12.2 The Structure of DNA

The Components of DNA

Deoxyribonucleic acid (DNA) is a nucleic acid molecule consisting of nucleotides connected by covalent bonds into long chains.

Nucleic acids are long somewhat acidic molecules first discovered in cell nuclei. They are made up of small sub-units, called nucleotides. The nucleotides of DNA are made up of three main parts: deoxyribose (a 5-carbon compound), a phosphate group and a nitrogenous base.

DNA has four different nitrogenous bases: adenine (A), guanine (G), cytosine (C) and thymine (T). The nucleotides in DNA are connected by covalent bonds between the deoxyribose of one nucleotide and the phosphate group of the next. The bases are connected to the deoxyribose and stick out sideways. The bases are connected to each other as well. The nucleotides can be arranged in many different combinations, enabling them to carry genetic information.

Figure 12-2a, The Four Nitrogenous Bases of DNA: Adenine, Guanine, Cytosine and Thymine

Solving the Structure of DNA

Another important part of DNA is how it is arranged in 3D.

Erwin Chargaff (Austria-USA) found out that the ratio between adenine and thymine was more or less one to one in any piece of DNA, and the relationship between guanine and cytosine was the same. This discovery lead to “Chargaff’s Rule”: [A]=[T] and [G]=[C].

During the first part of the 1950’s Rosalind Franklin (Britain) started studying the structure of DNA using X-ray diffraction. She used an X-ray to get photographs of stretched out DNA in a thin glass tube. One photograph taken in May of 1952 would become very important later. The photograph showed that DNA was shaped in a helix (coil), that there were two strands in the structure, and that the bases were located in the centre of the DNA molecule.

James Watson (USA) and Francis Crick (Britain) were studying the structure of DNA at this time as well. They used cardboard and wire to model the shape of DNA, but never quite managed to do it. In 1953, Watson saw Franklin’s X-ray photograph. Using the clues from the photo Watson and Crick created a model that showed the specific shape and properties of DNA. This model was the double helix.

The Double-Helix Model

A double helix looks like a coiled latter. In DNA, two chains of nucleotides are coiled around each other, explaining Chargaff’s rule and how the strands are connected.

Figure 12-2b, DNA

One interesting part of the double helix is that the strands go in opposite directions; they are antiparallel, allowing all the bases to come together at the centre. Each chain of double helix carries a number of nucleotides, put together similar to a four-letter alphabet.

Watson and Crick could not explain what held the two strands in the double helix together in the beginning. After a while, they realised that hydrogen bonds could be created between some nitrogenous bases. This provided little, but enough, force to keep the double helix together. There is a reason for there being a weak force though; DNA must be able to separate.

The hydrogen bonds can, however, only be formed between A and T, and G and C. This is called base pairing and proves Chargaff’s rule.




12-2a The Four Nitrogenous Bases of DNA: Adenine, Guanine, Cytosine and Thymine. Digital image. WEBSITE FOR DR. KAISER’S MICROBIOLOGY COURSE. 02 Nov. 2005. Web. 08 Dec. 2012. <http://student.ccbcmd.edu/~gkaiser/goshp.html>.

12-2b DNA. Digital image. Forbes. 22 Nov. 2011. Web. 08 Dec. 2012. <http://www.forbes.com/sites/daviddisalvo/2011/11/22/whats-your-dna-worth/>.