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. Actively scan device characteristics for identification. Use precise geolocation data. Select personalised content. Create a personalised content profile. Measure ad performance.
Select basic ads. Create a personalised ads profile. Select personalised ads. Apply market research to generate audience insights. Measure content performance. Develop and improve products. List of Partners vendors. Share Flipboard Email. Regina Bailey. Biology Expert. Regina Bailey is a board-certified registered nurse, science writer and educator. Updated February 07, Of course, this is Watson and Crick's incredible realization back in , but it will stand in history as probably one of the most significant scientific moments of all time.
Francis S. Collins, M. Featured Content. Having outlined the general principles of nucleic acid structures, we will now focus on how these principles influence the formation of specific structures found in DNA. The helical structure of DNA arises because of the specific interactions between bases and the non-specific hydrophobic effects described earlier.
Its structure is also determined through its active synthesis; that is, duplex DNA is synthesised by specialist polymerases upon a template strand. We outlined earlier the general principle of base pairing. Now we will go into more detail about why these base pairs arise within a duplex DNA.
The specificity of Watson-Crick base pairing results from both the hydrogen-bonding factors we described earlier and steric restrictions imposed by the two deoxyribose—phosphate backbones. Thus spatial considerations limit each base pair to being between a purine and a pyrimidine. Note that mispairings, such as that between two purines, do occur during replication, but such mispairings distort the helix and are readily detected and corrected. Apart from the spatial considerations, specific requirements apply for the formation of hydrogen bonds between the bases in helical DNA, and the final positions of the hydrogen atoms within each base pair will be influenced by the positions of the bases after stacking interactions have occurred.
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.
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