To understand the Double Helix, one must first understand the function of DNA.
In the beginning of the 1950s, biologists knew that DNA carried the pattern of heredity. The DNA molecule looks like a spiral ladder (geneticists call this shape a double helix), but unlike any ladder you’ve ever seen, the rungs are formed by base molecules, which occur in pairs. These sequences of base pairs embody the genetic coding that determines the composition of the organism in question (always hated your nose? Blame your DNA).
DNA is the holder of all the information needed to recreate a living being. It is made up of bases, as mentioned before, which consist of sugar, phosphate and nitrogen. There are four nucleotides that are given one letter abbreviations as shorthand for the four bases (remember, these form the “rungs” of the double helix ladder).
A-adenine
T-thymine
G-guanine
C-cytosine
There are two of these bases on each “rung” of the “ladder.” Each of these bases can only pair with one other base: adenine always pairs with thymine, and guanine with cytosine. The result is the thousands of DNA strands that are in our bodies. Of course, these sequences are not the only thing an organism is made of, but they are found in every cell inside the organism and they tell the organism how to form. As part of the coding process, DNA is transcribed into mRNA, which is then translated into the proteins that form an organism. DNA is made of nucleotides, and each nucleotide consists of a 5-carbon sugar called deoxyribose (the acronym DNA is actually short for deoxyribonucleic acid…fun to say, isn’t it?).
The two strands along the sides of DNA, or the “sides” of the ladder, consist of two polynucleotide chains held together by weak thermodynamic forces—the strands are held together only by H-bonds and by hydrophobic interactions between each pair of bases. These strands, along with the bases discussed earlier, form the entire DNA molecule. Within the DNA helix, the adenine/thymine bond is made of two hydrogen bonds, and guanine forms three hydrogen bonds with cytosine.
So, to sum it up, the double helix includes the following features:
- Two DNA strands that form a helical spiral, winding around a helix axis in right-handed spiral.
- The two polynucleotide chains run in opposite directions.
- The sugar-phosphate backbones of the two DNA strands wind around the helix like the railing on a spiral staircase.
- The bases of the single nucleotides are on the inside of the helix and are stacked on top of each other like the steps in a spiral staircase.
The helix axis is most visible from a view directly down the axis. The sugar-phosphate backbone is on the outside of the helix where the polar phosphate groups work together with the polar environment. The nitrogen containing bases are within the structure, stacking perpendicular to the helix axis.
The most important feature of the double helix regarding function is that all the information in it is redundant. For example if one base is lost, the complementary base on the opposite strand still contains the information. This is the basis of one strand DNA repair. In this course a damaged or missing base is replaced using the information on the opposed strand. The redundancy of the helix is what makes the replication process possible.
The structure was first described by James Watson and Francis Crick in 1953 and is now the most famous structure of genetics. Today we know that DNA determines each organism’s physical characteristics—both those that are visible and those that are not (visible traits are known as phenotypes; but a trait may not show up in the phenotype even if it is still present in the genotype). We now know that there are 35,000 genes in each human DNA molecule, together comprised of over 3 billion precise sequences. It was not until 2001 that the Human Genome Project had gotten fully underway, but we have this project to thank for the extent of DNA knowledge we possess today.
By Jessica Maughan