10.3 Structure and Function of RNA

RNA Structure and Function

RNA is the versatile counterpart to DNA, playing central roles in gene expression and cellular function. Messenger RNA carries genetic instructions, transfer RNA delivers amino acids, and ribosomal RNA catalyzes protein synthesis. But RNA's roles extend well beyond these three types.

RNA's single-stranded structure allows it to fold into complex three-dimensional shapes, giving it catalytic and regulatory abilities that DNA doesn't have. Some viruses even use RNA as their genetic material, and the "RNA world" hypothesis suggests RNA may have been the molecular foundation of early life.

Structure and Role of Ribonucleotides

Ribonucleotides are the monomers (building blocks) of RNA. Each one has three components:

• A nitrogenous base: adenine (A), guanine (G), cytosine (C), or uracil (U). Note that uracil replaces the thymine (T) found in DNA.
• A pentose sugar: ribose. Compared to the deoxyribose in DNA, ribose has a hydroxyl group ($-OH$) at the 2' carbon. That extra $-OH$ is a big deal: it makes RNA more chemically reactive, less stable than DNA, and able to fold into diverse functional shapes.
• A phosphate group attached to the 5' carbon of the sugar.

Ribonucleotides link together through phosphodiester bonds to form a single-stranded polymer. Each bond forms between the 3' hydroxyl group of one nucleotide and the 5' phosphate group of the next, giving the strand a consistent 5' to 3' directionality.

Even though RNA is single-stranded, it can fold back on itself and form secondary structures through intramolecular base pairing:

• A pairs with U via two hydrogen bonds
• G pairs with C via three hydrogen bonds

These base-pairing interactions create structures like hairpin loops and stem-loops. Beyond that, additional long-range interactions between different regions of the molecule produce complex tertiary structures, which are essential for the function of molecules like tRNA and ribozymes.

RNA vs. DNA: Key Features

RNA and DNA share some core features but differ in important ways.

Similarities:

• Both are nucleic acids built from nucleotide monomers
• Both share three nitrogenous bases: adenine (A), guanine (G), and cytosine (C)
• Both have a sugar-phosphate backbone with 5' to 3' directionality

Differences:

Functions of Major RNA Types

Messenger RNA (mRNA) carries genetic information from DNA to the ribosome, where it directs protein synthesis.

• Produced by transcription, in which RNA polymerase reads a DNA template strand and synthesizes a complementary RNA strand
• In eukaryotes, the mature mRNA has three key features: a 5' cap (helps with ribosome recognition), a coding region containing codons that specify amino acids, and a 3' poly(A) tail (protects against degradation)
• Before translation, eukaryotic pre-mRNA undergoes splicing to remove non-coding sequences (introns) and join coding sequences (exons)

Transfer RNA (tRNA) serves as the adapter molecule that matches codons on mRNA to the correct amino acids during translation.

• Each tRNA has an anticodon (a three-nucleotide sequence) that base pairs with a complementary codon on the mRNA
• Before participating in translation, a tRNA must be "charged" with its correct amino acid. This process, called aminoacylation, is carried out by aminoacyl-tRNA synthetase enzymes. Each synthetase recognizes a specific tRNA and attaches the appropriate amino acid.
• The cloverleaf secondary structure of tRNA, with its multiple stem-loops, is a classic example of how RNA folding creates function

Ribosomal RNA (rRNA) is the main structural and catalytic component of the ribosome.

• In prokaryotes, ribosomes consist of a large subunit (50S) containing 23S and 5S rRNA, and a small subunit (30S) containing 16S rRNA. Together they form the 70S ribosome.
• The rRNA doesn't just provide scaffolding. The 23S rRNA actually catalyzes peptide bond formation during translation, making the ribosome itself a ribozyme.
• The 16S rRNA in the small subunit is also used widely in microbiology for phylogenetic classification of bacteria and archaea, since its sequence is highly conserved.

RNA as Genetic Material

Most cellular organisms use DNA as their genetic material, but many viruses use RNA instead. The type of RNA genome determines how the virus replicates:

• Single-stranded RNA (ssRNA) viruses like poliovirus, influenza virus, and SARS-CoV-2 carry genomes that can function directly as mRNA (positive-sense) or must first be copied into a complementary strand (negative-sense, as with influenza) before translation.
• Double-stranded RNA (dsRNA) viruses like rotavirus carry segmented genomes. Each segment is transcribed into mRNA for protein synthesis.
• Retroviruses like HIV take a unique approach: their ssRNA genome is reverse transcribed into DNA by the enzyme reverse transcriptase. That DNA integrates into the host genome, where it's transcribed to produce both viral proteins and new RNA genomes.

The RNA world hypothesis proposes that before DNA and proteins existed, early life depended on RNA to do both jobs: store genetic information and catalyze chemical reactions. The fact that RNA can do both of these things today (as seen in ribozymes and RNA viruses) is the strongest evidence supporting this idea.

Regulatory and Catalytic RNA Functions

Beyond mRNA, tRNA, and rRNA, cells contain a variety of non-coding RNAs with regulatory and catalytic roles.

Catalytic RNAs (ribozymes) can catalyze chemical reactions without any protein component. Two well-known examples: self-splicing introns (Group I and Group II), which excise themselves from pre-mRNA, and the peptidyl transferase activity of the 23S rRNA in the ribosome.

RNA interference (RNAi) is a powerful gene-silencing mechanism. Double-stranded RNA is processed into small interfering RNAs (siRNAs), which guide a protein complex called RISC to complementary mRNA targets, leading to their degradation or translational repression.

Small nuclear RNAs (snRNAs) combine with proteins to form small nuclear ribonucleoproteins (snRNPs), which assemble into the spliceosome. The spliceosome is the molecular machine responsible for removing introns from pre-mRNA in eukaryotes.

MicroRNAs (miRNAs) are short non-coding RNAs (roughly 22 nucleotides) that fine-tune gene expression. They bind to complementary sequences in the 3' untranslated region of target mRNAs, leading to translational inhibition or mRNA degradation. A single miRNA can regulate hundreds of different mRNA targets, making miRNAs important players in development, cell differentiation, and disease.