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TLS Online TPP Program

#Question id: 2858

When lactose enters the cell, it is converted to allolactose. It is allolactose (rather than lactose itself) that controls the Lac repressor. Paradoxically, the conversion of lactose to allolactose is catalyzed by b-galactosidase, itself encoded by one of the lac genes. How is this possible? Which statement is incorrect according lac genes expressions?

#Unit 2. Cellular Organization

  1. The expression of the lac genes is leaky, even when they are repressed, because, allolactose binds to a part of the Lac repressor distinct from its DNA-binding domain.

  2. In the absence of allolactose, the repressor is present in a form that binds its site on DNA (and so keeps the lac genes switched off).

  3. Allolactose has altered the shape of the repressor, the protein can no longer bind DNA, and so the lac genes are no longer repressed.

  4. This leakiness ensures that there is a low level of b-galactosidase in the cell even in the absence of glucose, and so there is enzyme poised to catalyze the conversion of lactose to allolactose.

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TLS Online TPP Program

#Question id: 11160

#Unit 9. Diversity of Life Forms

A phylogenetic tree constructed using sequence differences in mitochondrial DNA would be most valid for discerning the evolutionary relatedness of
TLS Online TPP Program

#Question id: 11161

#Unit 9. Diversity of Life Forms

The lakes of northern Minnesota are home to many similar species of damselflies of the genus Enallagma that have apparently undergone speciation from ancestral stock since the last glacial retreat about 10,000 years ago. Sequencing which of the following would probably be most useful in sorting out evolutionary relationships among these closely related species?
TLS Online TPP Program

#Question id: 11162

#Unit 9. Diversity of Life Forms

Which statement represents the best explanation for the observation that the nuclear DNA of wolves and domestic dogs has a very high degree of homology?
TLS Online TPP Program

#Question id: 11163

#Unit 9. Diversity of Life Forms

The reason that paralogous genes can diverge from each other within the same gene pool, whereas orthologous genes diverge only after gene pools are isolated from each other, is that
TLS Online TPP Program

#Question id: 11164

#Unit 2. Cellular Organization

Cellular membranes are self-sealing— if they are punctured or disrupted mechanically, they quickly and automatically reseal. What properties of membranes are responsible for this important feature?
TLS Online TPP Program

#Question id: 11165

#Unit 2. Cellular Organization

Membrane lipids in tissue samples obtained from different parts of a reindeer’s leg have different fatty acid compositions. Membrane lipids from tissue near the hooves contain a larger proportion of unsaturated fatty acids than those from tissue in the upper leg. What is the significance of this observation?