BMSC 220 Topic 7

• -carried out on ribosomes with tRNA's serving as adaptors between mRNA template and AA being incorporated into protein
• -each AA coded for by a triplet of nucleotide sequence in mRNA

• -always Methionine
• -AUG codon

• -61 possible codons
• -each codon is for specific AA or stop sequence 
• -AA can have more than one codon for it
• -3 stop codons
• -always codes AUG for methionine at the start

-tRNA with a specific AA covalently bound at 3' end

• -green part is aminoacyl tRNA
• -red UAC is anticodon

• -ribosomal RNA
• -form the components of ribosomes 
• -can be up to 10,000,000 (25% of dry weight)

• -50S large sub unit
• -30S small sub unit

• -60S large subunit 
• -40S small subunit

• -assembled in nucleolus of the nucleus 
• -ribosomal proteins are imported to the nucleus and attach to pre-rRNA. the pre-rRNA splits in two, one part the 40S the other the 60S subunit

• -synthesized by RNA polymerase III
• -possess unique 3 nucleotide sequences(anticodon) that pair the tRNA with the specified AA
• -70-80 nucleotides long and have characteristic cloverleaf structures based on intramolecular base pairing

-anticodon is on opposite side of molecule from actual AA binding site

• -20
• -each one recognizes the correct AA
• -the correct codon on the mRNA

• -AA is activated
• -AA is added to tRNA
• -error rate is about 1/1000

• -classical term for the smallest genetic unit for a specific trait
• -essentially the same as a gene in today's context

• -polycistronic
• -monocistronic

• -initiation
• -elongation
• -termination

-releasing factor

-elongation factor

-initiation factor

-look for a specific nucleotide sequence, Shine-Dalgarno sequence

• -40S subunit, various initiation factors (eIFs) and charged tRNA(charged with methionine) assemble
• -tRNA with methionine always binds to 40S subunit before the subunit binds to mRNA

-40S, tRNA^met, eIF complex meets up with mature mRNA and binds to the 5' m7G cap

• -entire complex sitting at the 5' end of mRNA slides along mRNA until it finds the start AUG codon
• -requires ATP for scanning procedure

• -complex disassembles (tRNA^met and 40S stay on mRNA) initiation factors leave
• -60S subunit binds

• -aminoacyl-A site-tRNA arrival site
• -peptidyl-P site-polypeptide (pause) site
• -exit-E site-tRNA leaves ribosome

-EF are complexed to GTP's and escort aminoacyl tRNA to the ribosome (A site)

• -within 40S subunit ensures proper aminoacyl tRNA pairs with the codon
• -third step in specificity (after AA-tRNA pairing and simple base pairing)

• -tRNA^met sits in AUG start

• -next tRNA comes to A 

• -peptide bond formation between first two AA
• -AA chain is transferred from P site tRNA to A site tRNA

• -ribosome moves downstream (tRNA moves from A site to P site=translocation)
• -A site is ready for next charged tRNA
• -energy dependent process(GTP)

-stop signal on the mRNA arrives in A site

• -step 2-releasing factor (purple blob) sits in A site at UAA codon (3 stop codons possible)
• -step 3-polypeptide chain is released

-release factor causes structure to come apart (ribosomal subunits can be reused)

• -some proteins must be cleaved to become an active protein
• -ex. insulin

-chemical modification of a polypeptide chain after translation

• -method of post translation modification
• -phosphate addition on serine, threonine or tyrosines by protein kinases 
• -phosphotases act to reverse phosphorylation

• -method of post translation modification
• -carbohydrate addition to protein to form glycoprotein

• -method of post translation modification
• -lipid addition to form lipoprotein

• -controlled at level of transcription and translation, which determines the amount of protein/enzyme that is made by the cell
• -also controls how quickly a protein is degraded

-marker in eukaryotic cells that targets cytosolic and nuclear proteins for rapid proteolysis(degradation)

-large, multi subunit protease complexes that recognize and degrade polyubiquinated proteins