Chapter 38: Translation Flashcards
What is responsible for carrying the information from genes to tell ribosomes how to produce polypeptides (proteins) in the cytoplasm
mRNA
What associates with ribosomal proteins to form ribosomes for translation purposes?
rRNA: direct the catalytic steps of protein synthesis (translation)
Make up half the mass of the ribosomes (other 1/2 is proteins)
What transfers AA’s to ribosomes for protein synthesis
tRNA
____________ is a process by which a ___________ reads the sequence of codons in a strand of mRNA and uses the information to produce a polypeptide from amino acids
- Translation
- ribosome
Differences in prokaryotic/eukaryotic ribosomal units
Prokaryotic: 70S
Large: 50S
Small: 30S
Eukaryotic: 80S
Large: 60S
Small: 40S
Each subunit contains proteins and several molecules of rRNA
What is the tRNA region at the 3’ end that is single stranded and binds to the amino acid sequence?
- Amino acid binding site (acceptor stem)
What is tRNA region that contains the sequence of three bases that is complementary to a codon in the mRNA?
***Anticodon loop: BIND TO COMPLIMENTARY CODONS in mRNA
Each type of tRNA contains a different anticodon (and each is carrying a specific amino acid)
***Once the tRNA is bound to an AA, what enzyme reads the tRNA sequence and adds the appropriate AA to the stem and “charging” it? What does this enzyme require?
Aminoacyl‐tRNA syntheses: Requires ATP to add an AA to the 3’ end of tRNA —> makes aminoacyl tRNA
**There are 20 different types in a cell
**What is a single mRNA that is bound and read by multiple ribosomes simultaneously called?
Polyribosome (polysomes)
Steps of translation
- Binding of AA’s to tRNA
- Initiation of translation
- Elongation
- Termination
In translation initiation, what is the initiation complex?
Ribosome + mRNA + tRNA getting together so translation can begin
Transcription initiation:
The initiation complex typically binds to the_________ and moves in the _______ direction to scan to find the start codon. This is aided by ______________, which are proteins that bind to small ribosomal unit and help the initiation complex (ribosome/mRNA/tRNA) to assemble.
- mRNA 5’ cap
- 3’
- initiation factors
transcription initiation:
Before the start codon is found, ______ binds to the small subunit and helps to keep it apart from the large subunit. Once the start codon is found, the _________ joins, forming to initiation complex.
**1. IF3
2. Large subunit
**IF THE RIBOSOMES DO NOT start at the 5’ mRNA cap, they will instead bind directly to a specific nucleotide sequence on the mRNA, what are these sequences in prokaryotes vs eukaryotes?
prokaryotes: Shine-Dalgarno sequence
Eukaryotes: Kozak sequence
Prokaryotes are SaD (Shine-Dalgarno)
What proteins bound to the 3’ poly(A) tail interact with the initiation factors on the 5’ cap to keep mRNA from being degraded?
poly(A) binding proteins (PABP): otherwise, RNA is easily degraded
Translation: Elongation:
Ribosomes have ____ mRNA binding site and ____ tRNA binding sites
- ONE mRNA
- THREE tRNA
Translation: Elongation:
IMPTtRNA sites on the ribosome:
- What site holds the tRNA (anticodon match) that just arrived to the ribosome (exposed codon)?
- What site holds the tRNA that contains the growing polypeptide chain? Where does the peptide bond form? What forms the N-terminus and C-terminus of the polypeptide?
- What site holds the tRNA that is not longer carrying its AA and is about to exit the ribosome?
- A (aminoacyl) site
- P (peptidyl) site: peptide bond forms between A and P site
N-terminus: methionine
C-terminus: other AAs - E (exit) site
What occurs when a stop codon enters the A site of the mRNA? What are the 3 stop codons? What proteins recognize the stop codon and where are they located? What do these proteins do?
- TERMINATION
**2. UAA, UAG, UGA (U Are Annoying, U Go Away, U Are Gone) - release factors: located in P site of the ribosome
- They prevent peptide bond formation and cause the release of the last tRNA
What type of polysomes synthesize membrane and secretory proteins? Where are they synthesized and where do they go after?
- Membrane-bound polysomes (bound to ER): ONLY EUKARYOTES
- ER
- Transported through golgi body to cell membrane or endosome
What type of polysomes synthesize proteins required by organelles, enzymes and structural proteins? Where are they synthesized and where do they go after?
- FREE polysomes (in the cytoplasm)
- Cytoplasm
- Target sites via diffusion through cytoplasm (DO NOT REQUIRE SPECIALIZED TRANSPORT)
Why is translation regulation important?
Important for the cellular response to stressors, growth cues, and differentiation
**How is translation regulated through ribosomal recruitment on the initiation codon?
Ways to regulate ribosome recruitment:
**1. The accessibility of ribosomes to the Shine‐Dalgarno or Kozak sequence (mRNA sequences that ribosome binds to)
**2. Point mutations on the Shine‐Dalgarno or Kozak sequence for initiation factor recognition (no longer recognized by ribosome)
- Altering initiation factor activity (changes in expression levels should be equal amts)
phosphorylation: activates IFs
How is translation regulated through modulation of peptide elongation?
- By phosphorylating threonine 56 on the ribosome —> eEF2, which moves tRNA from A to P site, can no longer bind —> this stops elongation
- Reduction of tRNA pools by altering cellular oxygen supply
**How is translation regulated through termination of protein synthesis?
- By altering termination sequence
UAAU: 80% efficient vs UGAC: 7% efficient - Using noncoding-tRNAs to bind to stop codons instead of release factors (works if they match 2/3 bases within the stop codon —> wobble): can stop translation or cause it to continue
**How is translation regulated through ribosome biogenesis?
Mutations in any step of making ribosomes can lead to issues like ribosomopathy genetic dxs
Ex: bone marrow failure syndromes, cancer, reduced RBCs, muscle wasting
How many codons are in the genetic code? What is the start codon?
64 triplet codons: only make 20 different AAs and 3 nonsense codons (stop codons)
Leucine is encoded by 6 codons
Start codon: AUG
** The degeneracy (redundancy) of the genetic code is what accounts for synonymous mutations —> AKA because more than one codon can make the same AA
**What is a change in the DNA sequence that DOES NOT affect the AA or protein?
Silent mutation
What is a change in the DNA sequence that results in a different AA?
Missense mutation
What is a change in the DNA sequence that gives rise to a STOP codon instead of an AA?
NONsense mutation: gives rise to UGA, UAG, UAA
What is the hypothesis that says the last nucleotide of a codon can often be changed with NO effect on the AA allowing for a little flexibility?
The Wobble hypothesis/effect: the pairing at the 3rd codon position is slightly more relaxed
Aminoglycosides, Erythromycin, and **Chloramphenicol selectively interact with the _____ subunit in prokaryotic (bacterial) and eukaryotic cells? Result?
Interact with the large subunit so it CANT bind to small subunit —> prevents RIBOSOME ASSEMBLY, stopping translation and protein synthesis
70S: bacterial
80S: eukaryotic
Streptomycin and Neomycin irreversibly bind to the bacterial ____ subunit on the ribosome. Effect?
- 30S subunit —> prevent RIBOSOME ASSEMBLY, stopping translation and protein synthesis
Cycloheximide binds the ribosome and inhibits eEF2‐mediated translocation, therefore tRNA cannot move from ______ to ______. What does this block?
- A site to P site
- BLOCKS ELONGATION PHASE of translation
IMPT What occurs when bacteria acquires antibiotic resistance?
Typically the bacteria will cause mutations of the genes for ribosomal proteins or mutations in ribosomal RNA. Therefore, the antibiotics can no longer STOP translation/protein synthesis and the bacteria continue to thrive
**The bacteria will cause a point mutation to occur in the ribosome, stopping the drug from interacting with it. Therefore, translation is NO LONGER inhibited and the bacteria do not die
Antibiotics that inhibit translation and protein synthesis by affecting tRNA
- _________ inhibits peptidyl‐tRNA hydrolysis and peptide bond formation
- ________ blocks the A site on the ribosome, preventing the binding of aminoacyl tRNAs
- ________ causes premature dissociation of the peptidyl‐tRNA from the ribosome
- Blasticidin S: cant break or form peptide bonds
- Tetracycline: charged tRNA quickly pops out of the A site
- Clindamycin: stops the polypeptide chain from continuing to grow
**WHAT antibiotic has a structure like that of the tyrosinyl aminoacyl‐tRNA (tyrosine). It binds to the ribosomal A site (“charged tRNA”) and participates in peptide bond formation to produce peptidyl‐puromycin and stop further translation
Puromycin**
What types of non-coding RNAs are involved in post‐transcriptional regulation via mRNA degradation or translational repression
siRNA: small interfacing RNAs
miRNAs: microRNAs
What type of non-coding RNAs are involved in chromatin modifications, transcriptional regulation or translational regulation?
Long RNAs: double or single stranded
*** What type of non-coding RNA plays an essential role in the RNA
interference pathway (RNAi)? What is this pathway and its importance?
- siRNAs: small interfacing RNAs
- RNAi: a mechanism of GENE SILENCING
*****3. importance: development, maintenance of the genome, defense against viruses, VALUABLE RESEARCH TOOL because it allows the prevention of translation of SPECIFIC GENES
RNAi pathway:
- The RNAi interface pathway is initiated by the enzyme ______ which cleaves double stranded RNA (dsRNA) into fragments
- An ___________ complex is then formed by base pairing between complementary mRNA and 1 of the 2 strands of each new fragment
- RISC formation is followed by degradation of the complementary mRNA by the enzyme __________________, creating “silenced mRNA”
- dicer
- RNA‐induced silencing complex (RISC)
- endonuclease argonaute
MicroRNAs (miRNA) and cancer:
- The RNA can act as a drug to target mutated genes or restore tumor suppressor RNAs (destroys bad genes, fixes the RNA that stops the tumor)
- The RNA can be drugged/targeted to inhibit oncogenic RNA (stops bad RNA from replicating)
- The biogenesis of the offending RNA can be targeted and destroyed (kills bad RNA)
What are chemically modified, single‐stranded nucleic acids designed to specifically bind to and inhibit microRNA (miRNA) molecules? How do they affect miRNA?
- Anti-microRNAs (anti-miRNAs)
- STOPS endogenous miRNA from finding it’s target mRNA
