RNA Structure and Function

RNA Structure and Function

• RNA is a product of DNA transcription
• A cell contains 10 times more RNA than DNA
• Multiple types of RNA play various roles in gene expression
• RNA differs from DNA in chemical structure, physical properties, and biological roles

Aims of the Lecture

• Describe the physical and functional features of RNA
• Explain the biological functions of RNA within a cell
• Outline the roles of untranslated regions (UTRs)

RNA Chemical Structure

• RNA uses ribose instead of deoxyribose in its structure
• The 2'-hydroxyl group in ribose makes RNA less stable compared to DNA
• The base uracil replaces thymine in RNA
• Uracil lacks the methyl group present in thymine

Biological Roles of RNA

• Transferring information from DNA to proteins (messenger RNA, mRNA)
• Protein synthesis (ribosomal RNA, rRNA; transfer RNA, tRNA)
• Involved in processing messenger RNA (small nuclear RNA, snRNA; small nucleolar RNA, snoRNA)
• Involved in splicing introns from pre-messenger RNA
• Important as catalytic RNA (self-splicing introns; natural/artificial ribozymes)

RNA Content of a Cell

• A single cell contains ~10 pg (10^-11 g) of RNA
• 80-85% is ribosomal RNA
• 10-15% are low molecular weight species (tRNAs, small nuclear RNAs)
• 1-5% is messenger RNA

RNA Physical Properties

• Generally single-stranded
• Can form complex structures through base-pairing (secondary and tertiary structure)

RNA Secondary Structure Formation

• For each type, palindrome sequence and self-complementary base-pairing plays a key role to form.

RNA Regulation

• The amount/usage of RNA is important to function
• Regulating synthesis: This is rapid, as shown by the lac operon
• Regulating degradation: Examples like transferrin receptor mRNA
• Regulating translation efficiency: Proteins control usage of certain RNAs, as seen in ferritin mRNA

Eukaryotic mRNA-Encoding Genes

• mRNA is transcribed from DNA
• mRNA contains 5' UTR, coding region, and 3' UTR sequences
• UTRs are untranslated and play significant regulatory roles
• mRNA moves from the nucleus to the cytoplasm for translation

The Transferrin Receptor

• Iron in the blood binds to transferrin and is transported into cells through a receptor
• If sufficient intracellular iron is present, the mRNA is degraded

RNA 2° Structure Regulation

• Intracellular iron (Fe²⁺) regulates transferrin receptor (TfR) mRNA degradation
• Iron Response Element (IRE)-Binding protein (IRE-BP) binds to mRNA regions in response to iron availability
• The binding/release status of IRE-BP controls mRNA stability

RNA Secondary Structure and Regulation of Translation

• Ferritin binds Fe²⁺
• Low iron causes IRE-BP to bind to the IRE sequence - preventing translation
• High iron causes IRE-BP to release from the IRE sequence - allowing for ferritin translation

Summary

• RNA 2° structure is important for catalysis, organization, and recognition
• RNA 2° structure impacts stability (as seen with the transferrin receptor) and translation (as seen with ferritin)

Intended Learning Outcomes

• Define and describe the basic chemical and functional properties of RNA and explain how it differs from DNA
• Understand how RNA forms secondary structure and builds three-dimensional structures
• Understand the role of secondary/tertiary structure in RNA's biological roles