The DNA Model
While cells are the fundamental unit of life, at their centre is a nucleus that contains a complex molecular structure called deoxyribonucleic acid (DNA), which carries the genetic information for all cellular forms of life. Therefore, the DNA model is the first topic to be addressed, although the diagram below attempts to provide some initial overview, this is only the start of a very complex set of structures and interactions.
DNA is a class of molecules formed from a long chain of nucleotides. Each nucleotide consists of three components: a base made of cytosine (C), guanine (G), adenine (A) or thymine (T) connected to a sugar molecule, which in turn is connected to a phosphate molecule. The connection between the sugar and phosphate molecules forms the backbone of one half of the DNA strand of nucleotides.
However, the ability of DNA to store, and ultimately to transmit, information is linked to the way each strand of DNA nucleotides is connected to a paired strand of DNA nucleotides via each base component, which then helps to form the double-helix structure of DNA. The bases link across the two strands in just two permutations, i.e. cytosine (C) with guanine (G) and adenine (A) with thymine (T).
The structure of DNA, as outlined above, helps explain two key properties:
- It can be replicated with each strand acting as a template for
its complementary strand.
- It can store information in terms of the sequence of the nucleotides along each strand.
The specific ordering of sections of base sequences along a single strand of DNA constitutes the genetic code in the form of a gene structure. The gene is normally considered to be the basic physical and functional unit of heredity, which consists of a specific sequence of nucleotides at a given position on a given chromosome. The nucleus of each cell in the human body contains approximately 1.8 metres of DNA in total, which is sub-divided into structures called chromosomes. The DNA within a chromosome is wound around proteins called histone, which provide structural support and play a role in controlling the activities of the genes. A strand 150 to 200 nucleotides long is typically wrapped around a core of eight histone proteins to form a structure called a nucleosome.