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.
Solution 2
(a) Although we know the genotypes of the parents, we have no information concerning the genotype of the immediate maternal ancestor that was responsible for the organization of the egg cytoplasm from which our parental individuals developed. Therefore, we are unable to determine what phenotypes these individuals exhibit. Let us assume for the purpose of diagramming part (b) that the maternal parent is dextral and the paternal parent is sinistral.
(b) Let D = dextrally organized cytoplasm; S = sinistrally organized cytoplasm.
Practice 3
Slow-growing yeast cells called neutral petites lack normal activity of the respiratory enzyme cytochrome oxidase associated with the mitochrondria. Petites can be maintained indefinitely in vegetative cultures through budding, but can sporulate only if crossed to wild type. When a haploid neutral petite cell fuses with a haploid wild-type cell of opposite mating type, a fertile wild-type diploid cell is produced. Under appropriate conditions, the diploid cell reproduces sexually (sporulates). The four ascospores of the ascus (Fig. 6-4) germinate into cells with a 1 : 1 mating type ratio (as expected for nuclear genes), but they are all wild type. The petite trait never appears again, even after repeated backcrossings of both mating types to petite. The mitochondrial factors for petite are able to perpetuate themselves vegetatively, but are "swamped," lost, or permanently altered in the presence of wild-type factors. Neutral petite behaves the same in reciprocal crosses regardless of mating type. Assume that a neutral petite yeast has the chromosomal genes for normally functioning mitochondria, but has structurally defective mitochondria. Another kind of yeast is known, called segregational petite, which has structurally normal mitochondria that cannot function because of inhibition due to a recessive mutant chromosomal gene. What results would be expected among the sexual progeny when the neutral petite crosses with the segregational petite?
Solution 3
The diploid zygote receives from the segregational petite parent structurally normal mitochondria that should be able to function normally in the presence of the dominant nuclear gene from the neutral petite parent. Sporulation would probably distribute at least some structurally normal mitochondria to each ascospore. The nuclear genes would segregate 1 normal : 1 segregational petite. Let shaded cytoplasm contain defective mitochondria.
Practice 4
A condition called "poky" in Neurospora is characterized by slow growth due to an abnormal respiratory enzyme system similar to that of petite yeast. The poky trait is transmitted through the maternal (protoperithecial) parent. A chromosomal gene F interacts with poky cytoplasm to produce a faster growing culture called "fast-poky," even though the enzyme system is still abnormal. Poky cytoplasm is not permanently modified by transient contact with an F genotype in the zygote. It returns to the poky state when the genotype bears the alternative allele F'. Gene F has no phenotypic expression in the presence of a normal cytoplasm. If the maternal parent is fast-poky and the paternal (conidial) parent is normal, predict the genotypes and phenotypes of the resulting ascospores.
Solution 4
Let shaded cytoplasm contain poky mitochondria. The chromosomal alleles segregate in a 1 : 1 ratio, but poky cytoplasm follows the maternal (protoperithecial) line. The recovery of poky progeny indicates that poky cytoplasm has not been altered by its exposure to the F gene in the diploid zygotic stage.
Practice 5
Commercial corn results from a "double cross." Starting with four inbred lines (A, B, C, D), a single cross is made between A and B by growing the two lines together and removing the tassels from line A so that A cannot self-fertilize, and thus receives only B pollen. In another locality the same procedure is followed for lines C and D. The yield of single-cross hybrid seed is usually low because the inbred parent lacks vigor and produces small cobs. Plants that germinate from single-cross seed are usually vigorous hybrids with large cobs and many kernels. It is undesirable for the single-cross hybrid to self-fertilize, as this inbreeding process commonly produces less vigorous progeny. Therefore, a double cross is made by using only pollen from the CD hybrid on the AB hybrid. Detasseling is a laborious and expensive process. A cytoplasmic factor that prevents the production of pollen (male-sterile) is known. There also exists a dominant nuclear gene R that can restore fertility in a plant with male-sterile cytoplasm. Propose a method for eliminating hand detasseling in the production of double-cross hybrid commercial seed.
Solution 5
Let S = male sterile cytoplasm; F = male fertile cytoplasm. The ABCD double-cross hybrid seed develops on the large ears of the vigorous AB hybrid. When these seeds are planted they will grow into plants, half of which carry the gene for restoring fertility so that ample pollen will be shed to fertilize every plant.
View Full Article
From Schaum's Outlines of Genetics. Copyright © 2010 by The McGraw-Hill Companies. All Rights Reserved.
Celebrate Memorial Day! Worksheets and Activities About American History 
May Workbooks are Here!
Get Outside! 10 Playful Activities
7 Parenting Tips to Take the Pressure Off 
Add your own comment