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The Molecular Biology of Eukaryotes Practice Problems

By — McGraw-Hill Professional
Updated on Aug 23, 2011

Review the following concepts if needed:

The Molecular Biology of Eukaryotes Practice Problems

Practice 1

A single cell, the fertilized egg, is totipotent; i.e., it has the capacity to produce a complete, normal adult individual. Repetitive mitotic divisions convert the zygote into the multicelled organism. During this cellular proliferation, many cells differentiate into types with different morphologies and physiological functions. These differences are associated with the different kinds of proteins made by these cells. For example, the protein hormone insulin is made only by the beta cells in the islets of Langerhans in the pancreas, whereas hemoglobin is made only by erythropoietic cells.

(a)  Explain how different proteins are made by different cell types, given your knowledge of development process regulation.

(b)  Are differentiated cells totipotent? Devise an experiment that might provide a positive answer to this question.

(c)  In an experiment of the kind described in part (b), if the egg nucleus is exposed to ultraviolet light, a positive result might be due to failure of the radiation to destroy the native egg nucleus. Propose an experiment that might prove that this was not the cause of the positive result.

Solution 1

(a)  Different groups of genes are silenced or activated in each cell type, leading to a specific program of gene expression specific to that cell type. Translational regulation of mRNA molecules may also occur.

(b)  Remove (by micropipette) or destroy (e.g., by radiation) the nucleus of a fertilized egg. Then transplant a diploid nucleus from a differentiated cell of the same species into the enucleated egg. If a complete, normal adult organism can develop from such an egg, then development in this species must be totipotent. We cannot generalize these results to all species because different species may not give similar results in such transplant experiments.

(c)  Transplant a conspecific (same species) nucleus containing a genetic marker that differs from that of the recipient individual. If all cells of the resulting adult organism contain only the marker of the transplant, the native egg nucleus must have been destroyed by the ultraviolet light treatment.

Practice 2

The direction in which the shell coils in the snail Limnaea peregra can be dexteral like a righthand screw or sinistral like a left-hand screw. The maternal genotype organizes the cytoplasm of the egg in such a way that embryological cleavage divisions of the zygote will follow either of these two patterns regardless of the genotype of the zygote. If them other has the dominant gene s+, all her progeny will coil dextrally; if she is of genotype ss, all her progeny will coil sinistrally. This coiling pattern persists for the life of the individual. Limnaea is a hermaphroditic snail that can reproduce either by crossing or by self-fertilization. A homozygous dextral snail is fertilized with sperm from a homozygous sinistral snail. The heterozygous F1 undergoes two generations of self-fertilization.     (a)  What are the phenotypes of the parental individuals?     (b)  Diagram the parents, F1, and two selfing generations, showing phenotypes and genotypes and their expected ratios.

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