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Topic 3.3: DNA Structure

Topic 3.3: DNA Structure. Assessment Statements. 3.3.1: Outline DNA nucleotide structure in terms of sugar (deoxyribose), base, and phosphate 3.3.2: State the names of the four bases of DNA 3.3.3: Outline how DNA nucleotides are linked together by covalent bonds into a single strand

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Topic 3.3: DNA Structure

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  1. Topic 3.3: DNA Structure

  2. Assessment Statements • 3.3.1: Outline DNA nucleotide structure in terms of sugar (deoxyribose), base, and phosphate • 3.3.2: State the names of the four bases of DNA • 3.3.3: Outline how DNA nucleotides are linked together by covalent bonds into a single strand • 3.3.4: Explain how a DNA double helix is formed using complementary base pairing and hydrogen bonds • 3.3.5: Draw and label a simple diagram of the molecular structure of DNA

  3. Nucleotides are the building blocks of DNA • DNA (deoxyribonucleic acid)—long molecule • Subcomponents: • Nucleotides • Each nucleotide of DNA is composed of a phosphate group, a sugar (deoxyribose), and a nitrogenous base

  4. Nucleotides are the building blocks of DNA • The four possible nitrogenous bases of DNA: • Adenine, thymine, cytosine, and guanine

  5. Each strand of DNA is composed of nucleotides covalently linked • DNA described shape of a double helix • Each of the nucleotides in a single strand are covalently bonded together

  6. Hydrogen bonds help form the double helix • Imagine a twisted ladder • Two sides of the ladder are made up of the phosphate and deoxyribose sugars • The rungs of the ladder are made up of the nitrogenous bases (2)

  7. Complementary Base Pairing • Adenine pairs with Thymine • Held together with two hydrogen bonds • Cytosine pairs with Guanine • Held together by three hydrogen bonds

  8. Topic 3.4: DNA Replication

  9. DNA replication involves ‘unzipping’ • Cell must to prepare for cell division by doubling the DNA content of the cell • Process all DNA Replication • Among the variety of molecules present in the nucleoplasm are two types that are important for DNA replication • Enzymes needed for replication (Helicase and DNA polymerase) • Free Nucelotides (nucleoside triphosphates)

  10. DNA replication involves ‘unzipping’ • One of the first events of DNA replication is the separation of the double helix into two single strands • Remember the double helix is held together by the hydrogen bonds between the complementary base pairs • The enzyme that initiates this separation into two single strands is called helicase • Helicase begins at a point in or at the end of a DNA molecule and moves one complementary base pair at a time, breaking the hydrogen bonds so the double-stranded DNA molecule becomes two separate strands

  11. DNA replication involves ‘unzipping’ • The unpaired nucleotides on each of these single strands can not be used as a template to help create two double-stranded DNA molecule identical to the original

  12. Formation of two complementary strands • In the environment of the nucleoplasm, there are many free-floating nucleotides. • These nucleotides are available to form complementary pairs with the single-stranded nucleotides of the unzipped molecule • The free nucleotides come and join the complementary pairs on the single stranded molecule. • Then the DNA Polymerase catalyzes the formation of the covalent bond between the two new nucleotides

  13. Formation of two complementary strands • After the DNA polymerase forms the covalent bond another nucleotide comes and joins the next complementary base pair. • Both separated strands act as a template for the formation of another new strand • The leading strand is going in one direction • The lagging strand is going in the other direction

  14. Significance of complementary base pairing • The pattern of DNA replication ensures that two identical copies of DNA are produced from one • This also means that no DNA molecules is every new • Every DNA molecule after replication consists of a strand that was ‘old’ now paired with a strand that is ‘new’ • Described as a semi conservative process because half of a pre-existing DNA molecule is always conserved (saved)

  15. Animations • http://www.johnkyrk.com/DNAreplication.html • http://www.wiley.com/college/pratt/0471393878/student/animations/dna_replication/index.html • http://student.ccbcmd.edu/biotutorials/dna/dnareppr.html • http://nobelprize.org/educational/medicine/dna/a/replication/lagging_ani.html

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