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| Deoxyribose Nucleic Acid (DNA) |
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| With the exception of a few viruses, DNA forms the genetic material in all living organisms. In the prokaryotic cells DNA occurs in the cytoplasm. In the eukaryotic cells it is confined largely to the nucleus and forms the main component of chromosomes. A small amount of DNA is found in the cytoplasm in the organelles like mitochondria and plastids. It is called extra- nuclear DNA. |
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| DNA is a polynucleotide of adenine, guanine, cytosine and thymine. Uracil nucleotides are absent. The DNA molecule is composed of two polynucleotide chains. |
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| The structure of the DNA molecule and the arrangement of nitrogenous bases in the two polynucleotide chains, is very well explained by the double helix model proposed by Watson and Crick in 1953. |
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| According to the double helix model, DNA molecule shows the following salient features |
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 The two polynucleotide chains are coiled around each other like a spiral stair case (double helix). |
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 The cross-rungs (steps) are formed by the nitrogen bases while phosphates and sugars form the uprights. |
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 The two-polynucleotide chains run in opposite directions (antiparallel). One chain runs in the 3-5 direction. While for the other chain 3-5 direction is opposite. |
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 The average distance between the two chains is 20A0. |
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 One full turn of the helix, called gyre, measures 34A0 in length. |
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 The distance between two successive sugar molecules is 3.4A0. Thus each gyre accommodates 10 nucleotides. |
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| fig. 14.8 - Double Helix of DNA |
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| fig. 14.9 - Base Pairing in DNA |
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 The nitrogen bases of the two opposite chains exhibit highly specific base pairing. A purine in one chain always pairs with a pyrimidine in the opposite chain. |
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 Among purines adenine(A) pairs only with the pyrimidine thymine (T)and vice versa. Similarly guanine (G) pairs only with cytosine (C) and vice versa. |
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 There are two weak hydrogen bonds between A and T or T and A. There are three weak hydrogen bonds between G and C or C and G. |
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 The total amount of purines is equal to the total amount of pyrimidines (A+G)=(C+T). |
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 The two polynucleotide chains of DNA molecule are not identical to each other but complementary to each other. |
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| The fact that the total amount of purines will be equal to the total amount of pyrimidines, was first enunciated by Chargaff in 1950. |
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| The most significant feature of DNA molecule is its capacity to make its own identical copies. This property is called replication. It occurs regularly during the synthetic phase of the cell cycle, every time a cell divides. |
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| During replication, the DNA molecule unwinds itself from one end. The two chains separate. With the help of an enzyme called DNA polymerase free nucleotides of DNA become attached to the exposed nitrogen bases of the two chains, as per the complementary base pairing. The uncoiling and complementary base pairing continues in the 5-3 direction till the other end of the DNA molecule is reached. Thus, each separated chain develops a new complementary chain. Each of the resulting daughter molecules has one complete strand received from the parent molecule while the other one is newly synthesised. Hence, replication is described as semiconservative type. |
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| fig. 14.10 - Replication of the DNA double helix |
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| DNA serves the following functions: |
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 It is the genetic material in all prokaryotes and eukaryotes. |
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 It is capable of replication by which it can be faithfully passed on to successive generations. |
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 It is involved in the synthesis of RNA. |
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 It provides the code for protein biosynthesis. |
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 It is involved in mutations and genetic recombinations, which bring about variations. |
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