Exam 3 Flashcards
Where in a cell does splicing occur?
The nucleus
Where in a cell does transcription occur? (eukaryotes)
The nucleus
Where in a cell does translation occur? (eukaryotes)
The cytoplasm
Definition: Translation
Translating the ‘language’ of nucleic acids into the language of proteins (nucleotides into amino acids)
What is codon usage bias?
Different species prefer different codons for the same amino acid
True or False: The AUG codon for methionine is the most common start codon in both eukaryotes and prokaryotes
TRUE
–> In prokaryotes, a specialized form of methionine is used for the initiating methionine known as N-formyl methionine, though
True or False: The start codon in translation signifies the C terminal of the protein
FALSE: The start codon signifies the N-terminal of the protein
What are the three stop codons?
UAA, UAG, UGA
What is the difference between methionine (Met) and N-formylmethionine (fMet)?
N-formylmethionine has a H-C=O group on its N-group
–> Draw this!
True or False: Despite the slight difference between Met and fMet, eukaryotes can’t recognise fMet as foreign
FALSE: The eukaryotic immune system recognizes proteins with fMet as foreign
Where is fMet used (which types of cells)?
In prokaryotes, mitochondria, and chloroplasts
What establishes the reading frame in translation?
The initiator AUG
Definition: Reading frame
The reading frame is the phase in which 3 nucleotides are read into amino acids. There are 3 potential reading frames and the AUG specifies which frame is used.
Definition: Open reading frame
An open reading frame is the series of codons that come between the initiation AUG and the first Stop codon
What are the three types of point mutations?
Silent, Missense, and Nonsense
Describe a silent mutation
A point mutation that does not end up making a difference in the amino acid that is coded (due to redundancy of the genetic code)
Describe a missense mutation
A point mutation that results in one amino acid being changed
–> Normally not that big of a problem
Describe a nonsense mutation
A point mutation that changes a regular codon to a stop codon, resulting in premature ending of the protein
–> Usually results in a loss of function, may or may not be a big deal
What are tRNAs?
- A class of noncoding RNAs made by RNA Pol III
- Adapters that link the sequence of the RNA to the sequence of the protein
What is at the 3’ end of tRNA?
A single-stranded ‘CCA’ at the 3’-end that links to the amino acid
What is the anticodon of tRNA?
A sequence of 3 nucleotides that can base pair with the codon of the mRNA
How does tRNA fold into its cloverleaf shape?
- By intramolecular base pairing with 4 regions that are double-helical
True or False: tRNAs have about 10% of their bases modified before they leave the nucleus
TRUE
Why are tRNAs chemically modified?
Modifications are crucial for tRNA structure, function, and stability
- Modifications of specific positions affect the behavior of tRNAs during translation
- Some modifications, such as pseudouridines, make the tRNA more rigid whereas others such as dihydrouridines make it more flexible
- Some modifications are essential for ‘charging’ the tRNA with correct amino acid
True or False: Hypermodified tRNAs are targeted for degradation
FALSE: HYPOmodified tRNAs are targeted for degradation
What does Wobble Base Pairing do?
Allows some tRNAs to recognize more than 1 codon because it doesn’t require a perfect match in the 3rd position of the codon
–> i.e. non-standard Watson-Crick pairing
What is the first step of decoding the genetic code? What enzyme does this?
- Linking the correct amino acid to the tRNA is the first step
- This is done by aminoacyl tRNA synthetases
What is an aminoacyl tRNA?
A tRNA with the amino acid attached to it
Describe the bond that forms between the amino acid and the 3’ end of tRNA
- An ester bond forms between the acid of the amino acid and the 3’ hydroxyl of the ribose at the 3’-end of the tRNA.
- This is a high energy bond.
- This energy is used later to link the amino acid to the growing protein chain.
- Because of the high energy bond, an amino acid linked to a tRNA is sometimes said to be ‘activated’
What is an adenylated amino acid?
An amino acid that is prepped to be added to a tRNA
True or False: One ATP equivalent is needed to esterify a tRNA
FALSE: TWO ATP equivalents are needed to esterify a tRNA
Describe the steps taken to attach an amino acid to a tRNA
- The energy of ATP hydrolysis (ATP –> AMP, so 2 phosphates taken off) is used to attach the AA to AMP, forming an adenylated amino acid.
- An ester linkage forms between the adenylated AA and the 3’-OH of the tRNA. This yields the activated AA
How do aminoacyl tRNA synthetases recognize their cognate tRNAs?
Aminoacyl tRNA synthetases recognize their cognate tRNAs by interacting primarily with the anticodon and the acceptor stem
Aminoacyl tRNA synthetases have 2 separate pockets that must both recognize the amino acid
- -> Active site pocket that attaches the AA (preferential binding to correct AA)
- -> Editing (proofing) pocket that exclude the correct AA, while allowing access by closely related AA, if incorrect AA binds while adenylated or bound to tRNA, the bond is hydrolysed
Describe the functions of the proofreading pocket of aminoacyl tRNA
- The correct AA has the highest affinity for the active site pocket of its specific tRNA. The pocket also eliminates most AA that are too large. However, it is not as good at eliminating those that are too small or are closely related (ex: Leu and Ile)
- After the amino acid is esterified to its tRNA, it is switched to a second pocket for editing/proofreading. This involves hydrolysis of the incorrect bond. Editing (proofing) pocket excludes the correct AA, while allowing access by closely related AA, if incorrect AA binds while adenylated or bound to tRNA, the bond is hydrolysed
What are the two enzymatic activities of tRNA synthetases?
i) Synthesis of the amino acid to the tRNA
ii) A proofreading activity
How does the protein grow after tRNA with the proper amino acid attached has bound to mRNA?
- The peptide grows from the NH2 to the COOH terminus
- The peptide bond is formed by the nucleophilic attack of the amino group of the incoming aminoacyl-tRNA on the ester bond of the peptidyl tRNA (tRNA with a growing peptide chain attached to it)
- Energy is provided by the high energy ester bond in the aminoacyl tRNA.
What is the bacterial ribosome made up of? (#S rRNA)
-70S rRNA made up of 50S and 30S subunits
–> 16S rRNA in the 30S subunit can be checked for presence of bacteria
What is a eukaryotic ribosome made up of? (#S rRNA)
-80S rRNA made up of 60S and 40S subunits
–> 18S rRNA in the 40S subunit can be checked for presence of eukaryotic cells
–> Also 28S rRNA comes from the 60S subunit
What are the names of the binding pockets in a ribosome?
A- Aminoacyl-tRNA
P- Peptidyl-tRNA
E- Exit site
What is a ribozyme?
A mix of RNA and protein
–> ribosomes are ribosymes, with 2/3 RNA and 1/3 protein
What do rRNAs do?
- Responsible for ribosome’s overall structure
- Ability to position tRNAs on the mRNA
- Ribosome’s catalytic activity in forming peptide bonds
True or False: Ribosomal proteins bind to the outside of the rRNA mass with protrusions into the rRNA mass and are not direct participants in catalysi
TRUE
Describe the five steps of translation initiation in prokaryotes
1) Small ribosomal subunit binds to the mRNA
2) A special initiator tRNAmet (met-tRNAiMet), a tRNA with Met or Formyl-Met in bacteria covalently bound to it, is brought into the P site
- – AUG is the only codon in for which there are two tRNAs, an initiator Met-tRNAimet and a Met-tRNAmet for internal Met codons.
3) The initiating start codon is recognized
4) The large ribosomal subunit joins the small subunit
5) The tRNA with the 2nd amino acid is brought in to the A site
Describe start codon recognition in prokaryotes
- Requires Shine-Dalgarno sequence
- -> This is because bacteria have no 5’ cap to to direct ribosome binding
- The 3’ end of the 16S rRNA base pairs with the Shine-Dalgarno sequence
- -> The nearest AUG downstream of this binding site is the beginning of translation
True or False: The Shine-Dalgarno sequence is very well conserved
FALSE: It’s really poorly conserved!
What accounts for the polycistronic nature of bacterial mRNAs?
There are multiple Shine-Dalgarno sequences in a bacterial mRNA, meaning there are multiple ribosome binding sites and therefore many different proteins can be made from only one mRNA
What is the advantage of having polycistronic mRNAs in prokaryotes?
Having all components of a metabolic pathway can be regulated together (i.e. lac operon)
–> Also, smaller genome
True or False: The start codon in eukaryotic mRNAs is a part of the consensus sequence (part of the Kozak sequence)
TRUE! The start codon is NOT downstream of the consensus sequence like it is in prokaryotes
True or False: Most eukaryotic mRNAs are polycistronic
FALSE: Most eukaryotic mRNAs are NOT polycistronic (most prokaryotic mRNAs ARE polycistronic though)
Describe eukaryotic translation initiation (basic)
-In eukaryotes, the small ribosomal subunit with met-tRNAimet starts at the 5’-Cap and scans the mRNA for a start codon located within an optimal setting, i.e. with a good Kozak sequence. Thus, 5’-UTRs can vary greatly in length and the first AUG is not necessarily the start site for translation, although it is for ~90% of the mRNAs
Describe eukaryotic translation (in-depth)
- Initially, Met-tRNAimet is loaded onto the small subunit along with initiation factors such as eIF2, which binds GTP to use for energy. eIF2 delivers Met-RNAimet to the P site.
- Of all the tRNAs, only Met-tRNAimet is capable of binding to the small ribosome without the complete ribosome.
- This complex then binds to the 5’end of the mRNA, which is recognized because of the eIF4E & eIF4G proteins bound to the 5’-Cap.
- Notes:
- – The little “e” stands for eukaryotic. Similar initiation factors exist in prokaryotes.
- – The mRNA must have both a Cap and a poly(A) tail to be recognized for translation.
- – There are many more initiation factors.
Describe eukaryotic assembly of the full ribosome
- After the small subunit binds to the mRNA, it scans for the first AUG in a good context, i.e. the Kozak sequence
- eIF2 hydrolyzes GTP when contact is made with the AUG start codon.
- After locating the initiating AUG, eIF2 and other initiation factors dissociate, and the large subunit binds.
- The first aminoacyl-tRNA binds as specified by the codon and the first peptide bond is formed.
What is the first elongation factor to bind in prokaryotic translation?
EF-Tu
EF-Tu (elongation factor thermo unstable) is a prokaryotic elongation factor responsible for catalyzing the binding of an aminoacyl-tRNA (aa-tRNA) to the ribosome. It is a G-protein, and facilitates the selection and binding of an aa-tRNA to the A-site of the ribosome. As a reflection of its crucial role in translation, EF-Tu is one of the most abundant and highly conserved proteins in prokaryotes.
Describe eukaryotic/prokaryotic elongation (in-depth)
- EF-Tu in bacteria and EF1 in eukaryotes deliver aminoacyl tRNAs to the A site.
- The accuracy of the anticodon-codon interaction is verified by proofreading
- When accurate, a new peptide bond is catalyzed by peptidyl transferase (= 28S rRNAin 60S large subunit of eukaryotes, 23S rRNAin 50S large subunit in prokaryotes)
- Large subunit translocation: The large subunit moves and shifts the tRNAs into the E- and P-sites. This is facilitated by EF-G in bacteria and EF-2 in eukaryotes
- Small subunit translocation: The small subunit plus its bound mRNA shift 3 nucleotides over and the A site is available again. This is called indexing.
What does EF-2 do in translation? What does EF-Tu do?
EF2: Involved in indexing
EF-Tu: Involved in bringing in the aminoacyl tRNA
How many ATP equivalents are needed for each peptide bond formation in translation?
Four!
- 2 high energy phosphates for the aminoacyl tRNA synthetase to adenylate the AA
- 1 high energy phosphate - EF-Tu/EF1~GTP -
- 1 high energy phosphate - EF-G/EF2~GTP -
Which of the given statements regarding the initiation of translation in eukaryotes is/are true:
a. The first AUG in the mRNA is the initiator AUG.
b. The large ribosomal subunit is responsible for identifying the initiating AUG
c. The small and large ribosome subunits bind simultaneously to the mRNA to begin translation.
d. The mRNA must have eIF2, eIF4E, eIF4G, and polyA binding proteins bound to it before initiation occurs.
D) The mRNA must have eIF2, eIF4E, eIF4G, and polyA binding proteins bound to it before initiation occurs.
What does protein chain termination require?
-Protein release factors (these are proteins themselves)
–> These bind where there is a stop codon, because no tRNAs recognize stop codons
How does a protein release factor terminate translation?
- The release factors cause hydrolysis of the last peptide bond.
- Hydrolysis of that bond causes dissociation of the ribosomes from the mRNA
- The ribosomes are then free to initiate translation of another mRNA Peptide
True or False: Protein release factors are structurally distinct from tRNAs
FALSE: Protein release factors perform molecular mimicry, and look very similar to tRNAs in structure (also makes sense b/c they need to look like tRNAs in order to get into the ribosome)
What is a polysome/polyribosome?
An mRNA covered with multiple ribosomes is called a polysome or polyribosome
True or False: A Single mRNA May Be Translated Simultaneously by Many Ribosomes
TRUE
–> Called a polysome
True or False: Only prokaryotes use polysomes
FALSE: Both prokaryotes AND eukaryotes use polysomes
–> This dramatically increases the amount of protein made from a single mRNA
What is ribosome recycling?
When a protein is done being made, the ribosome dissociates, but because of the close proximity of the end of the protein and the beginning of the translation site, the ribosome goes right back and attaches at the beginning to synthesize another protein
What is read-through (translation)?
When an amino acid is inserted where a stop codon is
What is programmed translational recoding?
Deliberate alteration of the reading of the mRNA caused by the structure of the mRNA (stem loops, pseudoknots)
Describe programmed translational recoding
- Recoding alters the reading of individual codons and thus alters the colinearity between the mRNA and the protein
- Recoding occurs due to structural elements in the mRNA
- Recoding can occur in up to 80% of the translation of a particular mRNA
- Can have both -1 and +1 frameshifting (or +2) of the ribosome depending on the mRNA. Can get up to 4 different proteins from 1 mRNA!
- -> Often called programmed frameshifting or programmed ribosomal frameshifting.
What amino acid can be created from serine via programmed read-through?
Selenocystiene
–> This insertion requires a specific selenocysteine translation factor
Why is translational frameshifting so important to viruses?
- In HIV and some other viruses, a stop codon is bypassed by a translational frameshift, enabling the virus to make the reverse transcriptase (polymerase) needed for the virus to replicate its genetic content. Without translational frameshifting there would be no HIV AIDS
Would you classify the gag/polymerase mRNA in HIV as being polycistronic or monocistronic? Why?
Tends to be called polycistronic because there are 2 proteins made. However, there is only one RNA, so it could be argued that it is monocistronic
How does tetracycline work?
Blocks binding to the A site in translation
How does streptomycin work?
Prevents transition from the initiation to elongation stage
How does chloramphenocol work?
Blocks peptide bond formation
How does erythromycin work?
Blocks translocation (indexing)
How does puromycin work?
- It Mimics an Amino Acyl tRNA
- Puromycin is a mechanism based inhibitor. It enters the A site, forms a peptide bond, but cannot elongate further because it has an amide where it should have an ester
True or False: In eukaryotes, only the 5’ cap (not the poly-A tail) is needed to initiate translation
FALSE: Both the 5’-Cap and the poly-A tail are required to initiate effective translation
What is nonsense-mediated mRNA decay?
Nonsense-mediated mRNA decay degrades mRNAs with a nonsense (stop) codon in the wrong place, which usually occurs because of improper splicing
What post-transcriptional modifications must mRNA have in order to be transported to the nucleus (eukaryotes obviously)?
- CBC: Cap binding complex
- EJCs: exon junction complexes
- PABPs: poly-A binding proteins
- Nuclear Export Receptor
What must be removed from mRNA before it can be transported to the cytoplasm (eukaryotes)?
Nonsense codon in frame upstream of an exon junction complex
Describe the Steps In Nonsense-Mediate mRNA Deca
- When the 5’-end of the mRNA emerges from the nuclear pore, a ribosome begins a ‘test’ round of translation.
• As the ribosome moves along the mRNA, it displaces the exon junction complexes (EJCs) that are bound to each splice site.
• The normal Stop codon will be in the last exon and there will be no more EJCs, so the mRNA passes inspection and is released to the cytosol for typical translation.
• However, if the ribosome reaches a Stop codon while there are still EJCs, that mRNA is rapidly degraded.
• Thus, the first round of translation checks the ‘fitness’ of the mRNA for translation
When is nonsense-mediated decay triggered?
- Nonsense-mediated decay is triggered when a Stop codon is found before all the EJCs are stripped off.
- -> This is elicited by the binding of Upf proteins to each remaining EJC. However, this mechanism is triggered only when the aberrant Stop codon is in the same reading frame as that of the normal protein
–> Is 5’ exonucleolytic decay (starts at 5’ end)
True or False: Translation requires coordination between the mRNA, tRNAs, and the ribosome.
TRUE
What do newly synthesized proteins require before they are functional?
- Correct folding (conformation)
- Cellular localization (protein trafficking)
- Proteolytic processing
- Modifications
What is Co-Translational Protein Folding?
Proteins Folding into Subdomains as They Are Extruded from the Ribosome
–> This folding is spontaneous as a result of noncovalent thermodynamic forces (hydrogen bonding, electrostatic interactions, van der Waals forces).
How do chaperone proteins work?
- Chaperone proteins reversibly bind to hydrophobic regions of nascent proteins and proteins in intermediate stages of folding.
- Chaperones can stabilize folding intermediates, promote correct folding, maintain proteins in an unfolded state so they can cross membranes, help unfold misfolded segments, prevent misfolding, and prevent protein aggregation.
- -> A big function of chaperones is to unfold proteins that are destind for degradation.
Describe ‘molten globule’ proteins
Each domain of a nascent protein quickly attains a molten globule (loosely folded) state as a result of non-covalent interactions.
Compare and contrast Hsp 60 and Hsp 70
- Both hsp60 and hsp70 have a high affinity for hydrophobic patches on polypeptide chains and both utilize ATP to regulate protein folding.
- However, the hsp60 and 70 proteins function differently. Hsp 70 acts early in the life of many proteins, often before they leave the ribosome, and hsp60 acts after they are released
Describe Hsp70’s mechanism
- Each hsp 70 binds to stretches of 4 – 5 hydrophobic amino acids as the protein exits the ribosome.
- The hydrolysis of ATP helps promote conformational changes
- The binding of a new ATP to hsp70, causes hsp 70 to dissociate from the protein
- This cycle can repeat a number of times.
- -> Occurs while the protein is being expelled from the ribosome
- -> Only binds to hydrophobic regions
What does a chaperonin do?
- Chaperonins sequester proteins to allow them time to refold by forming a “basket” with hydrophobic sides
- Confinement of a protein in the basket gives the protein time to refold without interference from other cellular proteins. Confinement lasts for ~15 seconds
- If the protein isn’t correctly folded the first time, the cycle repeats.
- The cap on the basket requires ATP
What is a proteasome?
A protease complex that degrades misfolded proteins or other proteins that are targeted for destruction by ubiquitinatio
Describe the structure of a 26S proteasome
- 19S Regulatory ‘cap’. Acts as a gate. Recognizes the protein to be degraded and threads it through this cap, which unfolds the protein so it can be digested.
- -> Where the hydrolase is
- 20S Catalytic core contains four stacks of heptamericrings with protease activity
- -> Proteases face the inside so they can’t degrade all proteins in the cell!
True or False: Proteasomes are only present in the cytoplasm of the cell
FALSE: Proteasomes are present in the cytoplasm AND the nucleus of cells (if it’s a nucleated cell)
True or False: Proteases are processive
TRUE: They clip proteins every 6 to 7 amino acids
What is ubiquitin?
Ubiquitin is a 76 amino acid protein that is highly conserved among eukaryotes
What is an isopeptide bond?
An isopeptide bond is an amide bond that is not present in the main chain of a protein. The bond forms between the carboxyl terminus of one protein (ubiquitin) and the amino group of a lysine residue on another (target) protein
When is a protein recognized by the 19S cap of the proteasome (for degradation)?
When the linkage is between Lys48 of one ubiquitin and Gly76 of another ubiquitin and when there are at least 4 ubiquitins attached, the target protein is recognized by the ubiquitin receptor in the 19S Cap of the proteasome.
What amino acid is always at the C-terminal end of a protein that is set to be degraded?
Glycine
Name the three enzyme families required for ubiquitination and their functions
- E1: ubiquitin-activating enzymes. Create an activated E1-bound ubiquitin through a high energy thioester link
- E2: ubiquitin-conjugating enzymes. Accept the ubiquitin from E1.
- E3: ubiquitin ligases or E3 ligases. E3 ligases recognize the degradation signal (degron) in the protein to be ubiquitinated. E3 ligases thus confer the specificity.
True or False: E1 (activating enzyme) is the most specific of the ubiquitination enzymes
FALSE: E3 is the most specific (have over 600 different genes coding for them)
What is regulated degradation/regulated proteolysis?
Some types of E3 ubiquitin ligases regulate the mass of critical cellular proteins during different phases of the cell cycle (cyclins), development, etc
-> A type of degradation by the proteasome
List the three contexts that the Ubiquitin-Proteasome System Degrades Proteins
- Degradation of misfolded proteins
- Regulated gene expression by controlled proteolysis (ex. Control of cell cycle proteins)
- Hydrolysis of foreign proteins for production of antigens in the process of generating antibodies
- -> For all of these, the specificity is primarily provided by the E3 ligases.
In what two ways is regulated degradation by a proteasome induced?
1) Activation of a ubiquitin ligase complex
2) Activation of a degradation signal on a protein
Give a summary of correct protein folding
- Proteins begin folding co-translationally into their final conformation.
- Molecular chaperones such as Hsp70 and Hsp60 assist in protein folding.
- The ubiquitin ligase system marks incorrectly folded proteins and proteins whose mass is ‘regulated’ for degradation by the proteasome by polyubiquitinating the target protein.
- Other types of ubiquitination serve other purposes in the cell such as regulation of gene expression
Describe how translation is down-regulated in the presence of an infection or stress
- This is largely accomplished by the phosphorylation of eIF2 by protein kinases that are activated in response to the stress.
–> Reminder: when eIF2 is bound in a complex with GTP, it mediates the binding of met-tRNAi to the small ribosomal subunit. This is the first step in the initiation of translation, so the perfect step to regulate
What does Exchange of GDP for GTP on eIF2 Require?
- Exchange of GDP for GTP on eIF2 Requires a Guanine Nucleotide Exchange Factor (GEF)
- GDP and GTP are allosteric modulators of eIF2, and eIF2 is only active when GTP is bound to it. GDP binds very tightly to eIF2, so eIF2B, an accessory protein known as a guanine nucleotide exchange factor(GEF), is needed to get GDP off so that GTP can bind and activate eIF2.
What happens when When eIF2 Is Phosphorylated?
- It Binds Too Tightly to eIF2B So That eIF2 Is Not Released and Cannot Bind GTP
- Stress induces various protein kinases that can phosphorylate eIF2.
- This mechanism is part of what puts cells into Go as protein synthesis decreases to about 20% of that in proliferating cells.
True or False: ~90% or eukaryotic mRNAs are translated beginning with the first AUG downstream from the 5’ Cap
TRUE
For the eukaryotic genes that are not translated beginning with the first AUG downstream from the 5’ cap, where does translation begin?
- Translation of many of the remaining mRNAs begins at internal ribosome entry sites (IREs).
- -> IREs are specialized RNA sequences of several hundred nucleotides long that fold into specific structures that bind many of the proteins that are used to initiate normal 5’ Capdependent translation.
- -> This unusual structure allows the translational machinery to bypass the 5’ Cap structure.
- -> Used primarily by viruses or during stress. It allows for translation under conditions where 5’ Cap-dependent translation is turned off or reduced. Helps the cell cope with stress, and it helps virus proteins get translated preferentially.
Compare the Initiation of Translation Using the 5’ Cap vs. IREs
- IRE-initiated translation requires only a subset of the normal initiation factors and those assemble directly on the IRE rather than the Cap.
- Some viruses produce a protease upon infection that cleaves eIF4G so it will no longer bind to eIF4E. This halts host translation. However, the clipped eIF4G will bind to IREs in the viral mRNAs, so viral proteins are preferentially made.
How can bacterium to adapt to rapid environmental changes?
Bacterial mRNAs typically have a very short half-life, usually less than ~ 2 min.
What is the function of an exonuclease in mRNA?
Exonucleases degrade mRNA from the 3’- to 5’direction, are usually responsible for mRNA degradation in bacteria
What is responsible for mRNA degradation in eukaryotes?
- Most eukaryotic mRNAs are degraded from the 3’-end by a deadenylase and from the 5’-end after decapping by a 5’- to 3’-exonuclease.
– Some mRNAs are internally cleaved by endonucleases. The stability of these mRNAs is specifically regulated by sequences in the mRNA
True or False: Degradation of Eukaryotic mRNAs Is Often Co-Translational
TRUE
–> As an mRNA is being translated, a deadenylase associates with the poly-A tail, shortening it over time. Once the tail is shortened to about 25 A’s, then the mRNA is decapped and exonucleases start 5’-to-3’ and 3’-to-5’ degradation
What are the two mechanisms of mRNA degradation in eukaryotes?
- Mechanism 1: The loss of the Cap leads to rapid 5’-to-3’ degradation.
- Mechanism 2: After the poly-A tail is reduced to about 25 A’s, there is rapid 3’-to-5’ degradation.
- -> Mechanisms 1 and 2 can occur on the same mRNA at the same time.
Describe iron transport and storage in eukaryotic cells
- Ferritin and the transferrin receptor are largely responsible for regulating iron levels in our bodies.
- Ferritin binds iron within cells, stores it in the cells, and releases it as needed.
- Transferrin carries iron in serum and when more iron is needed in cells, the transferrin receptor takes up transferrin + Fe3+ (ferric ion) and thus brings iron inside.