Question

Imagine that you've isolated the complete human growth hormone gene directly from the human genome. After running through all the steps described in Lecture 30 for cloning and gene expression in bacteria, you find that no human growth hormone is expressed. What is the most likely explanation?

a. Bacteria cannot carry out splicing.
b. Human DNA can be maintained in cloned form only for brief periods in bacteria.
c. Bacteria lack a nucleus for proper transcription of eukaryotic genes.
d. Bacteria cannot translate human mRNA coding sequences.
e. Human DNA cannot be cloned in a bacterium.

Answer 1

a. Bacteria cannot carry out splicing.

Step-by-step explanation:

Human genes contain non-coding sequences called introns. Transcription of genes produces primary transcripts that undergo post-transcriptional modifications to produce mature mRNA. These modifications include removal of introns, the addition of 3' poly-A tail and 5' cap. The splicing of introns from primary transcripts is followed by ligation of protein-coding sequences exons to produce mature mRNA. The mRNA leaves the nucleus and serves as a template for protein synthesis.

Splicing of introns requires specific machinery such as several small nuclear ribonucleoprotein complexes, etc. which are not present in a prokaryotic cell. Therefore, a eukaryotic gene like the gene for human growth hormone is not expressed in a bacteria cell as these prokaryotic cells cannot carry out the splicing of introns from pre-mRNA.