A biologist isolated a gene from a human cell, inserted it into a plasmid, and inserted the plasmid into a bacterium. The bacterium made a new protein, but it was nothing like the protein normally produced in a human cell. Why? (Explain your answer.)
a. The bacterium had undergone transformation.
b. The gene did not have sticky ends.
c. The human gene contained introns.
d. The gene was not synthesized from scratch.

Verified step by step guidance

1. Understand the problem: The biologist inserted a human gene into a bacterial plasmid, and the bacterium produced a protein that was different from the one normally produced in human cells. The question asks why this happened, and we are given four possible explanations to evaluate.
2. Step 1: Recall the differences between eukaryotic and prokaryotic gene expression. In eukaryotes (like humans), genes often contain introns (non-coding regions) that are removed during RNA processing. Prokaryotes (like bacteria) lack the machinery to process introns, so they cannot correctly express genes that contain introns.
3. Step 2: Analyze the options: (a) Transformation refers to the uptake of foreign DNA by a bacterium, which did occur in this case, but it does not explain why the protein was different. (b) Sticky ends are important for inserting the gene into the plasmid, but if the gene was successfully inserted, this is not the issue. (c) If the human gene contained introns, the bacterium would not be able to process them, leading to an incorrect protein. (d) The gene not being synthesized from scratch is irrelevant to the problem.
4. Step 3: Focus on option (c): The presence of introns in the human gene is the most likely explanation. Since bacteria cannot process introns, the mRNA transcribed from the gene would include these non-coding regions, resulting in a faulty or incomplete protein.
5. Step 4: Conclude that the correct answer is (c), and explain that to express a human gene correctly in bacteria, scientists often use complementary DNA (cDNA), which is synthesized from processed mRNA and lacks introns.

Key Concepts

Gene Expression

Gene expression is the process by which information from a gene is used to synthesize a functional gene product, typically a protein. In eukaryotic cells, this involves transcription of DNA to mRNA, followed by translation of mRNA to protein. Bacteria, however, have different mechanisms for gene expression, which can lead to differences in the proteins produced when eukaryotic genes are expressed in prokaryotic systems.

Introns and Exons

Introns are non-coding sequences within a gene that are transcribed into pre-mRNA but are removed during RNA splicing, leaving only exons, which code for proteins. Bacteria lack the machinery to splice introns out of eukaryotic pre-mRNA, meaning that if a human gene with introns is inserted into a bacterium, the bacterium may not produce the correct protein, as it cannot process the gene properly.

Transformation in Bacteria

Transformation is a process by which bacteria take up foreign DNA from their environment and incorporate it into their own genome. This allows for genetic modification and the expression of new traits. However, the success of transformation and the functionality of the expressed protein depend on the compatibility of the inserted gene with the bacterial cellular machinery, which can differ significantly from that of eukaryotic cells.