RNA is the versatile cousin of DNA, playing crucial roles in gene expression and cellular function. From messenger RNA carrying genetic instructions to transfer RNA delivering amino acids, RNA molecules come in various forms with diverse responsibilities.

RNA's unique structure allows it to fold into complex shapes, enabling catalytic and regulatory functions. Unlike DNA, RNA can serve as genetic material for some viruses and may have been the original basis for life in the "RNA world" hypothesis.

RNA Structure and Function

Structure and role of ribonucleotides

• Ribonucleotides serve as the building blocks of RNA molecules
  • Consist of three components: a nitrogenous base, a pentose sugar (ribose), and a phosphate group
  • Nitrogenous bases in RNA include adenine (A), guanine (G), cytosine (C), and uracil (U) which replaces thymine (T) found in DNA
  • Ribose sugar contains an additional hydroxyl group (-OH) at the 2' position compared to deoxyribose in DNA enabling RNA to fold into various functional shapes
• Ribonucleotides are linked together by phosphodiester bonds forming a single-stranded RNA molecule with a 5' to 3' directionality
  • Bonds form between the 3' hydroxyl group of one ribonucleotide and the 5' phosphate group of the next
• RNA can form secondary structures through base pairing interactions
  • Adenine (A) pairs with uracil (U) via two hydrogen bonds while guanine (G) pairs with cytosine (C) via three hydrogen bonds
  • Base pairing allows RNA to fold into various functional shapes such as hairpin loops and stem-loops
• RNA molecules can also form complex tertiary structures through additional interactions between different regions of the molecule

RNA vs DNA: Key features

• Similarities between RNA and DNA:
  • Both are nucleic acids composed of nucleotide monomers containing the nitrogenous bases adenine (A), guanine (G), and cytosine (C)
  • Both have a sugar-phosphate backbone with 5' to 3' directionality
• Differences between RNA and DNA:
  • Sugar component: RNA contains ribose while DNA contains deoxyribose
  • Nitrogenous bases: RNA uses uracil (U) instead of thymine (T) found in DNA
  • Strand number: RNA is typically single-stranded while DNA is double-stranded
  • Stability: RNA is less stable than DNA due to the presence of the reactive 2' hydroxyl group
  • Function: DNA primarily serves as the genetic material while RNA has diverse roles in gene expression (mRNA), amino acid delivery (tRNA), and catalysis (ribozymes)

Functions of RNA types

• Messenger RNA (mRNA) carries genetic information from DNA to ribosomes for protein synthesis
  • Produced through transcription where RNA polymerase synthesizes a complementary RNA strand from a DNA template
  • Consists of a 5' cap, coding region containing codons that specify amino acids, and 3' poly(A) tail
  • Undergoes RNA splicing to remove introns and join exons before translation
• Transfer RNA (tRNA) acts as adapter molecules that translate the genetic code from mRNA into amino acids
  • Contain an anticodon loop that base pairs with the complementary codon on mRNA
  • Undergo aminoacylation where they are "charged" with their corresponding amino acids by aminoacyl-tRNA synthetases
• Ribosomal RNA (rRNA) serves as the main structural and catalytic component of ribosomes
  • Ribosomes are composed of a large (50S) and small (30S) subunit in prokaryotes, each containing specific rRNAs (23S and 16S respectively)
  • rRNAs facilitate ribosome assembly and catalyze peptide bond formation during protein synthesis

RNA as genetic material

• Some viruses use RNA as their genetic material instead of DNA
  • Single-stranded RNA viruses (poliovirus, influenza virus, SARS-CoV-2): genome serves as both genetic material and mRNA for protein synthesis
  • Double-stranded RNA viruses (rotavirus): genome segments are transcribed into mRNA for protein synthesis
  • Retroviruses (HIV): single-stranded RNA genome is reverse transcribed into DNA, integrates into host genome, and is transcribed to produce viral proteins and new RNA genomes
• RNA world hypothesis proposes that early life relied on RNA as both genetic material and catalytic molecule before DNA and proteins emerged
  • RNA can store genetic information and catalyze chemical reactions supporting its crucial role in the origin and evolution of life

Regulatory and Catalytic RNA Functions

• Catalytic RNA (ribozymes) can perform enzymatic functions, such as self-splicing introns and peptide bond formation in ribosomes
• RNA interference (RNAi) is a regulatory mechanism that uses small RNA molecules to silence gene expression
• Small nuclear RNAs (snRNAs) play a crucial role in pre-mRNA splicing as components of the spliceosome
• MicroRNAs (miRNAs) are short non-coding RNAs that regulate gene expression by binding to target mRNAs and inhibiting translation or promoting degradation