<span>Cytosine is a building block of DNA, it is a component to DNAs base pairs and pairs with guanine (these pairs make up the rungs of its double helix)... it is not Ribose as plain Ribose has a hydroxyl group where a hydrogen atom should be if it were to be found in DNA and be deoxyribose (hence deoxyribonucleic acid and not ribonucleic acid since ribonucleic acid is RNA). Uracil is only found in RNA and actually takes Cytosine's place in the rungs of RNA and binds with guanine (although RNA is not in the conventional double helix shape and more resembles a protein in shape). Polypeptide is a long peptide chain and usually associated with proteins and not DNA....</span>
