DNA Replication • During DNA Replication, an exact copy of DNA is made. • Replication of DNA occurs during the S Phase of Interphase during the cell cycle. • Each strand of DNA holds specific information to create the other strand in the base-pairing pattern.
But….how does it replicate? • Conservative Model • Semiconservative Model • Dispersive Model
DNA Replication • Replication occurs in segments, call replication bubbles. • Each new strand consists of one old strand and one new strand, making it a semi-conservative process.
Steps to replciate DNA • There are numerous replication bubbles along a strand of DNA • Helicase unwinds the DNA at the replication fork. • Single strand binding proteins bind to the separate strands to stabilize them
Topoisomerase – helps relieve the strain ahead of the replication fork
Fig. 16-12b Origin of replication Double-stranded DNA molecule Parental (template) strand Daughter (new) strand 0.25 µm Replication fork Bubble Two daughter DNA molecules (b) Origins of replication in eukaryotes
Fig. 16-13 Primase Single-strand binding proteins 3 Topoisomerase 5 3 RNA primer 5 5 3 Helicase
RNA Primase creates an RNA primer (5-10 nts long) • DNA polymerase adds nucleotides to preexisting DNA chain. This is done in the 5’ to 3’ direction. (Leading strand) • Lagging strand is created discontinuously is segments. (Okasaki fragments )
Okasaki fragments 1000-2000nt in prokaryotes and 100-200 in eukaryotes • Nucleotides added 500nt/sec in bacteria and 50nt/sec in humans • DNA polymerase I removes the primer nucleotides and replaces with DNA nucleotides • DNA ligase joins the sugar phosphate backbones of Okasaki fragments
A closer look at DNA replication • How Nucleotides are added • Replication Fork
Thank goodness for spell check! • Proofreading Enzymes repair damaged or mismatched nucleotides • What causes the damage?
Interesting Fact!Chromosome s… • DNA and protein material • Bacterial chromosome consists of approximately 4.6 million nucleotides and 4400 genes • Humans have approximately 1000 gene/chromosomes.
Interesting Fact!Telomeres/Telomerase… • Protects our genes • TTAGGG repeats of 100x – 1000x • Telomerase protects cells from shortening during division by lengthening the ends. Found in gametes and a few somatic cells
What if telomerase was in every cell? • The fountain of youth!