They become water-soluble once they attach to a sugar and a phosphate to form a "nucleotide", the building block of DNA. To avoid water, the bases have to stack themselves in the center, while the sugar and phosphates stay outside see 1. But if the bases just stack themselves, this will still leave space between the bases through which water can sneak in see 1.
This space needs to be covered up. One obvious way to cover up this space is to bring in another chain to cover it up forming a straight ladder see 2. This is one of the reasons why DNA is double stranded. This still doesn't take care of the holes between the bases. One solution to get rid of the 'holes' is to skew the ladder to one side see 3. We can think about this as turning a ladder into a staircase. It turns out that although the skewed ladder closes the 'holes', it introduces a new problem.
In this arrangement, neighboring atoms bump into each other. To avoid bumping into each other, the staircase has to twist a little bit. This turns our staircase into a spiral staircase. This extra twist at the end is the reason for the helical shape. You'd think something as elegant and beautiful as the famed double helix would be easier to explain but it's not. Calladine et al. By Dr. Rama Balakrishnan, Stanford University. What is DNA made of? Featured content. Free courses.
All content. Course content. Torsional energy can be taken up by alternative DNA conformations. The fluidity of torsional stress along the DNA chain. DNA supercoiling and protein binding in the E. The DPS protein compacts the eubacterial chromosome during stress.
The histone fold and formation of the nucleosome. Nucleosomal DNA packaging into a 30 nm fibre: the role of histone H1. Core histone tail modification regulates DNA compaction. About this free course 12 hours study. Level 3: Advanced. Course rewards. Free statement of participation on completion of these courses. Create your free OpenLearn profile.
Course content Course content. Nucleic acids and chromatin Start this free course now. Free course Nucleic acids and chromatin. Answer There is no potential for hydrogen bonding between these bases.
Figure 9 The structure of B-DNA in space filling format left and with the DNA helix represented as tubes right , with the major and minor grooves indicated. Several atoms on the purine and pyrimidine bases those highlighted by yellow arrows are available for hydrogen bonding with specific amino acid side-chains in DNA binding proteins.
Long description. SAQ 9 Look back at the structure of the modified nucleoside 5-methylcytidine in Figure 4b. Answer The methyl group of 5-methylcytidine will project into the major groove. Answer 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.
In each case, a stretch of DNA 12 base pairs in length is shown. Previous Summary of Section 2. Next 3. A double helix has become the icon for many, many kinds of discussions about where science has been and where it's going. This really is an amazing structure. You can't stare at the double helix for very long without having a sense of awe about the elegance of this information molecule DNA, with its double helical form basically being the way in which all living forms are connected to each other, because they all use this same structure for conveying that information.
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.
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