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Inquiry Questions

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Self Test

1). Humans and pufferfish have a similar number of genes, yet the human genome is approximately nine times larger than the pufferfish genome. In what form is much of this extra DNA?
    a). introns
    b). exons
    c). retrotransposons
    d). RNA
Answer: a

2). Genome comparisons have suggested that mouse DNA has mutated about twice as fast as human DNA. What is a possible explanation for this discrepancy?
    a). Mice are much smaller than humans.
    b). Mice live in much less sanitary conditions than humans and are therefore exposed to a wider range of mutation-causing substances.
    c). Mice have a smaller genome size.
    d). Mice have a much shorter generation time.
Answer: d

3). How many pairs of chromosomes do chimpanzees carry?
    a). 23
    b). 46
    c). 24
    d). 48
Answer: c

4). Why was the genome of the protist P. falciparum difficult to sequence?
    a). This protist has a large genome.
    b). This organism hides inside red blood cells, making it difficult to obtain enough DNA for the sequencing project.
    c). The apicoplast structures in this protist inhibit sequencing reactions.
    d). The genome contains a high proportion of adenine and thymine.
Answer: d

5). All of the following are believed to contribute to genomic diversity among various species, except
    a). gene duplication.
    b). gene transcription.
    c). lateral gene transfer.
    d). chromosomal rearrangements.
Answer: b

6). What is the fate of most duplicated genes?
    a). gene inactivation.
    b). gain of a novel function through subsequent mutation
    c). they are transferred to a new organism using lateral gene transfer
    d). they become orthologs
Answer: a

7). Which of the following best describes pseudogenes?
    a). two functional genes within an organism that arose from the duplication of one gene
    b). genes that share the same ancestral sequence, but are found in different organisms
    c). sequences of DNA that are very similar to functional genes, but do not produce a functional product
    d). sequences of DNA that are very similar to inactive genes, but do produce a functional product
Answer: c

8). The Tbx5 gene is known to play a role in which process?
    a). notochord development
    b). limb formation
    c). eye formation
    d). sexual reproduction
Answer: b

9). Which of the following organisms is not considered to be a model genetic system?
    a). mice
    b). fruit flies
    c). humans
    d). yeast
Answer: c

10). Which of the following statements about Pax6 is false?
    a). Pax6 has a similar function in mice and flies.
    b). Pax6 is involved in eyespot formation in ribbon worms.
    c). Pax6 is required for eye formation in Drosophila.
    d). Pax6 is required for eyespot formation in planaria.
Answer: d

Test Your Visual Understanding

1). Pax6 is known to play a role in the formation of eyespots during regeneration of the ribbon worm. Beginning with the removal of the head in the above diagram, outline the regeneration process and eyespot formation of a normal, wild-type ribbon worm and in one that lacks Pax6 expression. How are they different?
Answer: Refer to figure 24.16 for regeneration of a normal, wild-type ribbon worm. By contrast, in ribbon worms that lack Pax6 expression, regeneration of the head region should proceed relatively normally, except that no eyespots would form.

Apply Your Knowledge
1. How might knowledge of the Oryza sativa genome help combat world hunger?
Answer: Along with wheat and a few other similar crops, rice comprises a large portion of the world's food and animal feed. As we learn more about the rice genome we may be able to generate a larger variety of genetically modified rice strains to help combat world hunger and malnutrition. Recently, strains of rice have been developed to increase yields by 35%, as well as to provide vitamins and minerals to improve the health of the people in third-world countries. For example, one newly developed strain of genetically modified rice is IR68144, with artificially high levels of iron, zinc (to prevent anemia), and Vitamin A (to prevent blindness).

2). Can a human embryo that exhibits polyploidy survive until birth? What is the difference between polyploidy and trisomy?
Answer: A polyploidy human embryo that is 3n would not be expected to survive to birth. In contrast to polyploidy, trisomy involves the addition of a chromosome to a diploid genome (2n +1). Three forms of trisomy can lead to a viable infant, although the trisomy of chromosome 21, which results in Down syndrome, is the only condition in which a significant number of individuals live longer than a year past birth. The other two viable trisomies, Patau syndrome (trisomy 13) and Edward's syndrome (trisomy 18) have severe birth defects and often die within the first three or four months of birth. It is believed that other forms of trisomy result in spontaneous abortion of the embryo or fetus early in pregnancy. These observations suggest that normal human embryonic development requires a precise diploid number of chromosomes.

3). In a paper by Halder et al., 1995, the authors use the inducible GAL4-UAS expression system to artificially overexpress the eyeless (Pax6) gene in tissues where it is not normally expressed. Why does this system use the yeast transcriptional activator, GAL4, which is not normally present in Drosophila?
Answer: The GAL4-UAS system was developed in 1993 by Andrea Brand and Nobert Perrimon. It is based on the yeast transcriptional activator, GAL4, and its upstream activation sequence (UAS). Drosophila does not have any endogenous GAL4 or GAL4 binding sites and therefore allows relatively tight spatial and temporal control over transcriptional activity of the gene of interest.

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