DNA Structure

The structure of the DNA double helix was proposed by James Watson and Francis Crick in 1953. They received the Nobel Prize for this discovery in 1962. This discovery of theirs is considered a major breakthrough in the progress of modern biology.

The basic structure of DNA is a double helix like a twisted ladder, complete with rungs. The Watson - Crick Model of DNA is now called B-DNA. There are several other conformations like Z-DNA and A-DNA.

Features of B-DNA

The B-DNA is a right-handed double helix. It is made up of two polydeoxyribonucleotide strands that are wrapped around each other on a common axis. The two strands are anti-parallel to each other with one running from 5' to 3'and the other running in the 3' to 5' direction. The turns on the double helix occur every 34A˚ or 3.4 nm after ten pairs of nucleotides, this is called the pitch; the distance between each pair 3.4A˚. The deoxyribonucleotides on each strand are linked to each other by phosphodiester bonds.

The twisting of these strands is in such a way that the grooves that form at the turns are of two types-major and minor. When proteins have to interact with the DNA, for example, during replication, these grooves come in handy. The proteins can attach to the DNA at the grooves without disturbing the strands too much structurally.

Each of these strands has a hydrophilic deoxyribose phosphate backbone with the hydrophobic bases stacked up on the inside. The deoxyribose bases that make up one strand bond with the bases on the other strand with either two or three hydrogen bonds. Adenine pairs with Thymine with the help of two hydrogen bonds while Guanine and Cytosine pair up using three. The GC bonds are stronger than the AT bonds. Since the deoxyribose bases pair up with complimentary not similar bases, the two strands are not identical to each other.

It is not possible for the other base pairing combinations to occur- for example, AG or TC or AG. This is because adenine and guanine are purines and these can only pair with pyrimidines thymine and cytosine respectively. Two purines would be too large to be fitted into the helix while two pyrimidines will not be close enough to form bonds. The complimentary base pairing is in accordance to Chargaff's rule which can simply be represented as A=T and GΞC.

The genetic information is stored on one of these strands. This is called the template strand or the sense strand. The sequence of these bases on the DNA backbone form the blueprint or recipe that carries all the information required to form components of a cell like proteins and RNA. The segments of DNA with this information are called genes. This information is called genetic information. The other strand is called the antisense strand.

These are the general characteristics of the B-DNA which is the predominant form under physiological conditions. There are at least five other forms that have been identified like A-DNA, C-DNA, D-DNA, E-DNA and Z-DNA.

Other conformations of DNA

The A-DNA is also a right-handed helix. However, it can accommodate eleven bases per turn compared to B-DNA's ten. The distance between two turns is 3.2nm and the distance between each base pair is 0.29nm.

The Z-DNA is a left-handed helix which can accommodate twelve bases per turn. The distance between two turns is 4.5nm and the distance between two base pairs is 0.37nm. The biological significance of the Z-DNA is not known but, it is being believed that it may be necessary during transcription.

DNA is in this relaxed double helical state only when the processes of transcription or replication are underway. At other times, it undergoes heavy coiling known as supercoiling. If the double helix is twisted in the same direction as the helix, it is called as positive supercoiling. The bases are held closer to each other. If the helix is coiled in the direction opposite to the direction of the helix, the bases are loosely held and can be easily accessed. This is called as negative supercoiling.

Other than the double helix structures, DNA can also exist in various other structures. This is essential for the recognition of DNA by proteins in some specific cases. These can include triple stranded DNA, four-stranded DNA, and bent DNA.

Triple stranded DNA will be less stable that the double stranded structure. This is because it will have extra bonds between the bases. This will result in more repulsion between the three strands. The four-stranded DNA contains a large amount of Guanine residues.

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