This complementary base-pairing enables the base pairs to be packed in the energetically most favorable arrangement in the interior of the double helix. 120–121) is paired with a single-ring base (a pyrimidine) A always pairs with T, and G with C ( Figure 4-4). In each case, a bulkier two-ring base (a purine see Panel 2-6, pp. Because these two chains are held together by hydrogen bonding between the bases on the different strands, all the bases are on the inside of the double helix, and the sugar-phosphate backbones are on the outside (see Figure 4-3). The three-dimensional structure of DNA- the double helix-arises from the chemical and structural features of its two polynucleotide chains. This polarity in a DNA chain is indicated by referring to one end as the 3′ end and the other as the 5′ end. Moreover, the two ends of the chain will be easily distinguishable, as one has a hole (the 3′ hydroxyl) and the other a knob (the 5′ phosphate) at its terminus. If we think of each sugar as a block with a protruding knob (the 5′ phosphate) on one side and a hole (the 3′ hydroxyl) on the other (see Figure 4-3), each completed chain, formed by interlocking knobs with holes, will have all of its subunits lined up in the same orientation. The way in which the nucleotide subunits are lined together gives a DNA strand a chemical polarity. A DNA molecule is composed of two (more.) DNA is made of four types of nucleotides, which are linked covalently into a polynucleotide chain (a DNA strand) with a sugar-phosphate backbone from which the bases (A, C, G, and T) extend. These same symbols (A, C, G, and T) are also commonly used to denote the four different nucleotides-that is, the bases with their attached sugar and phosphate groups.ĭNA and its building blocks. Because only the base differs in each of the four types of subunits, each polynucleotide chain in DNA is analogous to a necklace (the backbone) strung with four types of beads (the four bases A, C, G, and T). ![]() The nucleotides are covalently linked together in a chain through the sugars and phosphates, which thus form a “backbone” of alternating sugar-phosphate-sugar-phosphate (see Figure 4-3). ![]() In the case of the nucleotides in DNA, the sugar is deoxyribose attached to a single phosphate group (hence the name deoxyribonucleic acid), and the base may be either adenine (A), cytosine (C), guanine ( G), or thymine (T). 120-121), nucleotides are composed of a five-carbon sugar to which are attached one or more phosphate groups and a nitrogen-containing base. Hydrogen bonds between the base portions of the nucleotides hold the two chains together ( Figure 4-3). Each of these chains is known as a DNA chain, or a DNA strand. ![]() Mutagens damaged the DNA strand which leads to change in normal sequencing including base modification and double-strand breaks which is difficult to repair, as a results it eventually produced point mutations, insertions and deletion of DNA sequence.A DNA Molecule Consists of Two Complementary Chains of NucleotidesĪ DNA molecule consists of two long polynucleotide chains composed of four types of nucleotide subunits. Sugar-phosphate backbone is composed of alternating sugar and phosphate that defines directionality of the molecules which is negatively charged and hydrophilic to allow the DNA backbone to form bonds with water. The stacking interaction shows side-roll-twist, shift-tilt and rise which is more or less constant. Sugar-phosphate backbone produces energy minima matching by simple replacement of bases such as cytosine, guanine, adenine and thymine which shows mutual base position characteristic of the target sequence and it demonstrates one major degree of freedom consisting of torsion angles chi, delta, zeta and pseudorotation phase angle. The reason of this coiling structure is in order to protect the bases inside it protecting from being damaged from the environment and each turn of this helix is 34nm long in which 10 bases attached per turn. Sugar-phosphate backbone is linked to phosphodiester bond between carbon 4 and CH2 group that attached to phosphate ion thus make the DNA strand as double helix making it twist into a coil. A structural component of DNA that consists of 5-deoxyribose sugars and phosphate groups involved in carrying the genetic code.
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