Exam 2- Translation Flashcards
What are codons?
A set of three adjacent nucleotides called a triplet
Nucleotide options- A, U, G, or C
Total of 61 possible codons to code for 21 amino acids
Are very specific (unambiguous)
Describe start and stop codons.
Start Codon: AUG
- only codon that codes for methionine
- towards the 5’ end of DNA
Stop Codons: UGA, UAG, UAA
- Only exception in mitochondrial DNA, where UGA codes for Tryptophan
What are the 5’ and 3’ untranslated regions (UTR’s)?
5’UTR: Regulates translational efficiency
3’UTR: Controls mRNA stability. Stabilizing and destabilizing proteins bind to this end
A codon is “universal” and often degenerate. Explain.
Universal: they code for the same amino acid across multiple organisms with few exceptions.
Ex. AUG always codes for methionine
Degenerate: multiple codons can code for the same amino acid
Ex. GUU, GUC, GUA, and GUG all code for Valine
Codons are always specific to an amino acid
Describe Huntington Disease.
Insertion of a CAG codon in tandem repeats in the Huntington gene
Results in Huntington protein with several glutamine residues that accumulates and causes defects
Advanced stage symptoms include uncoordinated, jerky body movements, unsteady gait, mood change, and higher order cognition defect
Missense Mutation
Point mutation in which a single nt change results in a codon that codes for a different amino acid
Ex. Sickle cell anemia: single nt causes Glu to Val resulting in defective beta globulins
Nonsense mutation
A point mutation that results in a premature stop codon
Silent mutation
Mutation in a codon that results in the same amino acid and no observable effect
Ex. UCA (serine) —> UCU (serine)
Addition mutation
Addition of one or more nucleotides to a DNA sequence
Deletion mutation
Deletion of one or more nucleotides in a DNA sequence
Frame shift mutation
Insertion or deletion of 1 or 2 nt’s leading to a reading frame change and may lead to a truncated protein if causes a stop codon somewhere down the line
Note- loss or insertion of 3 does not alter reading frame
Ex. Cystic fibrosis loss of 3 nt’s causes loss of Phe in CFTR protein
Describe the tRNA synthetase catalyzed reaction of aminoacyl-tRNA formation
TRNA synthetase- is an enzyme that attaches the appropriate amino acid onto its tRNA
In initiation, tRNA synthetase charges an amino acid onto tRNA
- amino acid + ATP + amino acyl tRNA synthetase (E) —> E-AMP-amino acid (enzyme-substrate complex)
- E-AMP-amino acid + tRNA —> amninoacyl tRNA bound to amino acid + AMP + E
- carboxyl group of amino acid is now bound to the 3’ end of tRNA!
Describe the subunits of prokaryotic and eukaryotic ribosome complex.
Prokaryotic Ribosome:
50S (contains 5S and 28S RNA and 31 proteins)
30S (contains 16S RNA and 21 proteins)
Total Ribosome = 70S
Eukaryotic Ribosome:
60S large subunit (contains 5S, 5.8S, 28S, and 50 proteins)
- catalyzes formation of peptide bonds that link amino acids
40S small subunit (contains 18S RNA and 30 proteins)
Total Ribosome = 80S
The three sites of ribosome that is involved in translation/protein synthesis
E= Empty tRNA site for exit P= peptidyl tRNA (already extended, attached to a growing chain) A= incoming aminoacyl tRNA binding site
What are polysomes?
Cluster of ribosomes held together by a strand of messenger RNA that each ribosome is translating
Explain the functions of RER mediated protein synthesis.
RER has ribosomes studded on it
RER ribosomes make proteins for ER lumen, Golgi, Lysosomes, and Plasma membrane
Cytosolic ribosomes make proteins for cytosol, nucleus, mitochondria, and peroxisome
Three steps involved in protein synthesis
Initiation, Elongation, Termination
Explain Initiation
- mRNA associates with small ribosomal subunit and requires initiation factors
- charging of amino acid onto tRNA
In prokaryotes, Shine Delgado sequence (purine rich) binds 16S RNA and the whole mRNA aligns in position for translation. Fmet-RNA is the initiating tRNA
In eukaryotes, 5’-Cap is uses as an aligning factor with small subunit. Met-tRNA is the initiating tRNA
Elongation
Requires elongation factors that assist the peptide bond formation between carboxyl group of first aa and amino group of 2nd aa
The 28S RNA is catalytic (ribozyme), peptidyl transferase that catalyze the formation of peptide bonds
Termination
Involves release factors and a stop codon
What are the steps that use GTP and ATP during protein synthesis ?
Cleavage of 4 high energy bonds per 1 amino acid attached to the growing polypeptide chain
2 ATP for aminoacyl tRNA synthesis
1 GTP for aminoacyl tRNA to bind to A site
1 GTP for translocation
And at the end,
1 GTP for termination
Puromycin
Aminoacyl tRNA analog
Inhibits growth of both prokaryotic and eukaryotic proteins
Chloramphenicol
Prokaryotic peptidyl transferase (E that forms peptide bonds between adjacent aa’s using tRNA’s during translation) inhibitor
Tetracycline
Interacts with 30S subunit and blocks access of aminoacyl tRNA to the mRNA ribosome complex
Streptomycin
Interferes with initiation by binding to 30S
Clindamycin
Binds to 50S ribosome and inhibits elongation
Erythromycin
Binds to 50S ribosome and inhibits elongation
Diphtheria GToxxin
Inactivates eukaryotic elongation factor eEF-2 and prevents translocation
_____________ has its own ribosome with a lot of similarities to prokaryotic ribosomes, thus side effects can be seen in some prokaryote targeted drugs due to the cross reactivity with mitochondrial ribosomes
Mitochondria
