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Structure of DNA Learning Objectives By the end of this class you should understand: The experiments that determined the nature of genetic inheritance The structure and chemistry of DNA and RNA How to read and write complementary strands of DNA The process of DNA replication Why DNA and RNA can only be polymerized in one direction How telomeres work and why they are linked to aging Genetic Material We have known since Mendel that some chemical carries information from parent to child Determining which chemical was easier said than done! For a long time, many believed it was protein, since there are 20 amino acids and it was believed they made an “alphabet” Early Experiments Information was gleaned step by step “Nucleic acids” were found in the nucleus Nucleic acids contain phosphorus whereas protein contains sulfur Both are also made with carbon, hydrogen, oxygen, and nitrogen Griffith Experiments Hershey-Chase Experiments Chemistry of Inheritance These experiments established that nucleic acids carried the information for life Today it is taken as common knowledge The structure and function of nucleic acids still had not been determined until Watson and Crick And Franklin! Chemistry Recap All matter is made of many different kinds of atoms Depending on the number of protons, atoms may wish to share electrons in a covalent bond Carbon, oxygen, hydrogen, and nitrogen are very good at sharing electrons Metals are not good at sharing electrons Hydrogen Bonds When an atom shares its electrons unequally with another atom it may be slightly positive or slightly negative These small positive/negatives can attract each other in a hydrogen bond So called because hydrogen has a weak pull on electrons and so is often the positive pole Structure of DNA The various bonds between adjacent bases of DNA and their backbones are all covalent bonds, as are bonds within other biomolecules The two strands of DNA are held together by hydrogen bonds This should make sense since the DNA cannot be read unless it is first unzipped Bases (“letters”) of DNA Backbone of DNA Nucleic acid bases are attached to a special sugar to hold them in place Forms a sugar backbone The sugars are attached to each other with a phosphate Individual DNA bases with a sugar have three phosphates Phosphates carry energy Nucleic Acids There are two similar types of nucleic acids: DNA and RNA Deoxyribonucleic acid (DNA) Sugar backbone made with Deoxyribose Ribonucleic acid (DNA) Sugar backbone made with Ribose Why Deoxyribose? Deoxyribose is more stable than ribose Oxygen is reactive, and removing one oxygen helps prevent unwanted chemical reactions RNA is used for temporary copies, DNA is permanent Directionality DNA is written in one particular direction, just as we write left to write Or right to left in Hebrew.... Or top to bottom in Chinese.... New DNA/RNA bases can only be added to the 3' end of the DNA PROBLEM: Complementary strands run antiparallel Why Only 3' End? Remember individual DNA bases come with three phosphates on the 5' end Remember also phosphates store energy in their bonds The energy needed to attach a new DNA base is partially paid for by the two extra phosphates There is no way to use those phosphates when attaching to the 5' end! Complementary Strands Note the following sequences of complementary DNA: 5'-ACTTGCCATAGGA-3' 3'-TGAACGGTATCCT-5' If the DNA is opened, the template strand can be copied with new DNA (replication) or with RNA (for protein synthesis) 5'-GGCCATAACCG-3' 3'-CCGGUAUUGGC-3' (RNA strand) DNA Replication Occurs during S phase of mitosis/meiosis The complementary strands are opened and new DNA bases attach to each strand This is called semiconservative replication Semiconservative? DNA Replication DNA replication is accomplished with the use of DNA polymerase “Polymer” means many chemicals strung together “-ase” is a suffix used for enzymes (proteins that make chemical reactions happen faster) Okazaki Fragments Imagine painting a double yellow line on a road but you had to paint one line with forward strokes and the other with backwards strokes Forward one is easy, backwards one will be done in many small strokes that link together Animation: http://www.youtube.com/watch?v=teV62zrm2P0 RNA Transcription More of this in chapter 9 Essentially the same process as copying the leading strand of DNA only Uses RNA Polymerase This RNA copy can then result in production of proteins Telomere The tips of chromosomes have long repeated sequences of DNA that are noncoding but protect the rest of the DNA DNA replication typically fails to completely duplicate these telomeres They function as the aglets on your shoelaces Telomere Shortening Every time a cell undergoes mitosis it loses a bit of its telomeres Adult cells have shorter telomeres and telomere shortening may be linked to aging Dolly the sheep was cloned from an adult cell and so may have showed premature aging Telomerase Immortal cells such as stem cells and cancer cells express an enzyme called telomerase Lengthens telomeres and extends life of daughter cells Research in stem cells and telomerase may provide clues into aging process See you tomorrow!