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DNA Structure

DNA Structure. IB Topics 3 and 7. #1. DNA Structure (an overview). DNA has three main components 1. deoxyribose (a pentose sugar) 2. base (there are four different ones) 3. phosphate. #2. The Bases. They are divided into two groups Pyrimidines and purines

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DNA Structure

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  1. DNA Structure IB Topics 3 and 7

  2. #1. DNA Structure (an overview) • DNA has three main components • 1. deoxyribose (a pentose sugar) • 2. base (there are four different ones) • 3. phosphate

  3. #2. The Bases • They are divided into two groups • Pyrimidines and purines • Pyrimidines (made of one 6 member ring) • Thymine • Cytosine • Purines (made of a 6 member ring, fused to a 5 member ring) • Adenine • Guanine • The rings are not only made of carbon (specific formulas and structures are not required for IB)

  4. #3. Nucleotide Structure • Nucleotides are formed by the condensation of a pentose sugar, phosphate and one of the 4 bases • The following illustration represents one nucleotide

  5. #3. Nucleotide Structure • Nucleotides are linked together by covalent bonds called phosphodiester linkage

  6. #4. DNA Double Helix and Hydrogen Bonding • Made of two strands of nucleotides that are joined together by hydrogen bonding • Hydrogen bonding occurs as a result of complimentary base pairing • Adenine and thymine pair up • Cytosine and guanine pair up • Each pair is connected through hydrogen bonding • Hydrogen bonding always occurs between one pyrimidine and one purine

  7. #4. DNA Double Helix and Hydrogen Bonding • Complimentary base pairing of pyrimidines and purines

  8. #4. DNA Double Helix and Hydrogen Bonding

  9. #4. DNA Double Helix and Hydrogen Bonding • Adenine always pairs with thymine because they form two H bonds with each other • Cytosine always pairs with guanine because they form three hydrogen bonds with each other

  10. #5. DNA Double Helix • The ‘backbones’ of DNA molecules are made of alternating sugar and phosphates • The ‘rungs on the ladder’ are made of bases that are hydrogen bonded to each other

  11. #6. Antiparallel strands 5’ 3’ The strands run opposite of each other. The 5’ end always has the phosphate attached. 5’ 3’

  12. Assignment (in your notebook) • 1. Draw the structure of ribose and number the carbons • 2. Draw a schematic representation of a nucleotide. Label the sugar, base and phosphate. • 3. What are the complimentary base pairs to a DNA strand that has the following order A T A C C T G A A T? • 4. Draw a schematic representation of an unwound DNA double helix using the base pairs from your answer in question 3. • Include the number of hydrogen bonds between each base pair. Be sure to label all of the bases and the 5’ and 3’ ends of the structure.

  13. #6. When phosphodiester links are formed . . . • A. When the covalent bonds are formed between nucleotides the attach in the direction of 5’→3’ • B. The 5’ end of one nucleotide attaches to the 3’ end of the previous nucleotide

  14. #7. Nucleosome structure • Nucleosome are the basic unit of chromatin organization • In eukaryotes DNA is associated with proteins • (in prokaryotes the DNA is naked) • Nucleosomes = basic beadlike unit of DNA packing • Made of a segment of DNA wound around a protein core that is composed of 2 copies of each of 4 types of histones

  15. #7. Nucleosome structure • Nucleosomes have: • 8 histones in the core • DNA wrapped twice around the core • One histone holding the nucleosome together • A DNA ‘linker’ continuing towards the next nucleosome

  16. #7. Nucleosome structure • The DNA has a negatively charged backbone (because of the phosphate groups) • The proteins (the histones) are positively charged • The DNA and proteins are electromagnetically attracted to each other to form chromatin

  17. #8. Genes • Genes=units of genetic information (hereditary information) • Order of nucleotides make up the genetic code • Genes can contain the information for one polypeptide • Genes can also regulate how other genes are expressed • All cells of an organism contain the same genetic information but they do not all express the same genes • THIS IS CELL DIFFERENTIATION • Cells differentiate by genes that are activated

  18. #8. Genes • Repetitive sequences-part of the non-coding section of DNA • Function-unknown • Can be used in DNA profiling (DNA fingerprinting)

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