Campbell- Chap 16 pg 284 Inquiry Chap 25 DNA Replication Campbell- Chap 16 pg 284 Inquiry Chap 25

The structure of DNA is well suited to it’s function!

AA Summary of DNA Replication

The structure of DNA suggests a copying mechanism… Complementary Base pairing always A- T and C-G The 2 strands held together by weak Hydrogen bonds Can separate The 2 strands are antiparallel Orientated in opposite direction

Many enzymes & other proteins are also needed to carry out replication!

Why does DNA replicate? To pass on the genetic code! To ensure each new cell has a complete set of chromosomes.

DNA Replication is semi-conservative -> each new daughter strand will consist of one “old parental” strand and one “new copied strand”

Helpful Replication Clips 3D Animation: https://www.youtube.com/watch?v=TNKWgcFPHqw Animation: https://www.youtube.com/watch?v=VpmT7Lw_4v0 Ameoba Sisters: https://www.youtube.com/watch?v=5qSrmeiWsuc Crash Course: https://www.youtube.com/watch?v=8kK2zwjRV0M The immortal cells of Henriette Lacks: https://www.youtube.com/watch?v=22lGbAVWhro

STEP 1. “Unzipping/Unwinding” Enzyme: DNA Helicase Attaches to DNA Breaks H bonds between c base pairs DNA “unzips” or unwinds Forms “origin of replication”

Origins of Replication Replication starts here! “Bubbles” where 2 strands separate In eukaryotes there are 100s-1000s of these origins!

Replication Forks Y- shaped forks at the end of the replication origin Location where the new DNA strands begin to elongate

How does the anti-parallel nature of the strands affect replication? The new DNA strand can ONLY elongate in the 5’  3’ direction Nucleotides will ONLY get added to the free 3’ end

2. Elongating- Complementary base pairing DNA polymerases catalyse this step A RNA primer is required to start Primase enzymes join RNA nucleotides to create it

The lagging strand is made in a series of “Okazaki fragments” These pieces are joined into a single DNA strand by the enzyme DNA ligase

Nucleotide monomers form H bonds with exposed bases Via complementary base pairing A binds to T C binds to G

3. Joining of adjacent nucleotides Enzyme: DNA Polymerase This step creates the new sugar-phosphate backbone formation of sugar-phosphate covalent bonds (dehydration synthesis) between each nucleotide

4. Proofreading of new DNA Mismatch repair DNA polymerase can also fix mistakes made during replication Any incorrectly paired nucleotide is removed Excision repair Errors “cut out” by nuclease DNA polymerase fills the gap Ligase seals the nick There are many many DNA repair enzymes!

The ends of DNA pose a problem… DNA polymerasae can only add nucleotides to the 3’ end… There’s no way to complete the 5’ ends of the daughter strands Over time replicated strands get shorter & shorter Solution: Telomeres Typically, they are a repeated segment of TTAGGG that doesn’t code for any genes at the end of DNA

Replication Animation https://www.youtube.com/watch?v=yDkSWd_ZbbE