Human DNA Structure Model A base is a derivative of purine and pyrimidine, a key component of nucleic acids, nucleosides, and nucleotides. The main bases in DNA and RNA differ slightly. The crucial difference is that thymine is the primary pyrimidine base in DNA, rare in RNA. Conversely, uracil is the main pyrimidine in RNA, but rare in DNA. Besides the main bases, there are also a few rare bases in nucleic acids. Rare base structures vary, mostly methyl derivatives of main bases. tRNA often contains more rare bases, with some tRNAs having up to 10% rare bases. Purine and pyrimidine bases are nearly planar molecules, relatively insoluble in water, and exhibit strong ultraviolet absorption around 260 nm. Objectives: Discover the three-dimensional structure of DNA. Identify the fundamental units of DNA. Understand the principles of base pairing. Experimental Report: Rule 1: In a double-stranded DNA molecule, A=T and G=C. Namely, A+G=T+C or A+C=T+G. 5 bases 5 bases That is, the total number of purine bases equals the total number of pyrimidine bases, each accounting for 50% of the total number of bases. Rule 2: In a double-stranded DNA molecule, the ratio between the sum of two complementary paired bases is equal to the ratio in each single strand of the DNA molecule. (A1+A2+T1+T2) / (G1+G2+C1+C2) = (A1+T1) / (G1+C1) = (A2+T2) / (G2+C2). Rule 3: In a strand of DNA, the ratio between the sum of two non-complementary bases is equal to the reciprocal ratio in the other complementary strand, that is, the ratio in one strand of DNA is equal to the reciprocal ratio in its complementary strand. (A1+G1) / (T1+C1) = (T2+C2) / (A2+G2). Rule 4: In a double-stranded DNA molecule, the ratio of the complementary two bases to the total base is equal to the ratio of any single strand to the base, and equal to the ratio of this ratio in the transcribed mRNA. That is, double chain (A+T)% or (G+C)% = any single chain (A+T)% or (G+C)%=mRNA in (A+U)% or (G+C)%. Rule 5: The ratio between (A T) / (G C) of the sum of complementary pairs of bases in DNA molecules from different organisms is different, representing the specificity of DNA molecules in each organism.