Nucleic Acids: DNA and RNA Structure and Function
Nucleic Acids: Components
DNA
Deoxyribonucleic acid (DNA) is a macromolecule composed of nucleotide chains. It consists of two nucleotide chains. The base pairs are adenine (A) with thymine (T), and guanine (G) with cytosine (C). DNA is highly conserved across organisms, with the exception of some viruses. In eukaryotic cells, DNA resides within the nucleus. In prokaryotic cells, it’s found freely in the cytoplasm. In chloroplasts and mitochondria, DNA is associated with histone-like proteins, similar to how prokaryotic DNA interacts with proteins. Mitochondrial and chloroplast DNA share similarities with bacterial DNA.
DNA Structure
Primary Structure
The primary structure of DNA is its nucleotide sequence, which carries the genetic information.
Secondary Structure
The secondary structure is the double helix, formed by two coiled polynucleotide chains. Sugars and phosphates are on the periphery, while nitrogenous bases are inside, pairing A with T (two hydrogen bonds) and G with C (three hydrogen bonds). The strands are complementary, antiparallel (5′ to 3′ and 3′ to 5′), and plectonemically wound. They won’t separate unless there’s structural distortion.
Tertiary Structure
Bacterial DNA adopts a spatial arrangement, sometimes without histones, called supercoiling, due to tension from varying the number of double helix turns.
DNA Packaging
- 100 Å Chromatin Fiber (Nucleosomes): Found in resting cells. Consists of 100 Å diameter particles connected by DNA. Each particle, along with its spacer DNA, forms a nucleosome. Nucleosomes contain a histone octamer and 146 base pairs of DNA wrapped around it. Spacer DNA is about 54 base pairs long. A nucleosome with histone H1 is called a chromatosome.
- Sperm Chromatin: DNA associates with protamines, forming a crystalline structure.
- 300 Å Chromatin Fiber: The 100 Å fiber coils upon itself, with about six nucleosomes per turn. H1 histones cluster to form the fiber’s core.
Types of DNA
- Single-stranded DNA: Found in parvoviruses, linear or circular.
- Double-stranded DNA: Most common, often supercoiled. Linear in eukaryotic cells and some viruses (e.g., T4), circular in mitochondria, chloroplasts, and bacteria.
DNA can be associated with histones and protamines.
RNA
Ribonucleic acid (RNA) is a single-stranded molecule composed of nucleotides with ribose sugar and the bases A, C, G, and uracil (U). It also contains phosphate groups. Ribonucleotides are linked by phosphodiester bonds in the 5′ to 3′ direction. RNA’s catalytic function suggests it may have predated DNA in early life.
Types of RNA
- Transfer RNA (tRNA): Cloverleaf-shaped, with D, T, and anticodon arms, and an amino acid acceptor arm. About 50 types exist, transporting amino acids to ribosomes for protein synthesis.
- Messenger RNA (mRNA): Structure differs in prokaryotes and eukaryotes. Eukaryotic mRNA undergoes maturation, including intron removal, addition of a 5′ cap, and a 3′ poly-A tail for stability. Prokaryotic mRNA lacks the cap and tail and is polycistronic (codes for multiple proteins). mRNA carries genetic information from DNA to ribosomes.
- Ribosomal RNA (rRNA): Part of ribosomes. Prokaryotic ribosomes (70S) have 23S and 5S rRNA in the large subunit and 16S rRNA in the small subunit. Eukaryotic ribosomes (80S) have 28S and 5.8S rRNA in the large subunit and 18S rRNA in the small subunit.