The building blocks of DNA are four different nucleotides, represented by the letters that signify the nitrogen bases they contain - Adenine, Thymine, Cytosine, and Guanine. You will have had experience in transcribing the code in DNA into a code in mRNA and then translating that into a polypeptide at ribosomes. Of course you will also revise all the fine detail that I have omitted from the story. An understanding of DNA is necessary to fully understand much of the second area of study for the CAT 3 examination. This area, Variation, Natural Selection and Evolution, can be seen as the central organising principle that biologists use to understand the world.
Variation is at the core of evolution and selection.
Selection of dogs for specialised tasks would not be possible if all dogs were genetically identical. Fear about the reduction of diversity in many plants and animal species is related to the knowledge that as variability in a species is reduced then the ability of that species to respond to changes in the environment is also reduced. This will become clearer to you if you consider the usual steps considered in natural selection. Variation exists among members of a species. In adverse conditions in the environment, some individuals may die but others will survive.