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Warm-Up 2/15/12

Warm-Up 2/15/12. Draw a DNA molecule, with one strand having the sequence AGGAC Don’t forget to label the 5’ and 3’ ends! I am stamping your 12.1 Book Notes. Leave your book notes and pen/pencil on your desk and clear the rest…. Time for a Random Reading Quiz!!!. Section 12.2.

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Warm-Up 2/15/12

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  1. Warm-Up 2/15/12 Draw a DNA molecule, with one strand having the sequence AGGAC Don’t forget to label the 5’ and 3’ ends! I am stamping your 12.1 Book Notes

  2. Leave your book notes and pen/pencil on your desk and clear the rest…. Time for a Random Reading Quiz!!!

  3. Section 12.2 How is DNA packaged into chromosomes? How is DNA replicated during S phase of the Cell cycle?

  4. The Size of DNA • Organisms have a huge amount of DNA for such tiny cells • Ex: E. coli has 1.6 mm of DNA, but each bacterium is only 1.6 um in size! • Humans have about 1 meter of DNA in each cellhttp://www.sacred-texts.com/dna/hgp011k.htm • Eukaryotes have 1000 times the amount of DNA that prokaryotes have

  5. The Size of DNA • Eukaryotes have developed chromosomes to package large amounts of DNA into tiny nuclei of cells • Humans = 46 chromosomes per cell • Each chromosome has a MINIMUM of 30 million base pairs! • A special sequence of folding occurs to fit all of that DNA into chromosomes

  6. Packaging DNA into Chromosomes • Step One: DNA is coiled around histone proteins to form a nucleosome (tie 15 beads to your string) • Step Two: Nucleosomes are coiled into a thick fiber (bunch bead/string combos together)

  7. Packaging DNA into Chromosomes • Step Three: Thick fibers are again coiled to become a chromosome (wrap fiber you made around your fingers) • The result: 46 tightly packaged chromosomes that are ready to be moved during mitosis

  8. Cut the beads free from your string • Throw the string away and return the beads to your baggie

  9. DNA Replication • DNA is constantly being replicated (doubled) in the S phase of the Cell Cycle • Big Picture: • DNA molecule is split into two strands • An enzyme builds a new strand for each original strand • Result is two identical DNA molecules

  10. Notice, we are making two DNA molecules that have anti-parallel strands, with one strand 5’ to 3’ and the other 3’ to 5’

  11. DNA Replication: Nitty Gritty 1. The DNA is split into two strands at thousands of ‘origins of replication’ - the enzyme helicase breaks the hydrogen bonds to ‘unzip’ the molecule - a ‘bubble’ is created at each origin of replication, with a ‘replication fork’ on each end

  12. DNA Replication: Nitty Gritty 2. The enzyme DNA polymerase builds a new strand of DNA for each original strand, pairing A with T and G with C • DNA polymerase builds strands ONLY in the 5’ to 3’ direction • Let’s see how this works up close…

  13. DNA Replication: Nitty Gritty • Original strand #1 (blue) goes in the 5’ to 3’ direction, so the enzyme can start at the bottom and build a new strand (black) without interruption Original strand #1

  14. Original strand #2 goes in the wrong direction for the enzyme to build one new, continuous strand! • Instead, small fragments are joined by the enzyme ligase Original Strand #2

  15. Results of DNA Replication • Thousands of origins of replication (bubbles) join together • Finished product: two identical DNA molecules, each with an original strand and a copy

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