There are two types of nucleic acids, DNA and RNA. Remember what you learnt in unit 2 - both DNA and RNA are polymers of nucleotides. (ie. Chains of joined nucleotides). They form genetic material and are involved in the functioning of chromosomes and in protein synthesis.
VIDEO: DNA Structure
iii) Nitrogen base:
Guanine always bonds to Cytosine (Three hydrogen bonds)
This bonding of bases is called Complementary Base Pairing. The bases cannot bond any other way because 2 purines would overlap and 2pyrimidines would be too short to form the rungs of the ladder. The double strand is held in place by hydrogen bonds between the bases. It is the number and order as well as the type of the bases that determine what kind of organism will develop.
As a DNA strand lengthens, it twists into a double spiral called a Double Helix.