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3.3 – DNA Structure

3.3 – DNA Structure. Topic 3 – The Chemistry of Life. Nucleotides are the building blocks of DNA. DNA is an extremely long molecule Subcomponents of DNA are nucleotides Composed of three units: phosphate group, sugar ( deoxyribose or ribose), and a nitrogenous base

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3.3 – DNA Structure

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  1. 3.3 – DNA Structure Topic 3 – The Chemistry of Life

  2. Nucleotides are the building blocks of DNA • DNA is an extremely long molecule • Subcomponents of DNA are nucleotides • Composed of three units: phosphate group, sugar (deoxyriboseor ribose), and a nitrogenous base • The four nitrogen bases are adenine, thymine, cytosine, and guanine https://www.youtube.com/watch?v=FUA6_Ucw3i4

  3. DNA in form of double helix • Two strands shaped like a spiral staircase • Two strands connected by nitrogen bases • Sugar-phosphate backbone (outside of strand) • Phosphates and sugar molecules form covalent bonds https://www.youtube.com/watch?v=qy8dk5iS1f0

  4. Formation of double helix The DNA double helix or “twisted ladder” Sides of the ladder are the two strands of the DNA molecule. Strands are composed of phosphates and deoxyribose sugars. The “rungs” or steps in the “ladder” are the nitrogenous bases. DNA is double stranded, so there are two sides, and two bases making up each rung.

  5. Complimentary base pairing • adenine – thymine • cytosine – guanine Bases held together by hydrogen bonds • A-T (two hydrogen bonds) • C-G (three hydrogen bonds)

  6. Diagram of DNA - Draw and label a simple diagram of the molecular structure of DNA.

  7. 3.4 – DNA Replication

  8. Why does DNA need to replicate? • Must prepare for division • Exact copy of DNA for new cell Nuclear environment during interphase • DNA in chromatin (not tightly coiled chromosomes) • Enzymes: helicase and polymerase • Free nucleotides

  9. Unzipping the DNA molecule • The unpaired nucleotides on each of these single strands can now be used as a template to help create two double-stranded DNA molecules identical to the original. • An analogy comparing a zipper to this process can be used. * Helicase may catalyse the unzipping of DNA at a rate measured in hundreds of base pairs per second.

  10. Once unzipped DNA considered single strand. • Free-floating nucleotides then use rules of base pairing to create complimentary strands with accurate genetic code. • New strand forms from the covalent bonds between nucleotides which is catalysed by DNA polymerase.

  11. Significance of complimentary base pairing • Complementary nucleotides use unzipped second-stranded areas as templates. • No DNA molecule completely new. • Old strand now paired with a strand that is new.

  12. DNA replication is semi-conservative • DNA replication described as semi-conservative. • Half of a pre-existing DNA molecule is always conserved (saved). ** The concept of semiconservative DNA replication has some interesting repercussions. For example, one can argue that there never is such a thing as a ‘new’ DNA molecule. How long has your DNA been in you? In your family lineage?

  13. Questions?

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