Question: Genetically modified (GM) foods contain DNA from more than one source. The goal of creating GM foods is to modify the organisms to express new and useful traits. A few examples involve engineering crops to be resistant to bacterial or viral infections, to be resistant to herbicides, to ripen more slowly, to survive extreme temperature changes, to contain vaccines, or to create crops that are more nutritious.
One GM food that has received media attention over the years is golden rice. This GM product was first introduced in 2000 and was engineered to provide carotene that is a precursor to vitamin $\mathrm{A}$. The carotene in the rice provides the golden color. Vitamin A deficiency is a major problem in underdeveloped parts of the world, killing more than one million people a year, most of whom are children. This vitamin is critical for the development and maintenance of epithelial tissues, for vision, and for growth.
Rice plants normally produce carotene in the inedible parts of the plant. To be able to access and use the carotene, it was necessary to engineer the rice to produce it in the edible grains. In order to achieve this goal, two genes from daffodil plants, psy and $l y c$, and a bacterial gene called $c r t 1$ were added to the rice genome. These genes engineered a pathway that produces lycopene, which is eventually converted to carotene. Each of these genes was placed under the control of a grain-specific promoter so that the products were produced in the rice grains.
While golden rice was produced to provide carotene, which the human body can convert into the essential vitamin A, the rice provided much less carotene than was anticipated. Golden rice 2, a modified version released in 2005, contains a much larger amount of carotene and, as such, is a better source of the vitamin. Field tests were conducted in 2009, and human consumption trials were completed in 2012. However, neither version has been grown in large-scale quantities at this time.
When preparing GM organisms, it can be easier to get a eukaryotic cell to express bacterial genes than it is to get a bacterial cell to express eukaryotic genes. What might be the MOST reasonable explanation for this?

Options:
A. Bacteria are more simplistic organisms, and it is more difficult to get them to express complex eukaryotic genes.
B. The ribosomes of bacteria are different from eukaryotic ribosomes, and they have difficulty translating eukaryotic mRNA.
C. Eukaryotic genes contain introns that must be spliced out prior to translation in order to make a functional protein. Bacteria lack the ability to perform RNA splicing.
D. Bacterial cells recognize eukaryotic DNA as foreign and use nucleases to destroy it before it can be expressed.

Answer: Bacterial ribosomes are in fact different from eukaryotic ribosomes. However, the genetic code is universal so there should be no problem translating the proteins. When eukaryotic DNA is introduced to bacterial cells, it is contained in a vector such as a plasmid that is a normal component of many bacterial cells. We know that eukaryotic genes contain introns and prokaryotic genes do not. If a eukaryotic gene is placed in a prokaryotic cell, the introns cannot be removed and the mRNA produced will contain introns. When translated, the intended protein will not be produced. The correct answer is C.