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