The Structure of DNA

04/11/2010 § 1 Comment

A molecule of life (DNA) has to have a few characteristics in order to sustain the organism that holds it. These characteristics include: stability, complexity (which means that DNA can code information), the abilities to self-duplicate, change, modify itself, and store a lot of information.

The structure of DNA was recorded and finalized years ago by James Watson and Francis Crick (who were two very different men, but had the same goal). They knew (from past research experiments and reports written by people like Alfred Hershey, Martha Chase and Oswalt Avery) that DNA is responsible for carrying the genetic information that codes for an organism. They then asked further questions like how is DNA structured, what does DNA look like, how does DNA code itself and how does DNA copy information?

DNA is made out of three important subunits. A DNA molecule is made of a phosphate group, a five-carbon sugar and a base. A sugar and a phosphate make a sugar-phosphate, but if a sugar and base combine, a nucleoside is made. When we add the last subunit, the phosphate group, to the nucleoside then we get a nucleotide. (The phosphate group, sugar and base combined.)

The phosphate group and sugars are simply structural subunits that primarily hold the strands of DNA together. They are like the side rails of the DNA ladder. (Because the structure of DNA is like a spiraling ladder.) The patterns of phosphate-sugar-phosphate-sugar and on and on make the long chains or links that make DNA molecules very long and are held by covalent bonds. The bases, on the other hand, are like the rungs of the ladder and are the structures that code for an organism’s genetic information. There are five types of bases, all of which appear in DNA with the exception of uracil. The main four (that are in DNA) are adenine, guanine, cytosine and thymine. Adenine and guanine are purines, which are double-ringed structures. Cytosine and thymine are pyrimidines, structures that are single-ringed. Bases on two DNA strands are held by hydrogen bonds, which enable the DNA molecule to be “unzipped” and the information copied when necessary. As mentioned just previously, DNA has bases A, G, C and T but not U. RNA, on the other hand, has A, G, C, but replaces T with U. These bases code for the information an organism needs to make the appropriate proteins.

Prior to the findings of Watson’s and Crick’s DNA-model, Ervin Chargaff had first discovered (but had not noticed its importance immediately) that in DNA, somehow, the amount of adenine would always equal the amount of thymine and the amount of cytosine would always equal the amount of guanine. This important rule (known as Chargaff’s Rule) applies to all organisms, not just humans and animals. Ultimately, in all types of DNA universally, adenine = thymine and cytosine = guanine. This is complementary base pairing, meaning that each DNA strand is complementary to each other and holds the same information.

Watson and Crick also used measurements from Rosalind Franklin’s and Maurice Wilkin’s X-Ray images. With the measurements (a .34nm distance between two nucleotides and a 3.4nm distance for one full turn of the helix), Watson and Crick concluded that there must be 10 nucleotides per helix turn. Fitting one purine (adenine or guanine) to its matching pyrimidine (thymine or cytosine), they were able to build their spiraling ladder.

In class, we also started to talk about how the bases (after RNA has copied the information), can tell ribosomes what proteins to make. Later, we should learn that the possibilities of bases in their positions all codes for types of amino acids that later make the proteins, which will lead us to understanding our essay question: How does information produce meaning?

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