Understanding the Basics of the DNA Structure
DNA is a very large macromolecule composed of smaller building blocks. We can consider the structural features of DNA at different levels of complexity.
- Nucleotides are the building blocks of DNA (and RNA)
- A strand of DNA (or RNA) is formed by the covalent linkage of nucleotides in a linear manner.
- Two strands of DNA can hydrogen-bond with each other to form a double helix. In a DNA double helix, two DNA strands are twisted together to form a structure that resembles a spiral staircase.
- In living cells, DNA is associated with an array of different proteins to form chromosomes. The association of proteins with DNA organizes the long strands into a compact structure.
- A genome is the complete complement of an organism’s genetic material.
For Example: the genome of most bacteria is a single circular chromosome, while the genomes of eukaryotic species are ses of linear chromosomes.
Nucleotides Contain a Phosphate, a Sugar and a Base
Five different bases are found in nucleotides, although any given nucleotide contains only one base. The five bases are subdivided into two categories, the purines and the pyrimidines, due to differences in their structures.
The purine bases, adenine (A) and guanine (G), have a double-ring structure; the pyrimidine bases, cytosine (C), thymine (T), and uracil (U), have a single-ring structure.
Thymine is found only in DNA, while uracil is found only in RNA. Adenine, guanine, and cytosine are found in both DNA and RNA.