recap

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16.5 recap

One of the most important means of posttranscriptional regulation is alternative RNA splicing, which allows more than one protein to be made from a single gene. The stability of mRNA in the cytoplasm can also be regulated. MicroRNAs, siRNAs, mRNA modifications, and translational repressors can prevent mRNA translation. Proteins in the cell can be targeted for breakdown by ubiquitin and then hydrolyzed in proteasomes.

learning outcomes

You should be able to:

  • Explain the differences in the number of protein-coding genes and the number of proteins in a cell.

  • Explain how miRNAs regulate translation of mRNA.

  • Compare and contrast miRNAs and siRNAs.

  • Suggest a way that the structure of an mRNA molecule could affect its translation.

  • Describe the function of the proteasome in a cell.

Question 1

How do miRNAs and siRNAs regulate gene expression?

miRNAs and siRNAs can bind by base pairing to target mRNAs and prevent their translation because tRNA cannot bind; or the inhibitor RNAs can bind to pre-mRNA in the nucleus and lead to its hydrolysis by RNase; or the inhibitor RNAs can bind to DNA at the transcription site and block RNA polymerase from working for transcription.

Question 2

How does the three-dimensional structure of mRNA contribute to the regulation of its expression?

mRNA can fold back on itself by hydrogen bonding of complementary bases, forming looped structures. These structures can bind to proteins that then inhibit translation at the ribosome.

Question 3

What is the role of the proteasome?

The proteasome binds to proteins that are targeted with ubiquitin for breakdown. Within the proteasome are proteases that hydrolyze targeted proteins.

Question 4

Before the human genome was sequenced, most scientists thought it contained 80,000 to 150,000 protein-coding genes. The actual sequence revealed only about 21,000. How can mRNA explain this discrepancy?

On average, a human gene can form at least four different mRNAs by alternative splicing. Each of these mRNAs is translated to a unique protein. So the number of different proteins is much greater than the number of genes.

Question 5

Pharmaceutical companies are interested in developing miRNA drugs. How might they work in cancer? (Hint: See Figure 11.23, about oncogene and tumor suppressor proteins.)

miRNAs targeted to activated oncogenes will block the translation of target mRNAs that would make proteins that otherwise stimulate cell division.