Final answer:
Iron response elements form RNA hairpin structures in the untranslated regions of certain mRNAs, which are recognized by iron regulatory proteins to control the translation process depending on iron availability in the cell.
Step-by-step explanation:
Iron response elements (IREs) form RNA hairpin structures that are crucial in the regulation of iron metabolism. mRNAs for proteins involved in iron storage, such as ferritin, contain these IREs in their untranslated regions (UTRs). Iron regulatory proteins (IRPs), specifically IRP1 and IRP2, bind to these stem-loop structures, controlling the translation of the mRNA.
When iron levels are low, IRPs bind to the IREs, blocking the initiation of translation, and therefore, the synthesis of the protein. This mechanism ensures that iron is conserved within the cell when it is scarce. Unlike steroid hormone receptors, which bind to DNA sequences, IRPs interact with RNA molecules. Therefore, the option that iron response elements form RNA hairpin structures is correct.