DNA Replication
Review:
- Made up of monomers of nucleotides (sugar,
phosphate, nitrogenous base)
- The genetic code is in the order of the bases: A-T &
C-G held together by weak hydrogen bonds. Why?
- Each side of polynucleotides run antiparallel
 Replication happens in the S-phase of the cell cycle
DNA Replication is semiconservative
 Meselson—Stahl experiment using radioactive isotope
of Nitrogen (N15) and Nitrogen (N14)
- each strand copies a new daughter strand
 DNA does not unzip like a coat zipper (undo one end to the
next)
Opening up the DNA:
1. DNA Helicase = unwinds DNA at origins of replication
forming replication bubbles (many).
 Each end of the replication bubble = Replication fork
2. Single stranded binding proteins = stabilize DNA
separated single strands and prevent the double
strand from rejoining.
3. Topoisomerase = relieves stress of winded DNA ahead
of the replication fork as replication bubble grows.
Elongation: Because replication is semi-conservative,
this part is different for each side of the DNA.
 In general:
 New DNA is synthesized from 5’ – 3’ (parent
strand is read 3’ – 5’)
 Cannot initiate polymerization of nucleotides
without primers present.
 Now, specific to each side:
One side of DNA:
 Leading strand = continuous elongation toward
replication fork
4. One (RNA) Primer added first = series of RNA
nucleotides that are complement to parent strand of
DNA at the replication initiation site.
 Primase bonds nucleotides building RNA primer
5. DNA Pol III = synthesized nucleotides from 5’ – 3’
(new strand)
Other side of DNA:
 Lagging strand = discontinuous elongation in
pieces due to 5’ to 3’ direction; away from
replication fork
- Okazaki fragments = fragmented pieces of new
DNA added as lagging strand.
4. (RNA) Primer added (using primase) at the beginning
of each Okazaki fragment
5. DNA Pol III= bond nucleotides to make fragment
 When it reaches next primer, it disassociates
6. DNA Pol I = removes primer and adds DNA
nucleotides in its place.
7. DNA ligase – bonds Okazaki fragments
 Replication bubbles grow and join until two
parent strands are separated and fully
replicated = semi-conservative