Ch. 12: Mitochondria, Chloroplasts & Peroxisomes Online Quiz (Essay) Flashcards
Mitochondrial mRNAs have short poly-A sequences at their 3′ end. Poly-A is generally considered to be a feature of eukaryotic, not bacterial, mRNA. How can this observation be reconciled with an endosymbiotic origin for mitochondria?
The mitochondria of today are ___ the mitochondria of endosymbiotic origin. The mitochondrial genome today shows a mix of bacterial and eukaryotic traits.
NOT
Suppose that a student you are tutoring comes to you carrying two centrifuge tubes, each with a pellet at the bottom. She knows that one tube contains a pellet of mitochondria and the other a pellet of chloroplasts, but she cannot remember which is which. How would you help her?
The chloroplast pellet will be ___ (due to chlorophyl), while the mitochondria pellet will be colorless.
green
Presumably the original chloroplast genome coded for many more proteins than it does today. Some of these genes must have been transferred to the nucleus, with the resulting protein product imported into chloroplasts. Other genes must have originated in the nucleus and were then adapted for import into chloroplasts. In general, protein adaptations must have occurred for protein import into chloroplasts. What were these likely adaptations, and how could one tell whether the protein originated from a chloroplast or nuclear-coded gene?
In general, cytosolically synthesized and unclearly coded protein must have an ___ transit peptide that can be recognized by the chloroplast guidance complex. If it does not, the polypeptide will not be guided toward the translocon located in the outer chloroplast membrane. This sequence would need to be added by some kind of gene fusion. Machinery for recognition and translocation would need to be adapted in the outer chloroplast membrane. Homology comparisons could be done to determine whether the protein is more similar to a bacterial or a eukaryotic protein. If it is more similar to a bacterial protein, it presumably originated in the ___, irrespective of where the coding sequence resides today.
N-terminal, chloroplast
Zellweger syndrome is caused by defects in genes coding for peroxisomal protein import. Why are defects in such genes more likely to be lethal than a defect in a gene encoding a single enzyme present in the peroxisomal lumen?
A defect in import machinery affects the import of ___ proteins into the lumen of the peroxisome. A mutation in a single peroxisomal enzyme affects only that enzyme. Therefore, the import machinery mutation should have a greater effect and likelihood of being lethal.
multiple
All plant plastids contain the same genome as chloroplasts. However, chromoplasts, amyloplasts, and elaioplasts are clearly different from one another. How can these differences between plastids that all have the same internal genes be explained mechanistically?
All plastid development is under the coordinate control of genes within ___ the plastid and the nuclear genomes.
both
Explain the role of endosymbiosis in the evolution of mitochondria and chloroplasts.
Because of the prokaryotic nature of the organelle and the striking similarity between the genomes of mitochondria and some bacteria (most notably Rickettsia prowazekii), it has been hypothesized that mitochondria evolved from an ___ event in which a bacterium was endocytosed by a eukaryotic cell.
endocytic
What is the similar shared function of mitochondrial matrix processing peptidase (MPP) and chloroplast stromal processing peptidase (SPP)?
They are both peptidases that cleave ___ amino acid sequences that target polypeptides and proteins to the Tom and Toc complexes in mitochondria and chloroplast outer membrane and subsequently to the Tim and Tic complexes of the inner membranes.
N-terminal
What are “porins”?
Porins are transmembrane proteins that form large pores and are found in chloroplast and mitochondrial ___ ___, as well as in the outer membranes of Gram-negative bacteria.
outer membranes
At low pH, the chemical 2,4-dinitrophenol (DNP) is neutral and can diffuse freely across membranes, including those of mitochondria. At high pH, it gives off a proton, becomes negatively charged, and can no longer diffuse across membranes. What effect would DNP have on the proton gradient between the mitochondrial intermembrane space and matrix?
DNP would have the effect of dissipating the proton gradient across the inner membrane. It would be neutral at pH 7 in the intermembrane space and thus pass freely into the mitochondrial lumen, where it would encounter a high pH environment and release its proton. This would ___ the proton gradient and drastically decrease the overall ATP production of the cell as well.
neutralize
