Theme 2C Flashcards
What is translation?
The assembly of amino acids into polypeptides on ribosomes.
Where does translation take place?
In the cytoplasm for eukaryotes and everywhere in prokaryotes
What is tRNA’s role in translation?
It brings amino acids to the complex to be added to the polypeptide chain.
How is the sequence of amino acids determined?
By the sequence of codons in the mRNA. It is read from the 5’-3’ direction.
What direction is the polypeptide assembled?
From the N-terminal end to the C-terminal end
Non-polar amino acids
R groups usually contain -CH2 or -CH3
Uncharged polar amino acids
R groups usually contain -OH
Charged polar amino acids
R groups that contain acids or bases that can ionize
Aromatic amino acids
R groups contain a carbon ring with alternating single and double bonds
What are the special functional amino acids?
- Methionine: first amino acid in a polypeptide
- Proline: causes a kink in polypeptide chains
- Cysteine (S-S): disulfide bridge contributes to the structure of amino acids
What is the shape of tRNA and what causes it?
Winds into four double-helical segments = four-leaf clover. Caused by tRNA base-pairing with itself.
What is the anticodon and where is it?
A three-nucleotide segment that base-pairs with a codon in mRNAs. Located at the tip of one of the double-helical segments.
What is opposite from the anticodon?
A free 3’ end that links to the amino acid corresponding to the anticodon. Known as the acceptor stem.
What is charging?
When aminoacyl-tRNA adds the amino acid to the acceptor stem of the correct tRNA.
Charging rxn: amino acid + tRNA + ATP -> aminoacetyl-tRNA + AMP + PPi
How many aminoacyl-tRNAs are there?
20 for 20 different codons
How many sense codons are there?
61, written from 5’-3’ as they appear in mRNA
What are the rules of the genetic code?
- Codons are read from 5’-3’
- Codons are non-overlapping and the msg contains no gaps
- Msg is translated in a fixed reading frame set by the start codon
Are there 61 different tRNAs for the 61 codons?
No, because some tRNAs can read more than one codon. This is due to the base at the 5’ end of the anticodon, it can form H-bonds with more than one type of base located at the 3’ end = wobble.
What has no wobble?
The first two nucleotides in the anticodon are fixed.
What are ribosomes?
Ribonucleoprotein particles carry out protein synthesis by translating mRNA into chains of amino acids. Composed of the large subunit and small subunit.
What is the large subunit?
Contains the peptidyl-transferase center for the formation of peptide bonds.
What is the small subunit?
Contains the decoding center where charged tRNAs read and decode the codon of mRNA
What is the S (svedberg unit)?
The measure of sedimentation velocity and therefore mass.
Where are the subunits located?
They exist separately in the cytoplasm but join together on the mRNA.
What is the P site?
The peptidyl site, binds to the tRNA attached to the growing peptide chain.
What is the A site?
The aminoacyl site, binds to the tRNA carrying the next amino acid to be added.
What is the E site?
The exit site, binds the tRNA that carried the previous amino acid added.
What are the translation initiation steps?
- Met is brought to the P site of small ribosome subunit (requires GTP energy)
- Complex of initiator tRNA is brought to small ribosome subunit of the 5’ capped end. Then it scans along 5’-3’ until it reaches the start codon.
- Complementary base pairing occurs between anticodon of initiator tRNA and start codon.
- Large ribosome subunit binds to the small to form the initiation complex. Can now accept tRNA in the A site.
- GTP is hydrolyzed to GDP and translation begins
What are the translation elongation steps?
- Correct aminoacyl tRNA is loaded into the A site by EF (elongation factor)
- Peptidyl transferase in the large subunit forms a peptide bond between the carboxyl group of the growing chain and the amino group of amino acid in the A site.
- Ribosome translocates in which tRNA with the polypeptide shifts from A to P site and uncharged tRNA goes from P to E and is ejected.
- Next codon of mRNA is now in the A site and the next aminoacyl tRNA can be loaded in by EF-GTP
What are the translation termination steps?
- Stop codons are recognized by protein release factors
- The release factor binds to the A site and stimulates peptidyl transferase to cleave the polypeptide from the P site
- Ribosome subunits separate and detach from mRNA and empty tRNA and release factors also separate
What are the types of post-translational regulation of proteins?
- Phosphorylation
- Ubiquitination
- Proteolysis
What is phosphorylation?
The addition of phosphate to proteins by kinases can activate or inhibit their activity (example: phosphorylates activate Microtubule Associated Protein (MAP) to promote spindle assembly and mitotic entry)
What is ubiquitination?
The addition of ubiquitin molecules to proteins target them for destruction by the proteasome
What is proteolysis?
The specific cleavage of a protein can induce activity.
What is epigenetics?
Changes in gene transcription without changes in the DNA sequence. Modifications to lysines on histone tails affect transcription of genes (histone code).
What does histone acetyltransferase do?
Adds acetyl groups to histone tails to increase gene transcription by loosening DNA binding. Loosely bound caused by repulsion, thus makes it easier to access everything needed for transcription.
What does methylation of histone tails do?
Activate and repress transcription of genes. DNA can be methylated closer to the promoter to repress transcription.
What does chromatin remodelling complex do?
Displaces the nucleosomes from the promoter regions activating transcription. It gives more access to everything needed for transcription.
What does the removal of the stop codon in mRNA cause?
No increase/decrease in gene expression, longer polypeptide, regulated translationally
What does the increased ubiquitination of proteins cause?
Decreased gene expression, enhanced protein degradation, regulated post-translationally
What does complete inhibition of charging cause?
Decreased gene expression, no translation occurring, regulated translationally
What does a decrease in histone acetylation in promoter cause?
Decreased gene expression, DNA wound more tightly histone preventing access to transcriptional initiation complex, regulated post-translationally
What does the change to the amino acid attached to tRNA from serine to lysine cause?
No increase/decrease in gene expression, changes the amino acid sequence of the polypeptide, regulated translationally
