Becker's World of the Cell 9th Edition by Lewis Kleinsmith, Jeff Hardin, Gregory Paul Bertoni

Exercise 7

Gene Amplification. The best-studied example of gene amplification involves the genes coding for ribosomal RNA during oogenesis in Xenopus laevis. The unamplified number of ribosomal RNA genes is about 500 per haploid genome. After amplification, the oocyte contains about 500,000 ribosomal RNA genes per haploid genome. This level of amplification is apparently necessary to allow the egg cell to synthesize the 1012 ribosomes that accumulate during the two months of oogenesis in this species. Each ribosomal RNA gene consists of about 13,000 base pairs, and the genome size of Xenopus is about 2.7 × 10 9 base pairs per haploid genome.
(a)What fraction of the total haploid genome do the 500 copies of the ribosomal RNA genes represent
(b)What is the total size (in base pairs)of the genome after amplification of the ribosomal RNA genes What proportion of this do the amplified ribosomal RNA genes represent
(c)Assume that all the ribosomal RNA genes in the amplified oocyte are transcribed continuously to generate the number of ribosomes needed during the two months of oogenesis. How long would oogenesis Jiave to extend if the genes had not been amplified
(d)Why do you think genes have to be amplified when the gene product needed by the cell is an RNA but not usually when the desired gene product is a protein