RNA Modification and Protein Synthesis Flashcards
Generally, what posttranscriptional modifications are made to prokaryotic mRNA?
None
What enzymes transcribe rRNA in eukaryotes?
Mostly RNA Pol I but 5S rRNA by RNA Pol III
What enzymes act on pre-rRNA to yield final rRNA species?
Pre-rRNA is cleaved by a ribonuclease, and exonucleases produce final products
Where does eukaryotic rRNA synthesis/processing occur?
Nucleolus
What is the action of Rnase P?
It is an exonuclease that removes a 16 nucleotide sequences at the 5’ end of tRNA
What sequence is post-transcriptionally added to the 3’ end of tRNA that forms the acylation site?
CCA
What is the first modification step that happens to mRNA as it is produced?
5’ capping
What are the functions of 5’ capping?
Stabilizes mRNA and permits efficient initiation of translation; helps ribosome recognize and place mRNA
What is the first step of 5’ capping?
The gamma phosphate is removed
What molecule is first added to the 5’ end of the mRNA transcript in 5’ capping? By what enzyme? Through what kind of linkage?
GMP by guanylyl transferase via a 5’ to 5’ linkage
What is the final step of 5’ capping? What enzyme catalyzes this step?
Methylation of the guanine at the 7 position by guanine-7-methyltransferase
Where does 5’ capping occur?
Nucleus
What are the three funtions of polyadenylation?
Stabilizes mRNA, promotes exit from the nucleus, aids in translation
What enzyme catalyzes polyadenylation?
Polyadenylate Polymerase
Through what does Polyadenylate Polymerase bind to DNA?
The polyadenylation sequence near the 3’ end
What complex accomplishes mRNA splicing?
Spliceosome
What is at the 5’ end of an intron?
The splice donor site - GU here
What is at the 3’ end of an intron?
Splice acceptor site- AG here
What base is found at the branch site of an intron?
A
What is an snRNP? What are they made of?
small nuclear ribonucleoprotein complexes; snRNA
How do snRNAs facilitate intron removal?
By base pairing with consensus sequences at the exon/intron boundary
What chemistry results in the formation of the lariat structure in mRNA splicing?
The 2’ hydroxyl group of the A at the branch site attacks the phosphate of the G at the 5’ end of the intron forming a 2’-5’ phosphodiester bond
Approximately what percentage of genetic diseases result from splice site mutations?
15%
What disorder results from incorrect splicing of the beta globin gene?
Beta-thalassemia
What disease is an autoimmune response to one’s own snRNPs?
Systemic lupus erythematosus
What is the process of using different splice sites within the same gene to generate different proteins?
Alternative Splicing
What is the conventional direction for writing codon sequences?
5’–> 3’
How many codons are possible?
64
How many potential reading frames can each mRNA transcript have?
3
What is the intiation codon? What does it code for?
AUG; methionine
What are the 3 termination codons?
UAG, UGA, UAA
What are the four characteristics of the genetic code?
Specificity; universality; degeneracy; nonoverlapping and commaless
What is a missense mutation?
A single base pair mutation that results in the changing of the codon for one AA for that of a different AA
What is a nonsense mutation?
A single base pair mutation that results in the changing of the codon for one AA for that of a stop codon
What is a silent mutation?
A single base pair mutation that results in a codon that codes for the same AA
What is a frameshift mutation?
A single or double base pair insertion or deletion that results in a change in the reading frame?
What is the process by which a sequence of 3 bases that are noramlly repeated in tandem become amplified due to mistakes in DNA synthesis?
Triplet Repeat Expansion
What repeat expansion causes Huntington’s
CAG
How many tRNAs are there in humans?
50
What enzyme covalently attaches the AA to its respective tRNA?
Aminoacyl-tRNA synthetases
How many aminoacyl-tRNA synthetases are there?
20
What ribosomal subunit binds mRNA and ensures correct mRNA-tRNA base pairing?
Small Subunit
What ribosomal subunit catalyzes peptide bond formation?
Large subunit
What are the three ribosomal sites?
A (acceptor), P (peptidyl transferase), E (exit)
What is the energy cost of the addition of a single AA in protein synthesis?
2 ATP and 2 GTP
What is the directional relatioship of a codon and anticodon?
Antiparallel
What proposes that the last base of a codon is capable of non-tradiitional base pairing?
Wobble Hypothesis
In what direction are protein peptides syntheisezed?
Amino- to- carboxy terminal direction
What molecules assist in regulating and facilitating the formation of the full translational machinery?
Initiation factors
What prokaryotic mRNA sequence base pairs with the 16S rRNA in translation?
Shine-Delgarno sequence
What is the only aminoacyl tRNA to be inserted directly into the P site?
initiator met-tRNA
What is the chemistry of the peptide bond formation during translation elongation?
The carboxyl group of the polypeptide attacks the amino group of the AA-tRNA
What molecules recognize the stop codons and promote the hydrolysis of the bond between the protein and the final tRNA?
Release factors
What is a polysome?
More than one ribosome translating one mRNA transcript at a time
What is the mechanism of streptomycin?
Binds to 30S subunit, distorting it
What is the mechanism of tetracyclines?
Binds to 30S subunit, blocks entry of AA-tRNA to A site
What is the mechanism of chloramphenicol?
Inhibits peptidyltransferase
What is the mechanism of clindamycin/ erthromycin?
Binds to 50S subunit, blocks translocation
What is the mechanism of diptheria toxin?
Inactivates elongation factor, blocks translocation
What is the mechanism of cyclohexamide?
May bind to E site and block translocation
What is the mechanism of puromycin?
AA-tRNA analog, causes peptide chain termination
