Introductory Biology

9.2 The Structure of DNA

DNA lasts for your entire life carrying your information. DNA is structured in a spiral shape science that is its comfortable position. THe DNA back bone repeats from sugar to phosphate and are planar, stack up on top of another. DNA is composed of nucleotides subunits which are: deoxyribose sugar, phosphate group, and nitrogen-containing bases. The nucleotide is made up of phosphate, sugar and base. There is always a phosphate group because its an ion (positively/negatively charged) and attracted to water.

According to a magazine called Nature, “it has not escaped our notice that the specific pairing we have postulated to suppose to be true immediately suggested a possible copying mechanism for the genetic material.”

There are two types of nitrogen-containing bases: purines and pyrimidines. Adenine (A) and Guanine (G) are purines. As for Cytosine (C), Thymine (T), and Uracil (U) are pyrimidines. The factors of pyrimidines can be easily remembered from their similar pie shape and since the pie needs to be cut into pieces the “CUT” abbreviation may help as well.

Many scientists researched on the structure of DNA. Chargaff’s rule is A=T and G=C. Chargaff found that (A) and (T) are in a 1:1 ration and are in equal proportions so they may come together, but later he found that they are both the same.  Wilkins and Franklin’s X-ray diffraction photographs allowed them to find that DNA is either a double or triple helix of the repeating nucleotides. Linus Pauling was a chemist who knew about the helix and studied DNA with the X-ray. He stole Rosie Franklin’s photographs from the X-ray which helped him with his research on DNA. He found that the diameter does not vary: 10 nucleotides per turn, 34 angstroms (3.4 nanometers) is equaled to one measure of the height of one helical turn, 3.4 angstroms (0.34 nanometers) is the distance between one individual, and the measurement from the center to the edge/ diameter is 2 nanometers. He built a model of the triple helix, which was a stupid mistake because it all broke apart when water was combined with it. After his mistake, he started complementary base-pairing by making paper cut outs of the groups (G, T, A, C) and tried to pair them up, as he followed the Chargaff’s rule. When he added purine + purine it was too wide. When pyrimidine + pyrimidine was added, it was too narrow. Finally when he added purine + pyrimidine it was just right and the width was consistent with the x-ray data. CG and AT only fit if they are put in opposite ways and the weak hydrogen bond between them make a perfect fit. They are complementary in the charge: the AT is stabilized by 2 hydrogen bonds as for GC is stabilized by 3 hydrogen bonds. The molecule needs to be stable, able to code information, able to self-duplicate, capable of change and modification, and able to store a lot of information, as well.

 

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One thought on “9.2 The Structure of DNA

  1. Yurika

    Great job of summarizing 9.2 on the structure of DNA. You’ve covered all the main points and many of the details well.

    Mr. F.

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