Lecture 6 Flashcards
The nucelotide sequence of an mRNA is translated into the amino acid sequence of a protein via the
Genetic code
The genetic Code
The way that information is stored in the DNA and how it can be read to make a protein
UNIVERSAL code = all organisms use this genetic code (some organisms may use certain codons more frequently)
Redundant: Most amino acids (EXCEPT for 2) have more than one codon that codes for it
Methianine (Met): START codon
Coded by AUG
Codon
Contains 3 bases
Dictates what amino acid will be added to chain in the mRNA
Redundancy
Most codons code for more than 1 amino acid
In principle, an mRNA molecule can be translated into 3 possible reading frames
Reading frame: ribosome reads 3 bases at a time
Frame shift if you add 1 or 2 bases
tRNA molecules are molecular adaptors, linking amino acids to
Codons
Anticodon
Complementary to the codon in the mRNA
tRNA is TRANSCRIBED but never
Translated
Amino acid is ALWAYS added to the
3’ end (there is a conserved set of sequences on this end: CCA)
On the other end:
Anticodon loop (complementary to the codon)
Each aminoacyl-tRNA synthetase makes multiple contacts with its tRNA molecule
One per amino acid in cell
These are specific for the AMINO ACID, not the codon
Responsible for adding on amino acid to CCA on 3’ end of tRNA; specific to each amino acid in the cell
The genetic code is translated by
Aminoacyl-tRNA synthetases and tRNAs
This will connect the amino acid to the very last A residue of the CCA
ATP dependent
Amino acid and tRNA w/in aminoacyl-tRNA synthetase (specific to amino acid) -> releases ATP -> Linkage of amino acid to tRNA by high-energy ester bond -> anticodon in tRNA binds to its codon in mRNA -> Net result: Amino acid is selected by its codon in an mRNA
The eukaryotic ribosome is a large complex of 4 rRNAs and more than 80
Small proteins
rRNA is NOT translated to protein
Sibosomes are a COMPLEX of proteins and RNA
rRNA
SAME in prokaryotes and eukaryotes
Each ribosome has a binding site for an mRNA and 3 binding sites for tRNAs
Ribosome itself is considered a ribozyme (catalytic piece of RNA)
Ribosome is a
Ribozyme
Ribosomal RNA catalyzes peptide bonds to
Form
Fibozyme
Catalyst version of rRNA
Translation takes place in a 4-step cycle, which is repeated over and over during the Synthesis of a protein
Elongation:
1) 3 amino acids on protein; many tRNAs will attempt to attach, only the one that is complementary will attach
2) Peptide bond is formed; energy comes from the breakage of high-energy ester bond
3) Trigger conformational change in large ribosomeal subunit, resulting in a shift
4) Shifts down by 3 bases; 4th tTNA is in P site; empty tRNA is released; A site is left open for the next one
A site
Acceptor site
Initiation of protein synthesis in eukaryotes requires translation initiation factors and a special initiator tRNA
5’ cap binding protein and poly-A proteins get exchanged for other proteins to be used in translation
In eukaryotic cells
Methianine is ALWAYS the start codon
Small subunit has initiator tRNA (complex)
These initiation factors recognize and interact with cap scan for start codon
Load up at cap
This is waht it recognizes
Scan message to look for AUG (start)
mRNA binding at AUG ->
Small ribosomal subunit, with bound iniator tRNA, moves alond mRNA searching for first AUG
Initiation factors dissociate (basically only there to recognize processed messages that are ready to be translated
Large subunit will dissociate first
ALL tRNAs enter at the A site EXCEPT for initiator tRNA, must enter at P site instead
Translation initiation factors dissociate
Large ribosomal subuni binds
Initiator tRNA will NEVER be in the A site; ALWAYS starts in the P site
Monocistronic message
Eukaryotes
Only have ONE gen on RNA
Polycistronic message
Prokaryotes
MANY genes can be on one mRNA
Translations halts at a stop codon
Termination
1) Ribosome arrive at stop codon; there are NO tRNA’s involved
2) Realease/termination factor binds to the A site -> triggers the relaeas of everything (peptide chain, message and ribosoem dissociate); there is NO tRNA that would recognize a stop codon
Sae in ALL cells
Proteins are synthesized on polyrbosomes
Messages will be translated many (hundred of) times
Proteins are degraded by the proteasome AFTER translation
Proteins are targeted to be degraded
Degrading proteins within cell is important to get rid of damaged and old proteins
Proteasome: Chamber that gets rid of poroteins enzyme ccomplx
ubquitin marks proteins for destruction; proteasome just looks for ubiquitin tag
Proteases chop up proteins
Degrading proteins within cell is important process; cell gets rid of old/deteriorating proteins
Proteins marked by a polyubiquitin are degraded by the proteasome
Chain of proteins
Many proteins requrie post-transcriptional modifications to become fully functional
Most proteins will need to be modified
-Glycolysis, methylation, phosphorylation
