Why dna helical

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Next Video. Embed Share. Deoxyribonucleic acid, or DNA, consists of two polynucleotide chains. 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.

It should be noted that DNA is capable of adopting other higher-order structures, particularly in vitro , and we will now discuss some of these structures that may have biological roles to play. In the cell, DNA is, of course, found complexed with a variety of cellular proteins, many of which contribute to higher-order structures where the basic helix is folded into more condensed states. DNA can loop around itself and around proteins specific for this role. A number of different conformations and structures adopted by this versatile molecule have been identified and characterised, many of which have a particular purpose within the cell.

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Skip to main content. Search for free courses, interactives, videos and more! Free learning from The Open University. The double helix is a description of the molecular shape of a double-stranded DNA molecule. However, a crucial contribution that enabled this discovery was made by Rosalind Franklin, who was not acknowledged at that time.

After her death, Crick said that her contribution had been critical. The double helix describes the appearance of double-stranded DNA, which is composed of two linear strands that run opposite to each other, or anti-parallel, and twist together. Each DNA strand within the double helix is a long, linear molecule made of smaller units called nucleotides that form a chain.