Question

U937D cells express high levels of creatine kinase (CK‑B) mRNA but do not translate the mRNA into protein. Ribosomes bind the 5' end of the CK‑B mRNA; however, translation into protein is repressed in these cells.

U937D cells synthesize the CK‑B enzyme when researchers introduce numerous short segments of RNA containing 3' UTR consensus sequences into the cells. The total amount of CK‑B mRNA does not change after adding RNA containing 3' UTR sequences. Introducing short RNA segments without the 3' UTR consensus sequences does not stimulate CK‑B synthesis.

Which of the statements explains how the introduction of short RNA containing 3' UTR sequences allows CK‑B translation in U937D cells?
(a) Translational repressor proteins inhibit CK‑B translation by binding 3' UTR sequences in the CK‑B mRNA. These repressors bind the 3' UTR sequences in the introduced RNA instead of the CK‑B mRNA, allowing the ribosomes to freely translate CK‑B mRNA.
(b) The CK‑B mRNA molecule is too short for efficient translation. The added 3' UTR RNA sequences recombine with the existing CK‑B mRNA, creating a longer transcript that increases translation rates.
(c) The introduced RNA increases the intracellular RNA concentration, which triggers an increase in the number of ribosomes and accessory factors and improves translation efficiency.
(d) Ribosomes stall when the cell does not contain enough free nucleotides as substrate. Degradation of the additional 3' UTR RNAs provides nucleotides to increase translation rates.

Answer 1

Short RNA containing 3' UTR consensus sequences introduced into U937D cells function as competitive inhibitors for translational repressors, allowing CK-B translation to proceed.

Step-by-step explanation:

The introduction of short RNA containing 3' UTR consensus sequences into U937D cells allows for the synthesis of creatine kinase (CK-B) because these sequences act as competitive inhibitors for translational repressor proteins. The 3' UTR sequences in the CK-B mRNA normally bind to these repressor proteins, preventing translation. When short RNA segments with the 3' UTR consensus sequences are introduced, the repressor proteins preferentially bind to these decoys, allowing ribosomes to freely translate the existing CK-B mRNA into protein.