PART TWO
DNA SYNTHESIS ROBERT PERRY Malibu High School
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mitosis & cell division
WHY MUST DNA SYNTHESIZE COPIES OF ITSELF ? GENERAL ANSWER:
TO BE SURE THAT EVERY NEW CELL THAT IS “BORN” AFTER MITOSIS CONTAINS EXACTLY THE ______________ SAME NUMBER AND EXACTLY THE ____________ SAME KIND OF DNA NUCLEOTIDES AS THE PARENT CELL HAD.
WHY MUST DNA SYNTHESIZE COPIES OF ITSELF ? WHY DO LIVING THINGS NEED NEW CELLS?
A- GROWTH OF ORGANISM B- REPAIR & REPLACE DEAD OR WORN-OUT CELLS
HOW DOES DNA SYNTHESIZE COPIES OF ITSELF ?
weak = bonds
REVIEW: where are the weak bonds located?
DNA Synthesis Step One: weak hydrogen bonds are broken by the “zipper enzyme”
Step Two: free nucleotides fill-in on both strands.
original double helix
weak H bonds break replication fork
two identical new strands are formed
original double helix replication fork
free nucleotides are attracted to their opposites forming two new strands two identical new strands are formed
ORIGINAL STRAND NEW STRAND
aka, parent strand aka, daughter strand
DNA SYNTHESIS IS SAID TO BE “SEMI-CONSERVATIVE” BECAUSE IT CONSERVES ONE SIDE FROM THE PARENT DNA AND ONLY THE MATCHING SIDE IS NEW
THE 3 STEPS IN DNA SYNTHESIS
1.UNWINDING 2.BASE PAIRING 3.JOINING
STEP ONE
DNA HELICASE RESPONSIBLE FOR ---UNWINDING ---UNZIPPING
STEP TWO
IN YOUR PPT NOTE PACK
DNA POLYMERASE ADDS NEW NUCLEOTIDES TO “UNZIPPED” PARENT DNA STRAND.
STEP TWO continued...
STEP 3
LIGASE JOINS OKAZAKI FRAGMENTS TO MAKE ON NEW DAUGHTER DNA STRAND
DAUGHTER DNA DNA POLYMERASE leading strand
lagging strand
DIRECTION OF REPLICATION PARENT DNA
OKAZAKI FRAGMENTS DNA LIGASE
DNA POLYMERASE DAUGHTER DNA
RNA PRIMER
HELICASE
DNA SYNTHESIS - animated version: cut and paste into browser http://www.phschool.com/webcodes10/index.cfm? fuseaction=home.gotoWebCode&wcprefix=cbp&wcsuffix=4122
THE END