Why does dna form a helical structure

Of course, to make a double helix, we can simply fill in the other strand. Click here for a pretty animation. We can see the same thing with real DNA. Here is one nucleotide of DNA. Think of it as a asymmetric block, as before. Here's the next one stacked on top. Continuing to add them with the same relative orientation makes a helical single strand of DNA.

It would actually be difficult to arrange the relationship of one nucleotide to the next, given the asymmetry of the molecule, for repeated additions of nucleotide not to form a helical structure. Finally, we can add the second strand. I resolved to search for this text. As his first step in this search, Chargaff set out to see whether there were any differences in DNA among different species.

After developing a new paper chromatography method for separating and identifying small amounts of organic material, Chargaff reached two major conclusions Chargaff, First, he noted that the nucleotide composition of DNA varies among species. In other words, the same nucleotides do not repeat in the same order, as proposed by Levene.

Second, Chargaff concluded that almost all DNA--no matter what organism or tissue type it comes from--maintains certain properties, even as its composition varies.

In particular, the amount of adenine A is usually similar to the amount of thymine T , and the amount of guanine G usually approximates the amount of cytosine C. This second major conclusion is now known as "Chargaff's rule. Watson and Crick's discovery was also made possible by recent advances in model building, or the assembly of possible three-dimensional structures based upon known molecular distances and bond angles, a technique advanced by American biochemist Linus Pauling.

In fact, Watson and Crick were worried that they would be "scooped" by Pauling, who proposed a different model for the three-dimensional structure of DNA just months before they did. In the end, however, Pauling's prediction was incorrect.

Using cardboard cutouts representing the individual chemical components of the four bases and other nucleotide subunits, Watson and Crick shifted molecules around on their desktops, as though putting together a puzzle.

They were misled for a while by an erroneous understanding of how the different elements in thymine and guanine specifically, the carbon, nitrogen, hydrogen, and oxygen rings were configured. Only upon the suggestion of American scientist Jerry Donohue did Watson decide to make new cardboard cutouts of the two bases, to see if perhaps a different atomic configuration would make a difference.

It did. Not only did the complementary bases now fit together perfectly i. Figure 3: The double-helical structure of DNA. Complementary bases are held together as a pair by hydrogen bonds. Figure Detail. Although scientists have made some minor changes to the Watson and Crick model, or have elaborated upon it, since its inception in , the model's four major features remain the same yet today.

These features are as follows:. One of the ways that scientists have elaborated on Watson and Crick's model is through the identification of three different conformations of the DNA double helix. In other words, the precise geometries and dimensions of the double helix can vary. The most common conformation in most living cells which is the one depicted in most diagrams of the double helix, and the one proposed by Watson and Crick is known as B-DNA.

There are also two other conformations: A-DNA , a shorter and wider form that has been found in dehydrated samples of DNA and rarely under normal physiological circumstances; and Z-DNA, a left-handed conformation. Z-DNA was first discovered in , but its existence was largely ignored until recently.

Watson and Crick were not the discoverers of DNA, but rather the first scientists to formulate an accurate description of this molecule's complex, double-helical structure. Moreover, Watson and Crick's work was directly dependent on the research of numerous scientists before them, including Friedrich Miescher, Phoebus Levene, and Erwin Chargaff. Thanks to researchers such as these, we now know a great deal about genetic structure, and we continue to make great strides in understanding the human genome and the importance of DNA to life and health.

Chargaff, E. Chemical specificity of nucleic acids and mechanism of their enzymatic degradation. Experientia 6 , — Dahm, R. Human Genetics , — Levene, P. The structure of yeast nucleic acid. Ammonia hydrolysis. Journal of Biological Chemistry 40 , — Rich, A.

Zhang, S. Z-DNA: The long road to biological function. Nature Reviews Genetics 4 , — link to article. Watson, J. A structure for deoxyribose nucleic acid. Nature , — link to article. Wolf, G. Consequently, each base pair has a well defined position. Look back at Figure 5a and try to visualise pairing between A and C bases. Describe the result. Similarly, there is only potential for formation of a single hydrogen bond between G and T bases.

To satisfy steric restrictions of base pairing and to maximise the hydrophobic interactions between successive base pairs, the two polynucleotide chains in DNA are coiled around a common axis. If you take a closer look at the sugar—phosphate backbone in B-DNA, you can see that it spirals around the core. These grooves result from the geometry of the sugar-base structure and base-pair interaction, as shown in Figure 9b. Within the major groove, a large portion of the base is exposed and it will perhaps not surprise you to learn that this is where most protein—DNA interactions occur that depend upon the specific recognition of individual bases within the DNA.

Such interactions depend upon the formation of hydrogen bonds between amino acid side-chains in the protein and atoms in the bases that are not involved in base pairing; these atoms are identified in Figure 9c. You will see later in this unit how the accessibility of bases within the major groove permits protein—DNA interactions without interfering with base pairing.

Look back at the structure of the modified nucleoside 5-methylcytidine in Figure 4b. What will be the position of the methyl group in a B-form helix? The position of the methyl group on 5-methylcytidine in DNA is significant. Projecting into the major groove as it does, this group can potentially interfere with protein—DNA interactions. We will see the importance of this interference when we consider the influence of DNA cytidine methylation in the regulation of transcription, where the methyl group can directly interfere with the binding of transcription factors.

You will notice that, when compared to the A and B forms, Z-DNA is left-handed; that is, the backbone spirals the opposite way round the helical axis from that seen in the A and B forms. Due to the kinking of the backbone, the nucleotides themselves bulge out more, leaving only one groove which is equivalent to the minor groove in B-DNA.

These different DNA secondary structures have been demonstrated from crystal structures, but what do we know about the structure of DNA in vivo , in the cell? The B form is the lowest-energy state for the DNA duplex. In its native duplex state, when not denatured for transcription, replication or repair, the helical secondary structure of DNA in the cell is generally believed to be the B form.

However, there is a degree of fluidity in the structure adopted by DNA within an active cell, and other secondary structures such as A— and Z-DNA could exist. It is difficult to demonstrate the occurrence of Z-DNA in vivo , as it is believed to form only transiently within genomic DNA, though it is thought to occur in association with transcription.

In order to further prevent the nitrogenous bases from coming into contact with cell fluid, the molecule twists to reduce space between the nitrogenous bases and the phosphate and sugar strands. The fact that the two DNA strands that form the double helix are anti-parallel helps to twist the molecule as well. Anti-parallel means that the DNA strands run in opposite directions, ensuring that the strands fit tightly together.

This reduces the potential for fluid to seep between the bases. The double-helix shape allows for DNA replication and protein synthesis to occur. In DNA replication, the double helix unwinds and each separated strand is used to synthesize a new strand.

As the new strands form, bases are paired together until two double-helix DNA molecules are formed from a single double-helix DNA molecule. DNA replication is required for the processes of mitosis and meiosis to occur. The messenger RNA molecule is then translated to produce proteins.

RNA is also a nucleic acid but contains the base uracil instead of thymine. In transcription, guanine pairs with cytosine and adenine pairs with uracil to form the RNA transcript. After transcription, the DNA closes and twists back to its original state. Determining the structure of DNA was based in part on the work of many other scientists, including Rosalind Franklin.

Molecules with a helical shape have this type of X-shape pattern. Chargoff demonstrated that the concentrations of adenine in DNA are equal to that of thymine, and concentrations of cytosine are equal to guanine. With this information, Watson and Crick were able to determine that the bonding of adenine to thymine A-T and cytosine to guanine C-G form the steps of the twisted-staircase shape of DNA.

The sugar-phosphate backbone forms the sides of the staircase.