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Human DNA Structure Model

Bases refer to derivatives of purine and pyrimidine, and they are components of nucleic acids, nucleosides, and nucleotides. The main bases in DNA and RNA differ slightly, with the key distinction being that thymine is the primary pyrimidine base in DNA and is very rare in RNA; conversely, uracil is the primary pyrimidine base in RNA, while it is scarce in DNA.
Besides the major bases, nucleic acids also contain some rare bases that are present in very small amounts. The structures of rare bases are diverse, and most of them are methyl derivatives of the major bases. tRNA often contains more rare bases; some tRNA contain up to 10% rare bases. Purine and pyrimidine bases are nearly planar molecules and are relatively insoluble in water: they exhibit strong absorption in the UV region around 260 nanometers.

Experiment Objective: 1. What is the three-dimensional structure of DNA?

2. What are the basic units that make up DNA?

3. What principles govern base pairing?

Experiment Report:

Rule 1: In a double-stranded DNA molecule, A = T and G = C. That is: A + G = T + C or A + C = T + G.
5 types of bases
5 types of bases
In other words, the total number of purine bases equals the total number of pyrimidine bases, each accounting for 50% of the total base count.
Rule 2: In double-stranded DNA molecules, the ratio of the sum of the complementary base pairs in both strands is equal to the same 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 single strand of DNA molecule, the ratio of the sum of two non-complementary bases equals the reciprocal of this ratio in the complementary strand. In other words, the ratio in one strand of DNA equals the reciprocal of that ratio in its complementary strand. (A1+G1)/(T1+C1)=(T2+C2)/(A2+G2).
Rule 4: In double-stranded DNA molecules, the ratio of complementary bases to the total bases is equal to the ratio of any single strand to those bases, and also equal to the ratio in the mRNA transcribed from it. That is, (A+T)% or (G+C)% of the double strand equals (A+U)% or (G+C)% of any single strand, which equals (A+U)% or (G+C)% in the mRNA.
Rule 5: In the DNA molecules of different organisms, the ratio of complementary base pairs (A+T)/(G+C) varies, reflecting the specificity of each organism’s DNA molecule.