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What essential role does the root endodermis play during mineral absorption in plants?

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Like all cells, the endodermal cells have many transport proteins embedded in their plasma membrane; they let some solutes cross the membrane, but not others. Transport proteins of endodermal cells are control points, where a plant adjusts the quantity and types of solutes that reach the xylem. Because of the layer of suberin, the root endodermis has the ability to actively transport ions in one direction only.
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Figure shows the major structural elements of a chromosome of baker's yeast (Saccharomyces cerevi-siae ). Heiter, Mann, Snyder, and Davis (1985) determined the properties of some of these elements. They based their study on the finding that in yeast cells, plasmids (which have genes and an origin of replication) act differently from chromosomes (which have these elements plus centromeres and telomeres) during mitosis. The plasmids are not manipulated by the mitotic apparatus and segregate randomly between daughter cells. Without a selectable marker to force the host cells to retain them (see Fig. ), these plasmids are rapidly lost. In contrast, chromosomes, even without a selectable marker, are manipulated by the mitotic apparatus and are lost at a very low rate (about per cell division). Heiter and colleagues set out to determine the important components of yeast chromosomes by constructing plasmids with various parts of chromosomes and observing whether these "synthetic chromosomes" segregated properly during mitosis. To measure the rates of different types of failed chromosome segregation, the researchers needed a rapid assay to determine the number of copies of synthetic chromosomes present in different cells. This assay took advantage of the fact that wild-type yeast colonies are white whereas certain adenine-requiring (ade ) mutants yield red colonies on nutrient media. Specifically, ade cells lack functional AIR carboxylase (the enzyme of step 6a in Figure ) and accumulate AIR (5-aminoimidazole ribonucleotide in their cytoplasm. This excess AIR is converted to a conspicuous red pigment. The other part of the assay involved the gene , which encodes an ochre suppressor (a type of nonsense suppressor; see Box ) that suppresses the phenotype of some ade mutants. Heiter and coworkers started with a diploid strain of yeast homozygous for ade these cells are red. When the mutant cells contain one copy of , the metabolic defect is partly suppressed and the cells are pink. When the cells contain two or more copies of , the defect is completely suppressed and the cells are white. The researchers inserted one copy of SUP11 into synthetic chromosomes containing various elements thought to be important in chromosome function, and then observed how well these chromosomes were passed from one generation to the next. These pink cells were plated on nonselective media, and the behavior of the synthetic chromosomes was observed. Specifically, Heiter and coworkers looked for colonies in which the synthetic chromosomes segregated improperly at the first division after plating, giving rise to a colony that is half one genotype and half the other. Because yeast cells are nonmotile, this will be a sectored colony, with one half one color and the other half another color.(a) One way for the mitotic process to fail is nondisjunction: the chromosome replicates but the sister chromatids failto separate, so both copies of the chromosome end up in the same daughter cell. Explain how nondisjunction of the synthetic chromosome would give rise to a colony that is half red and half white.(b) Another way for the mitotic process to fail is chromosome loss: the chromosome does not enter the daughter nucleus or is not replicated. Explain how loss of the synthetic chromosome would give rise to a colony that is half red and half pink. By counting the frequency of the different colony types, Heiter and colleagues could estimate the frequency of these aberrant mitotic events with different types of synthetic chromosome. First, they explored the requirement for centromeric sequences by constructing synthetic chromosomes with different-sized DNA fragments containing a known centromere. Their results are shown below.(c) Based on these data, what can you conclude about the size of the centromere required for normal mitotic segregation? Explain your reasoning.(d) Interestingly, all the synthetic chromosomes created in these experiments were circular and lacked telomeres. Explain how they could be replicated more-or-less properly. Heiter and colleagues next constructed a series of linear synthetic chromosomes that included the functional centromeric sequence and telomeres, and measured the total mitotic error frequency as a function of size:(e) Based on these data, what can you conclude about the chromosome size required for normal mitotic segregation? Explain your reasoning.(f) Normal yeast chromosomes are linear, range from to in length, and have a mitotic error rate of about per cell division. Extrapolating the results from (e), do the centromeric and telomeric sequences used in these experiments explain the mitotic stability of normal yeast chromosomes, or must other elements be involved? Explain your reasoning. (Hint: A plot of log (error rate) vs. length will be helpful.)
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Question Text
What essential role does the root endodermis play during mineral absorption in plants?
TopicTransport in Plants
SubjectBiology
ClassClass 11
Answer TypeText solution:1
Upvotes16