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True or False:
the nuclear envelope and the ER are continuous.
true
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How are the ribosomes arranged on the ER (think of the diagram)?
Where are they located?
- in chains held together by mRNA, which they are translating
- -called polyribosomes, or polysomes
- ribosomes are on the outside
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Explain the steps of cell fractionation.
- 1) remove tissue. mince with blade into small pieces. suspend in cold isotonic homogenization medium.
- 2) homogenize the tissue fragment
- 3) Isolate subcellular component from homogenate by differential centrifugation.
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What is homogenate?
suspension of material from broken cells
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In centrifugation, what are you going to use to crush it?
a pestle
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At the first centrifugation, what are you going to have?
very large particles, such as nuclei adn unbroken cells
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What will you take after the first centrifugation?
What will you get?
- the postnuclear supernatant
- pellet (with large particles like mitochondria)
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At the last centrifugation, what will you get?
- pellet containing microsomes (small fragments of membrane as well as free ribosomes)
- - some microsomes will be from the SER and RER
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What are microsomes?
How will you tell the microsomes of the SER from those of the RER?
- fragments of the ER
- they look like little vesicles because membranes have no free edges
- use density gradient because RER is heavier
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Homogenization and initial fractionation steps can be done by ? Further separation can be done by __.
- differential centrifugation
- density gradient centrifugation
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Explain density gradient centrifugation.
developed in the mid-1950s to separate DNA strands stained with different heavy metals
layer less dense layers one after the other
they will sink until they find their proper densities
a fairly fine gradient is needed to separate it
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In the sedimentation steps of differential centrifugation, the particles will continue to do what as long as what and why?
- sediment
- subjected to centrifugation because the medium is less dense than the particles
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In density gradient centrifugation, the medium is composed of a __ and the particles move until they what?
- density gradient
- they reach a place in the tube equal to their own density, where they form bands
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What are the proteins synthesized by ribosomes bound to the RER destined for?
- Secretory proteins (ex. peptide hormones)
- other proteins released into the eR lume (Rough ER enzymes)
- Integral membrane proteins (glycoproteins)
- glycolipid- anchored plasma membrane proteins
- peripheral proteinis of the plasma membrane
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What are the proteins synthesized by ribosomes in the cytosol destined for?
- soluble cytosol proteins (enzymes of glycolysis)
- lipid-anchored membrane proteins (cytoplasmic face)
- peripheral proteins of the plasma membrane (cytosolic face)
- mito and chloroplasts proteins
- peroxisomes
- nuclear proitens
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How does a cell know where its destined to be?
surface receptors on the the ER that respond to a particular amino acid sequence
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