Human DNA structure model A base is a derivative of purine and pyrimidine, and is a component of nucleic acids, nucleosides and nucleotides. The major bases of DNA and RNA are slightly different. The important difference is that thymine is the main pyrimidine base of DNA, which is very rare in RNA. On the contrary, uracil is the main pyrimidine of RNA, but it is rare in DNA. In addition to the major bases, there are also very few rare bases in nucleic acids. The structures of rare bases are varied, mostly the methyl derivatives of major bases. TRNA often contains more rare bases, and some tRNA has a 10% base. Purine and pyrimidine bases are near-planar molecules, relatively insoluble in water: they have strong absorption in the ultraviolet region of about 260 nm. Objective: what is the three-dimensional structure of 1.DNA? 2. what are the basic units of DNA? 3. What are the principles for 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 也就是说,嘌呤碱基总数等于嘧啶碱基总数,各占全部碱基总数的50%. Rule 2: In a double-stranded DNA molecule, the ratio of 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 one strand of DNA, the ratio of the sum of two uncomplementary bases is equal to the reciprocal of the ratio in the other complementary strand, that is, the ratio in one strand of DNA is equal to the reciprocal of the 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 transcripted 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 of (A T) / (G C) of the sum of complementary pairs of bases in DNA molecules of different organisms is different, which represents the specificity of DNA molecules in each organism.