1. DNA Replication

2. Copying DNA Why replicate or copy DNA?
Make an exact copy of the genetic material for cell division
Where does replication occur?
Nucleus of eukaryotic cells
cytoplasm of prokaryotic cells
When does replication occur?
Before cell division: interphase (s phase) of mitosis/meiosis
Replication of DNA
base pairing allows each strand to serve as a template for a new strand
new strand is 1/2 parent template & 1/2 new DNA

3. How Replication Occurs: 1st step
Unwind DNA
DNA helicase enzyme
Breaks hydrogen bonds
unwinds part of DNA helix
Forms a replication fork
helicase
replication fork

4. Replication: 2nd step Build daughter DNA strand using DNA template5’
Replication: 2nd step
Build daughter DNA strand using DNA template
add new complementary nucleotides
Enzyme
DNA polymerase
Builds new DNA in 5’ to 3’ direction 3’
DNA
Polymerase 5’ 3’

5. Leading & Lagging strands
Okazaki
Leading & Lagging strands
Limits of DNA polymerase
can only build onto 3 end of an existing DNA strand  5
Okazaki fragments 5 5 3 5 3 5 3
ligase
Lagging strand 3
growing
replication fork 3 5
Leading strand  3 5
Lagging strand
Non-continuous Okazaki fragments
joined by ligase
“spot welder” enzyme 3
DNA polymerase
Leading strand
continuous synthesis

6. Editing & proofreading DNA
1000 bases/second = lots of typos!
DNA polymerase
proofreads & corrects typos
repairs mismatched bases
removes abnormal or mutated
repairs damage throughout life
reduces error rate from 1 in 10,000 to 1 in 100 million bases

7. Fast & accurate!
It takes E. coli bacteria<1 hour to copy 5 million base pairs in its single chromosome
Forms 2 replication forks
Human cells copy 3.2 billion bases & divide into daughter cells in only 8 hours
Forms multiple replication forks
remarkably accurate
~1 error per 100 million bases
~30 errors per cell cycle