The Nature Of Dna

THE NATURE OF DNA The hereditary material, DNA, has now been demonstrated in many prokaryotes and eukaryotes through an experimental procedure called transformation. This technique is generally applied to some specific character in which hereditary variants are available with two different types of phenotypic expression of that character. The nuclear localization of DNA can be established by using DNA-binding dyes such as FEULGEN or DAPI which stain the nuclear chromosome in cells and to a lesser extent also stain the mitochondria and chloroplasts. The chemical structures of the nucleic acids were elucidated by the early 1050s through the efforts of Phoebus Levine, followed by the work of Alexander Todd. With few exceptions nucleic acids are linear polymers of nucleotides whose phosphates bridge the 3’ and 5’ positions of successive sugar residues. The phosphates of these polynucleotides are acidic so that, at physiological pHs, nucleic acids are polyanions. THE THREE ROLES OF DNA Three key properties of DNA, that had to be fulfilled by hereditary material, has been elucidated by genetic studies. 1. Because essentially every cell in the body has the same genetic map, it is important that the genetic material must be faithfully replicated at every cell division. The structural features of DNA allow such faithful replication. 2. Because it must encode the constellation of proteins expressed by an organism , the genetic material must have sufficient informational content. 3. Because the hereditary changes, the so-called mutations, provide the raw material that evolutionary selection operates on, the genetic material must be able to change on rare occasions. THE DOUBLE-HELICAL NATURE OF DNA DNA’s base composition is governed by Chargaff’s rules—DNA has equal numbers of adenine and thymine residues i.e. A=T and equal numbers of guanine and cytosine residues i.e. G=C. DNA’s base composition varies widely among different organisms. It ranges from nearly 25% to 75% G+C in different species of bacteria. It is , however, more or less constant among the related species. RNA, which is usually occur as a single-stranded molecule, has no apparent constraints on its base composition. However, double-stranded RNA which comprises the genetic material of several viruses, obeys Chargaff’s rules. Conversely, in certain viruses with single-stranded DNA does not follow Chargaff’s rules. Such DNA molecules after entering into their respective host organism is replicated to form a double-stranded DNA molecule which then obeys Chargaff’s rules. DNA is composed of two side-by side chains of nucleotides twisted by virtue of hydrogen bonding into the shape of a double-helix.The backbone of each strand is consisted of a repeating units of sugar-deoxyribose sugar polymer.The sugar to phosphate bonds in this backbone are called phosphodiester bonds which is used to describe how a

nucleotide chain is organized. The carbons of sugar moiety are numbered 1’ through 5’. The linking between the 5’ carbon of a deoxyribose and the 3’ carbon of the adjacent deoxyribose is made by the phosphodiester bonds. This sugar to phosphate linking is said to have a 5’-to-3’ polarity which is crucial in understanding how DNA fulfills its roles. In the double-stranded DNA molecule these two strands are in antiparallel orientation relative to each other. So a strand of 5’-3’ direction is base paired with complementary DNA strand that is oriented in the 3’-5’ direction. The bases are attached to the 1’ carbon of each deoxyribose sugar unit in the backbone in one strand facing the inward toward a base on the other strand. This type of base pairing holds the DNA strands together in a double-helical fashion. Two types of base pairing are observed in DNA namely A-T and G-C pairing. The basis of these two types of base-pairing is hydrogen bonding. Each hydrogen atom I the amino group of purine and pyrimidine bases is slightly positive because the nitrogen atom by virtue of its electronegativity tends to attract the electrons involved in the N-H bond, thereby leaving the hydrogen atom slightly positively charged. The oxygen atom is sufficiently electronegative with its six unbonded electrons in its outer-shell. A hydrogen bond forms between a electropositive hydrogen atom and electronegative oxygen atom. Hydrogen bonds individually are quite weak (only 3% of the strength of a covalent bond) , but the large number of hydrogen bonds provide sufficient strength to hold the two strands together. This overall stability but potential for local separation or unwinding is important in initiating transcription and replication, when transient local unwinding is essential. Because the G-C pair has three hydrogen bonds, whereas A-T pairs are with two hydrogen bonds so the DNA molecules containing higher G-C content is more stable than that of containing lesser amount of G-C content. The temperature dependent separation DNA double-helix, a process called DNA melting or denaturation, depends mainly on G-C content of the respective molecule. The flat planar base-pair structures stack on top of one another at the centre of the double helix. Stacking adds to the stability of the DNA molecule by excluding the water molecules from the interior space. The repulsion of the electron cloud residing on the backbone phosphate groups destabilizes the double-helical structure, so DNA molecules are more stable dilute sodium chloride solution than pure water. Sodium ions present in the solution binds to the phosphate groups thereby neutralizing the existing electronic repulsion. Due to this base stacking and hydrogen bonding interaction two distinct sizes of the grooves are formed running around in a spiral. These are the major groove and the minor groove. DNA is a right handed helix, it has the same structure as a screw that would be screwed into place using a clockwise turning motion. The unit of measurement of DNA length is the base-pair (bp). A thousand base-pair is equal to one kilobase (kb) and a million is a megabase (Mb).