Nucleic acids store and transmit genetic information in cells. DNA holds the long-term genetic instructions, while RNA helps convert those instructions into functional proteins. Understanding their structure explains how they carry out these roles.
Nucleic Acid Building Blocks and Structure
Components of nucleic acids
Nucleotides are the monomers (individual subunits) of nucleic acids. Each nucleotide has three parts:
- Pentose sugar: differs between DNA and RNA
- DNA contains deoxyribose, which lacks a hydroxyl group (–OH) at the 2' carbon
- RNA contains ribose, which has that –OH group at the 2' carbon
- Phosphate group: attached to the 5' carbon of the sugar
- Nitrogenous base: linked to the 1' carbon of the sugar
- Purines (double-ring structure): Adenine (A) and Guanine (G)
- Pyrimidines (single-ring structure): Cytosine (C), Thymine (T) in DNA, and Uracil (U) in RNA
A quick way to remember: purines are the bigger bases (double ring), but the word is shorter. Pyrimidines are the smaller bases (single ring), but the word is longer.
DNA vs RNA
| Feature | DNA | RNA |
|---|---|---|
| Strands | Double-stranded | Typically single-stranded |
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, G, C | A, U, G, C |
| Primary role | Long-term genetic information storage | Protein synthesis, gene regulation |
| Structure | Double helix | Can fold into secondary structures via internal base pairing |
The missing –OH on deoxyribose makes DNA more chemically stable than RNA. That stability is why DNA serves as the long-term storage molecule, while RNA is shorter-lived and more versatile.
Structure of DNA
DNA consists of two polynucleotide strands wound around each other in a right-handed double helix, with roughly 10 base pairs per turn.
The two strands run in opposite directions (they're antiparallel): one runs 5' → 3' while the other runs 3' → 5'. This orientation matters for replication and transcription.
- The sugar-phosphate backbone sits on the outside of the helix. Phosphodiester bonds connect the 3' carbon of one nucleotide's sugar to the 5' carbon of the next.
- The nitrogenous bases point inward, stacking perpendicular to the helix axis. They're held together by hydrogen bonds between complementary pairs:
- A pairs with T through 2 hydrogen bonds
- G pairs with C through 3 hydrogen bonds
Because G–C pairs have three hydrogen bonds instead of two, DNA regions rich in G–C content are harder to separate (more thermally stable) than A–T rich regions.
Chargaff's Rule: In any DNA molecule, the amount of A equals T, and the amount of G equals C. This follows directly from complementary base pairing.
Nucleic Acid Function and Types of RNA
Nucleic acids in genetic information
DNA stores genetic information in the sequence of its bases. Genes are specific stretches of DNA that encode instructions for building proteins or functional RNA molecules.
During cell division (mitosis or meiosis), DNA is replicated so each daughter cell receives a complete copy. This ensures genetic information passes faithfully from one generation to the next.
RNA serves as the intermediary between DNA and proteins. The flow of information follows what's called the central dogma:
DNA → (transcription) → RNA → (translation) → Protein
DNA is transcribed into RNA in the nucleus, and that RNA is then translated into protein at the ribosome.
Functions of RNA types
Three major types of RNA work together during protein synthesis:
Messenger RNA (mRNA) carries the genetic instructions from DNA to the ribosome.
- Produced during transcription, using one strand of DNA as a template
- Travels from the nucleus to ribosomes in the cytoplasm
- Read in sets of three nucleotides called codons, where each codon specifies one amino acid
Transfer RNA (tRNA) acts as an adapter that matches codons to the correct amino acids.
- Each tRNA has an anticodon (a three-base sequence) that pairs with a complementary mRNA codon
- The correct amino acid is loaded onto the tRNA by enzymes called aminoacyl-tRNA synthetases
- tRNA delivers its amino acid to the ribosome, ensuring the protein is built in the right order
Ribosomal RNA (rRNA) is both a structural and catalytic component of ribosomes.
- Ribosomes (made of rRNA + proteins) are the cellular machines where translation occurs
- rRNA catalyzes the formation of peptide bonds between amino acids, making it a ribozyme (an RNA molecule with enzymatic activity)
- It also helps align mRNA and tRNA correctly during translation