The sympathetic nervous systemâs fight-or-flight response is your built-in mechanism for safely escaping a life-threatening emergency, such as a lion lurking behind a bush.
The basic building block of DNA is called a NUCLEOTIDE. A nucleotide is made up of one sugar molecule, one phosphate molecule and one of the four bases. Here is the structural formula for the four nucleotides of DNA. Note that the purine bases (adenine and guanine) have a double ring structure while the pyrimidine bases (thymine and cytosine) have only a single ring.
The nucleotides of DNA line up so that the sugar and phosphate molecules make two long backbones like the handrails of a ladder. To make the rungs of the ladder, two bases join together, between the sugar molecules on the two handrails. The phosphate molecules do not have any rungs between them.