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Genetics

Genetics. Instructor: Dr. Jihad Abdallah Lecture 9 Structure of DNA. In 1953, James Watson and Francis Crick proposed that DNA is a double-stranded molecule twisted into a helix ( the double helix structure )

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Genetics

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  1. Genetics Instructor: Dr. Jihad Abdallah Lecture 9 Structure of DNA

  2. In 1953, James Watson and Francis Crick proposed that DNA is a double-stranded molecule twisted into a helix (the double helix structure) • Each spiraling strand, comprised of a sugar-phosphate backbone and attached bases, is connected to a complementary strand by non-covalent hydrogen bonding between paired bases. • These two strands run in opposite directions to each other and are therefore anti-parallel. • The bases are adenine (A), thymine (T), cytosine (C) and guanine (G). • A and G are purines while T and C are pyrimidines • A and T are connected by two hydrogen bonds. G and C are connected by three hydrogen bonds.

  3. In the DNA molecule, the sum of Purines (A and G) is equal to the sum of the Pyrimidines (T and C) • The distance between two base pairs is 3.4 A° (Angistrom). • Each complete turn of the helix is 34 A° (10 base pairs). • In any segment of the molecule, alternating larger (major grooves) and smaller “minor grooves” are apparent along the axis. • The double helix measures 20 A° (2 nm) in diameter.

  4. DNA molecule

  5. Building units of DNA are called nucleotides

  6. (Pyrimidines ) (Purines) The double helix of the DNA is shown along with details of how the bases, sugars and phosphates connect to form the structure of the molecule.

  7. 3.4 A°

  8. Nucleotides • Nucleotides are the building blocks of DNA. Each nucleotide consists of 3 components: - Nitrogen base - Pentose sugar - Phosphate group

  9. Nitrogen bases • There are two kinds of nitrogen bases: • Purines: double ringed (include Adinine and Guanine) • Pyrimidines: single-ringed (include Thymine, Cytosine, and Uracil). Thymine is specific to DNA and Uracil is specific to RNA.

  10. Pyrimidines Purines

  11. Pentose sugars Found in RNA Found in DNA

  12. Polynucleotides • Are formed by the joining of mononucleotides. • The linkage between two mononucleotides consists of a phosphate group linked to two sugars forming a phosphodiester bond • The phosphate groups link the 3’ carbon of one deoxyribose molecule to the 5’ carbon of the next (3’ – 5’ orientation). • Joining of two mononucleotides forms a dinucleotide, joining of three nucleotides forms a trinucleotide and so on.

  13. Phosphate group forming 3` - 5` phosphodiester bond

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