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

SO which is it?

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

0.25 µm

Origin of replication Double-stranded DNA molecule

Parental (template) strandDaughter (new) strand

Bubble Replication fork

Two daughter DNA molecules

(b) Origins of replication in eukaryotes

Fig. 16-13

Topoisomerase

Helicase

PrimaseSingle-strand binding proteins

RNA primer

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5 3

3

3

• 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!