DNA is composed of nucleotides that form two helical strands bonded together. Each nucleotide contains a nitrogenous base, a sugar (deoxyribose), and a phosphate group. The nitrogenous bases on each strand bond with the bases on the other strand through hydrogen bonds, with adenine bonding with thymine and cytosine bonding with guanine. Together, the sugar-phosphate backbone and bonded nitrogenous bases form the double helix structure of DNA, which carries the genetic instructions for living organisms.

1. DNA STRUCTURE
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2. Overview
■ DNA is a thread-like chain of nucleotides carrying the genetic instructions used in the
growth, development, functioning and reproduction of all known living organisms and
many viruses.
■ DNA is nucleic acid, which consist of two bipolar strands coiled around each other to
form a double helix.
■ The two DNA strands are called polynucleotides since they are composed of simpler
monomer units called nucleotides.
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3. DNA
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4. Nucleotides
■ They are the building blocks of nucleic acids.
■ They are composed of:
– Nitrogenous base
– Sugar (deoxyribose)
– Phosphate
■ A nucleoside is a nitrogenous base and a 5-carbon sugar.Thus a nucleoside plus a
phosphate group yields a nucleotide.
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5. Nucleotide Structure
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6. Nitrogenous Bases
■ Divided into two groups:
– Pyrimidines (thymine and cytosine)
– Purines (adenine and guanine)
■ They are bound together, according to base pairing rules, with hydrogen bonds to
make double-stranded DNA.
■ ‘A’ pairs with ‘T’ and ‘C’ pairs with ‘G’.
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7. Nitrogenous Base Structure
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8. Deoxyribose
■ It is a monosaccharide with idealized formula H−(C=O)−(CH2)−(CHOH)3−H.
■ The nitrogenous base is attached to the 1′ ribose carbon.The 5′ hydroxyl of each
deoxyribose unit is replaced by a phosphate that is attached to the 3′ carbon of the
deoxyribose in the preceding unit.
■ Deoxyribose is generated from ribose 5-phosphate by enzymes called ribonucleotide
reductases.
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9. Deoxyribose Structure
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10. Phosphate
■ The sugar-phosphate backbone forms the structural framework of nucleic acids.
■ This backbone is composed of alternating sugar and phosphate groups, and defines
directionality of the molecule.
■ The sugar-phosphate backbone is negatively charged and hydrophilic, which allows
the DNA backbone to form bonds with water.
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11. Phosphate Structure
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12. Chromosomes
■ Within eukaryotic cells DNA is organized into long structures called chromosomes.
■ Most eukaryotic chromosomes include packaging proteins which, aided by chaperone
proteins, bind to and condense the DNA molecule to prevent it from becoming an
unmanageable tangle.
■ Within the eukaryotic chromosomes, chromatin proteins such as histones compact
and organize DNA.
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13. Chromosome Structure
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14. Properties
■ The structure of DNA is dynamic along its length, being capable of coiling into tight
loops, and other shapes.
■ In all species it is composed of two helical chains, bound to each other by hydrogen
bonds.
■ Both chains are coiled round the same axis, and have the same pitch of 3.4
nanometres.
■ The pair of chains has a radius of 1 nanometre.
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