Chemical Structure of DNA; Deoxyribose nucleic acid (DNA) is a molecule containing hereditary instructions. Eye color, height and predisposition for disease are examples of the kind of information stored in human DNA. It is packed tightly into structures called chromosomes, which are housed in the nuclei of cells.
Humans typically have 46 chromosomes in their somatic (body) cells; 23 inherited from the father, and 23 inherited from the mother. Chromosomes are made up of stretches of DNA called genes. These genes usually provide cells with directions to make proteins, which in turn make the body develop and function.
DNA is a very large molecule (macromolecule). The DNA in one somatic cell would stretch out to approximately 2 metres in length if unravelled. DNA not only needs to be able to hold important instructions, it needs to be stable and able to copy accurately — its simple structure allows for both.
DNA’s Double Helix Structure
The DNA molecule has two strands that are coiled round each other, and is often compared to a twisted ladder (see images 1 and 2). Both strands carry nucleotides, which might be considered the “rungs”. These are units that are repeated linearly along the molecule, and are sometimes referred to as DNA building blocks. A single nucleotide is made up of:
- Deoxyribose (sugar)
- A base chemical
The phosphate and sugar make up the outer part of the strand, which is hence called “the sugar phosphate backbone”. The bases are on the inside of the molecule, and can be one of the following four: chemicals adenine (A), thymine (T), cytosine (C) and guanine (G). These bases — A, T, C and G — are the alphabet of the DNA code.
Complementary Base Pairing and Replication
The bases on one strand pair up with the bases on the other strand, and are joined by hydrogen bonds. Adenine and thymine fit only with each other, and cytosine fits only with guanine. Therefore, if one strand had the sequence AAGGTTCC, the opposite strand would have the sequence TTCCAAGG.
This is referred to as complementary base pairing, and is the mechanism that allows for DNA to be copied accurately. If two DNA strands were to be pulled apart, the rules of base pairing would allow for each strand to act as a template for making two double helixes.
The Stability of the DNA Molecule
DNA is a stable molecule. The hydrogen bonds that link the base pairs together are relatively weak; however, they bring stability when stacked across the molecule. A possible analogy is that of a knitted sweater, where one lone stitch might be weak, but the whole sweater of stitches holds together