Untranslated regions (UTRs) of mRNAs play crucial roles in post-transcriptional regulation of gene expression. These regions, located at the 5' and 3' ends of mRNAs, influence translation efficiency, mRNA stability, and subcellular localization. The 5' UTR contains elements that control initiation of translation, such as upstream initiation codons, open reading frames (ORFs), and internal ribosome entry sites (IRES). The 3' UTR includes elements that regulate mRNA stability, such as AU-rich elements (AREs), and may also contain sequences that influence the incorporation of selenocysteine into selenoproteins. UTRs can also affect mRNA localization, with specific sequences guiding the transport of mRNAs to particular cellular compartments. The structure and function of UTRs vary among species, with 5' UTRs generally ranging from 100 to 200 nucleotides and 3' UTRs varying more widely. UTRs can contain introns, especially in the 5' UTR, and may be subject to alternative splicing, polyadenylation, and other regulatory mechanisms. The base composition of UTRs also differs, with 5' UTRs typically having a higher G+C content than 3' UTRs. UTRs can contain repetitive elements, such as short and long interspersed elements, which may contribute to regulatory functions. Translation efficiency is influenced by the structure of the 5' UTR, with stable secondary structures and upstream ORFs affecting the recognition of the initiation codon. The eIF4F complex, which includes eIF4E, eIF4A, and eIF4G, plays a key role in translation initiation by unwinding secondary structures in the 5' UTR. The presence of upstream AUGs can affect translation efficiency and may lead to the production of multiple proteins from the same mRNA through a process called 'leaky scanning'. IRES elements, found in some mRNAs, allow for cap-independent translation initiation. The stability of mRNAs is regulated by elements in the 3' UTR, such as AREs, which promote degradation in response to various signals. The degradation of mRNAs can also occur through endonuclease activity, independent of deadenylation and decapping. Nonsense-mediated mRNA decay (NMD) is another mechanism that can lead to the degradation of mRNAs containing premature stop codons. The subcellular localization of mRNAs is also regulated by UTRs, with specific sequences guiding the transport of mRNAs to particular cellular compartments. These sequences can be part of mRNA zip codes, which interact with zip-code-binding proteins to ensure proper localization. Overall, UTRs are essential for the regulation of gene expression at the post-transcriptional level, influencing translation, stability, and localization of mUntranslated regions (UTRs) of mRNAs play crucial roles in post-transcriptional regulation of gene expression. These regions, located at the 5' and 3' ends of mRNAs, influence translation efficiency, mRNA stability, and subcellular localization. The 5' UTR contains elements that control initiation of translation, such as upstream initiation codons, open reading frames (ORFs), and internal ribosome entry sites (IRES). The 3' UTR includes elements that regulate mRNA stability, such as AU-rich elements (AREs), and may also contain sequences that influence the incorporation of selenocysteine into selenoproteins. UTRs can also affect mRNA localization, with specific sequences guiding the transport of mRNAs to particular cellular compartments. The structure and function of UTRs vary among species, with 5' UTRs generally ranging from 100 to 200 nucleotides and 3' UTRs varying more widely. UTRs can contain introns, especially in the 5' UTR, and may be subject to alternative splicing, polyadenylation, and other regulatory mechanisms. The base composition of UTRs also differs, with 5' UTRs typically having a higher G+C content than 3' UTRs. UTRs can contain repetitive elements, such as short and long interspersed elements, which may contribute to regulatory functions. Translation efficiency is influenced by the structure of the 5' UTR, with stable secondary structures and upstream ORFs affecting the recognition of the initiation codon. The eIF4F complex, which includes eIF4E, eIF4A, and eIF4G, plays a key role in translation initiation by unwinding secondary structures in the 5' UTR. The presence of upstream AUGs can affect translation efficiency and may lead to the production of multiple proteins from the same mRNA through a process called 'leaky scanning'. IRES elements, found in some mRNAs, allow for cap-independent translation initiation. The stability of mRNAs is regulated by elements in the 3' UTR, such as AREs, which promote degradation in response to various signals. The degradation of mRNAs can also occur through endonuclease activity, independent of deadenylation and decapping. Nonsense-mediated mRNA decay (NMD) is another mechanism that can lead to the degradation of mRNAs containing premature stop codons. The subcellular localization of mRNAs is also regulated by UTRs, with specific sequences guiding the transport of mRNAs to particular cellular compartments. These sequences can be part of mRNA zip codes, which interact with zip-code-binding proteins to ensure proper localization. Overall, UTRs are essential for the regulation of gene expression at the post-transcriptional level, influencing translation, stability, and localization of m