible codons
The genetic code is degenerate - i.e. there are "synonyms" (multiple codons) for some amino acids
Three codons (UAG, UGA, and UAA) encode translation "stop" signals rather than amino acids
mRNA must be read in the correct reading frame to be decoded into the protein
Redundancy or degenerate coding Reference page 367
Amino acids
Organic molecule containing both an amino group and a carboxyl group
Building blocks of proteins Are added to the C-terminal end of a
growing polypeptide chain by the formation of a peptide bond
Peptide bonds – between the carboxyl group at the end of growing chain and a free amino group of incoming amino acid
Proteins are synthesized from its N-terminus to its C-terminus
tRNA
Adapter molecule that mediates recognition of the codon sequence in mRNA and allows its translation into the appropriate amino acid.
~ 80 nucleotides long Folds into 3D structure It has sites for amino-acid attachment and codon
recognition The codon recognition is different for each tRNA and
is determined by the anticodon region, which contains the complementary bases to the ones encountered on the mRNA.
Each tRNA molecule binds only one type of amino acid, but because the genetic code is degenerate, more than one codon exists for each amino acid.
Ribosomes
Small and Large subunits The site of translation Helps to maintain the correct reading
frame and to ensure accuracy Complex catalytic machine made up of 50
different proteins and several RNA molecules (rRNAs)
Produced in nucleolus Millions exist in cell
tRNA structuretRNA Molecule
Aminoacyl tRNA Synthetase
In Eukaryotes Unique synthetase for each amino acid
Proper Amino Acid by affinity or fit
Corrected by Hydrolytic editing
EP A
mRNA binding• two subunits, a large and a small
• the mRNA binds to the small subunit
• there are three sites of activity and tRNA binding within the large subunit
The Ribosome
How do Ribosomes work?Via 4 binding sites for RNA molecules
C G A U C A A U A CC G A U C A A U G C G
codon codon codon codon codon codon
The ribosome attaches to the RNA and scans for AUG,the start codon
The ribosome reads the mRNA three nucleotides at a time
Each group of three nucleotides is a single codon
Each codon specifies an particular amino acid
Start codon
TRANSLATION INITIATION
Initiation
Translation begins with the codon AUG A special tRNA is required to initiate
translation Initiator tRNA always carries the amino
acid methionine Initiator tRNA is loaded onto the small
subunit of the ribosome with the aid of additional proteins (eIFs) which are attached to GTPs
Initiation, cont.
Initiator tRNA binds small ribosomal subunit Small subunit then binds to 5’ end of an
mRNA molecule (recognized by 5’ cap) The small subunit then moves along mRNA
(5’-3’) searching for the first AUG eIF2 hydrolyzes GTP to GDP and detaches Large Subunit then assembles and
elongation can begin Bacteria use Shine-Dalgarno sequences to
initiate translation at any point on the mRNA.
Shine Dalgarno sequence Ribosome docking sequences
Upstream of AUG consensus sequence
Once the ribosome recognizes the start codon, protein synthesis begins
The ribosome promotes a chemical reaction to occur that joins two amino acids with a peptide bond
Amino acids are transferred to the ribosomes by tRNA molecules
tRNAs have an anticodon on one end and an amino acid on the other
The anticodon is a sequence of three nucleotides that complement a codon
Cont.
The anticodon determines which amino acid it carries to the ribosome
EF-Tu (EF-1) helps the fidelity of the process.
Each of the twenty amino acids pairs up with between 1 and 4 anticodons
The process continues, the ribosome moves along the mRNA to the next codon with the help of EF-G (EF-2)
A new tRNA recognizes the next codon.
U C A G C A A G A C
Met
U C A G U A A U G U C
Anti-codon
tRNA
Amino acid
• This continues until the ribosome reaches a STOP codon, which indicates the end of the gene•The ribosome & last tRNA fall off the mRNA & the amino acid chain is complete!
TRANSLATION ELONGATION
Elongation
Termination
One of the three STOP codons mark the end of translation
The stop codons are recognized by proteins known as release factors that do not specify any amino acids
The release factor triggers an addition of water to the end of the polypeptide chain the release of the new protein.
Protein Folding
Begins while Protein is still being synthesized
Guided by and made more efficient by molecular chaperones
The amino acid chain folds up into a 3-dimensional structure dictated by the order of the amino acids.
This unique structure gives the protein its unique function and allows it to do its work
Every protein has a unique order of amino acids
Proteins have many functions
Protein example: Antibiotics
Some antibiotics are peptides, others glycopeptides, others are amino acid derivatives
Inhibitors of prokaryotic translation, allowing for discrimination between prokaryotic and eukaryotic cells
Examples: Tetracycline, Streptomycin, Chloramphenicol, Erythromycin