Agenda Review homework DNA replication Begin HW (DNA Replication worksheet) Quiz will be Wednesday instead and will include DNA replication

#1 A, T, G or C

#2

#3 They match up perfectly to form hydrogen bonds (2 for AT, 3 for GC)

#4 The two bases can’t H-bond, so nothing holds them together. It will stick out

#5 H-bonds are weak! DNA must become separated to be copied or expressed

#6 GATTACA CTAATGT

#7 If 15% is Adenine, 15% is also Thymine That leaves 70% split between Guanine and Cytosine. So 35% each

#8 X-ray crystallography, helped determine shape

#9 Structure How it’s copied How it leads to proteins Sequence

#10 Why 99% of it is non-coding What that 99% does How the environment affects gene expression Where it came from/how it originated

DNA Replication

DNA Replication Must occur every time a new cell is to be made E.g. prior to mitosis or meiosis

DNA Replication Watson and Crick noticed the huge benefit of double strands Each strand can serve as a template for making the other

Semiconservative Model Each strand serves as a template for the creation of a new strand of DNA 2 DNA molecules are created, each containing 1 strand of the original DNA

Semiconservative Replication

DNA Replication is Remarkably Fast and Accurate! Humans have 46 chromosomes, and thus 46 DNA molecules About 6 billion base pairs DNA replication takes just a few hours, even in humans Only 1 error per 1 billion nucleotides

Step 1 – Separate the Strands Helicase breaks the hydrogen bonds

Step 2- Primase adds a short RNA primer to begin new strands

Step 3 DNA polymerase builds a new DNA strand

DNA is Antiparallel Strands run opposite directions DNA polymerase only works in one direction So the two strands are built in opposite directions

Leading vs. Lagging Strands Built in one piece Built in the overall direction of replication Only one primer needed Built in pieces called Okazaki fragments Built in the opposite direction of replication Many primers needed

Step 4 DNA ligase seals together Okazaki fragments

Step 5 - Proofreading Nucleases cut out (incise) the incorrect nucleotide DNA polymerase adds the correct nucleotide Ligase connects the new nucleotide to the strand

Topoisomerase Helicase Single Stranded Binding Proteins

5’ 3’ DNA Ligase Lagging Strand Okazaki Fragments Leading Strand DNA Polymerase Primase

Mutations are Rare… But Happen Most errors are fixed in proofreading, so DNA replication is incredibly accurate 1 error every billion nucleotides. We have 6 billion. Every time DNA is copied there are ___ errors We’ll talk about the significance of these mutations later