Post on 02-Jan-2016
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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
How is DNA packaged into chromosomes?
How is DNA replicated during S phase of the
Cell cycle?
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
cell http://www.sacred-texts.com/dna/hgp011k.htm
• Eukaryotes have 1000 times the amount of DNA that prokaryotes have
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
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)
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
• Cut the beads free from your string
• Throw the string away and return the beads to your baggie
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
• Notice, we are making two DNA molecules that have anti-parallel strands, with one strand 5’ to 3’ and the other 3’ to 5’
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
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…
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
• 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
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