9-7 Figure Q9-7 shows several possible substrates of exon shuffling. Horizontal lines and small filled circles represent chromosomes and centromeres, respectively. Exons are labeled A, B, C, and D. Homologous recombination or shuffling may take place at short, repeated homologous DNA sequences in introns; because DNA sequences have a polarity, the repeated sequences can be considered to have a head and a tail and thus are drawn as arrows. A recombinational crossover is indicated by a big X. Panel (A) shows that recombination between two direct repeats located on opposite sides of the centromere yields one circular product that contains a centromere and a second product that lacks a centromere and will therefore be lost when the cell divides. Panel (B) shows that recombination between inverted repeats flanking the centromere will keep the rearranged chromosome intact. This one requires a bit of thinking !
Draw the products of recombination when the repeated sequences are located on different chromosomes, as shown in panels (C) and (D). 6 Points
Will these products be faithfully transmitted during cell division? YES or NO
9-9 You are working in a human genetics laboratory that studies causes and treatments for eye cataracts in newborns. This disease is thought to be caused by a deficiency in an enzyme called galactokinase, but the human gene that encodes this enzyme has not yet been identified. At a talk by a visiting scientist, you learn about a strain of bakers yeast that contains a mutation called gal1– in its galactokinase gene. Because this gene is needed to metabolize galactose, the mutant strain cannot grow in galactose medium. Knowing that all living things evolved from a common ancestor and that distantly related organisms often have homologous genes that perform similar functions, you wonder if the human galactokinase gene can function in yeast. Since you have an optimistic temperament, you decide to pursue this line of experimentation. You isolate mRNA gene transcripts from human cells, use reverse transcriptase to make complementary DNA (cDNA) copies of the mRNA molecules, and ligate the cDNAs into circular plasmid DNA molecules that can be stably propagated in yeast cells. You then transform the pool of plasmids into gal1– yeast cells so that each cell receives a single plasmid. 6 Points
12 Points Total This Page
1) What do you think will happen when you spread the plasmid-containing cells on petri plates that contain galactose as a carbon source?
2) How can this approach help you find the human gene encoding galactokinase?
9-15 The genomes of some vertebrates are much smaller than those of others. For example, the genome of the puffer fish Fugu is much smaller than the human genome, and even much smaller than those of other fish, primarily due to the small size of its introns. 8 Points
A. Describe a mechanism that might drive evolution toward small introns or loss of introns and could therefore account for the evolutionary loss of introns according to the “introns early” hypothesis.
B. Describe a mechanism that might drive evolution toward more or larger introns and could thereby account for the evolutionary appearance of introns according to the “introns late” hypothesis.
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