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Nucleotide: Sugar, Phosphate, Nitrogenous Base

DNA vs. RNA

DNA

RNA

Phosphate Phosphate
Sugar: Deoxyribose Sugar: Ribose
Nitrogenous Bases: A (Adenine), G (Guanine), C (Cytosine), T (Thymine) Nitrogenous Bases: A (Adenine), G (Guanine), C (Cytosine), U (Uracil)
Double-stranded structure (double helix) Single-stranded structure
Genetic Material for Most Organisms Genetic Material for Some Organisms (like Viruses)

Adenine and Guanine are Purines.  Purines are double-ring structures, and thus they are big.

Cytosine, Uracil and Thymine are Pyrimidines.  Pyrimidines are single-ring structures, and thus they are small.

Chargaff's Rule:  In order to keep the diameter of the double helix consistent, a purine will always pair with a pyrimidine.       

Specifically:

DNA → DNA :  A pairs with T and C pairs with G.

DNA → RNA :  A pairs with U and C pairs with G.  T from DNA pairs with A. 

RNA → RNA :  A pairs with U and C pairs with G.

(Campbell/Reese.  Biology: 6th Ed. Copyright 2002 Pearson Ed. Inc.)