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CEllular Neuroscience > Translation > Flashcards

Translation Flashcards

1
Q

Where in the cytoplasm does translation occur?

A
  • Translation for proteins occurs on the ribosomes (either as part of the rough ER or free ribosomes).
  • The smooth ER synthesizes lipids, cholesterol and other cell membrane components
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2
Q

Initiation

A
  1. Formation of the 43S ternary ribosomal preinitiation complex
  2. Binding of mRNA to the 43S ribosomal preinitiation complex
  3. Joining of the 48S ribosomal preinitiation complex to the large 60S ribosomal subunit to form the 80S ribosomal complex
  4. The 80S ribosomal complex is what reads the first codon of the mRNA and pairs it with initiator tRNA in the ribosome
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3
Q

What does tRNA do?

A
  • tRNA is bound to amino acids on one end and has complementary sequence to the mRNA on the other.
  • the tRNA acts as a conduit to “translate” mRNA sequence to an amino acid
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4
Q

aminoacyl-tRNA synthetase

A

The enzyme aminoacyl-tRNA synthetase joins tRNA to amino acid before it even enters into the ribosome

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5
Q

elongation receptor sites

A

1) A (acceptor site) – where the amino-acid tRNA enters
2) P (peptidyl site) – where the polypeptide chain is being made
3) E (exit site) – where the “empty” tRNA leaves

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6
Q

mRNA translation to tRNA

A
  • mRNA translation is initiated with the binding of an initiator tRNA to the 80S ribosome
  • The initiator tRNA has a formylmethionine group attached to it
  • An incoming tRNA is delivered to the A site in the complex with elongation factor (EF)-Tu-GTP.
  • This EF factor is hydrolyzed in the ribosome, becoming EF-GDP, and this allows the tRNA-amino acid to bind to the growing chain of amino acids
  • Peptidyl transferase links the amino acid together
  • A separate type of EF factor, EF-G-GTP) is then needed to shift the growing amino acid chain from the A to P site
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7
Q

Termination

A
  • Elongation ends when a stop codon is reached, which initiates the termination reaction that releases the polypeptide. The 80S unit dissociates.
  • This “polypeptide” is the protein
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8
Q

Role of proteins made in the cytoplasm

A
  • Proteins that will have a role in the cytoplasm were made in free ribosomes
  • These proteins will get some post-translational modifications, but otherwise they are ready
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9
Q

Role of proteins made in the rough ER

A
  • Proteins that need to be secreted (some neurotransmitters) or are integrated into the membrane (receptors), were made on ribosomes attached to the endoplasmic reticulum
  • These proteins need further processing (a lot of it) before they are truly ready. Why?
    1. Marked for transport
    2. Proper insertion
      modifications added
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10
Q

Glycosylation

A

function unclear, different addition than with RER pathway

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11
Q

Phosphorylation

A

commonly, ser/thr/tyr can be phosphorylated to expose sites on the protein (e.g. PKC) that allow it to translocate to the membrane

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12
Q

The Secretory Pathway

A

1) Protein made in the ribosomes gets funneled into the ER
2) The ER further modifies the protein and sends it to the Golgi body
3) The Golgi body sorts this protein so it is ready for transport from the soma to the dendrites and/or axon

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13
Q

How is integral mRNA targeted to the rough ER

A
  • The mRNA of integral proteins will have a signal sequence, and when the mRNA is being made into a protein, its has the N-terminus signal sequence.
  • This sequence signals a signal recognition peptide (SRP) to the signal sequence of the protein as it is forming.
  • The SRP+protein+ribosome then bind to a SRP receptor on RER, during this time translation is halted.
  • The signal sequence is removed from the growing protein and translation can now resume within the RER
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14
Q

How does the polypeptide end up on the luminal side of the RER

A
  • The N-terminal signal sequence that was responsible for recruiting the SRP is removed by signal peptidases
  • Glycosylation (i.e. attaching of oligosaccharides) occurs on asparagine residues located on the polypeptide
  • For integral proteins, the arrangement of insertion/halt signals guides orientation of the protein as it forming. This process will determine the orientation of the protein in the plasma membrane.
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15
Q

COPII proteins

A
  • The vesicle has a coating of COPII proteins, which allow it to be targeted to the Golgi complex.
  • The COPII coating is recognized by microtubules, which help transport the vesicle from the RER to Golgi network.
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16
Q

For membrane proteins, the Golgi complex sorts and targets proteins, as well as adding other post-translational modifications.

A
  • Sorting occurs in the cis-Golgi network (CGN) and the trans-Golgi network (TGN). The TGN is where proteins are targeted to the plasma membrane or organelles.
  • Proteins that remain in the RER do so by being retrieved from the CGN because they contain specific sequences that recruit them back to RER
17
Q

how does the vesicle filled with protein actually move though the CGN to the TGN?

A

1) The vesicle fuses and buds off the Golgi cisternae (i.e. the folds), down from the CGN to TGN.
3) Proteins that coat the vesicles are responsible for helping the vesicle fuse and then bud off.

18
Q

budding process

A
  • First, in the cytosol a small GTP-binding protein known as ADP-ribosylation factor (ARF) releases its bound GDP and binds GTP.
  • The resulting ARF-GTP complex then binds to ARF receptors on the Golgi membrane.
  • Next, COP I coatomers bind to ARF and other proteins on the cytosolic face of the Golgi membrane, inducing budding of the transport vesicle.
  • Once the COP I vesicles are released from the donor membrane, the COP I coat depolymerizes (uncoats) and dissociates. Hydrolysis of the GTP bound to ARF proteins in the vesicles triggers this step.
19
Q

fusing process

A
  • Uncoating of transport vehicles exposes specific soluble NSF attachment proteins (SNAP) proteins on the surface of each type of vesicle. Each SNAP specifically binds to cognate SNARE (SNAP receptor) on the membrane of the target vesicle.
  • NSF and α-, β-, and γ-SNAP proteins then bind to the SNARE/SNAP complex, stabilizing the prefusion complex and/or catalyzing dissociation of the complex after vesicle fusion.
  • GTP-binding Rab proteins serve as regulators of vesicle targeting and fusion.
20
Q

how does the protein reach its final destination?

A
  • Once the protein is sorted in the Golgi body, it is released packaged into a vesicle. This vesicle then attaches to the protein transport machinery in order to reach its final destination.
  • Two types of axonal transport: fast transport of the proteins from the secretory pathway and slow transport of cytosolic proteins and cytoskeletal proteins

CEllular Neuroscience (8 decks)

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