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DNA Replication Structure of DNA • Understanding how DNA is built is important • The structure allows us to understand how the different processes fit can happen • There are several different processes that can happen with DNA Structure of DNA • DNA structure and DNA function are related • The way that DNA is built is very telling to how it operates • Let’s review and go deeper into how DNA is built Draw In Your Notes Time to Prime • Remember DNA is made from nucleotides • The nucleotides are made from a 5 carbon sugar • All five carbons of the sugar are numbered • Since there are 5 carbons they are numbered one through five Time to Prime • Each molecule of DNA has two distinct and different ends • Each end has a 3’ (three prime) and a 5’ (five prime) end • The 3’ and 5’ ends refer to the carbon on the sugar that is near the end Time to Prime • Notice how the 5’ end has a phosphate group that is left free • Also notice that the three prime end has a hydroxyl group left on the end • These are important for the next step of DNA replication Video • http://www.youtube.co m/watch?v=IN8pKyQz2 RQ DNA Replication Overview • What happens during the S Phase of the cell cycle? • Why does this need to happen for mitosis to happen? • How long does the S phase take? DNA Replication Overview • Watson and Crick correctly projected that making copies of DNA would be based on its structure • They said if you peeled apart DNA down the middle it would be easy to copy • This is because all of the unpaired bases must be paired with a specific base DNA Replication Overview • DNA replication is not much more complicated than Watson and Crick predicted • The basic idea is that DNA is split down the middle and each remaining strand is the template for a new strand • This is called the semiconservative model because the parental model is conserved in DNA replication DNA Replication Overview • Human DNA has over 6 billion base pairs • However it is copied in under three hours • During that three hours DNA is copied overwhelmingly accurately • Less than 1 mistake is made in every 10,000,000 base pairs copied Demo • To understand how accurate DNA replication is… lets do an experiment • Try this… • Have one partner sit facing the blackboard, one sit facing the opposite direction • The partner that faces the direction of the board will describe and the other partner will draw Lets see how many mistakes you make! Now Switch • If DNA Replication did that process 100 times a day it would take 10,000 days before it made a mistake • That about 27.4 years without a mistake • That is about as old as these celebrities Meghan Fox Dwight Howard Mr. Owdij DNA Replication Enzymes • There are many different enzymes that contribute to DNA replication • However there are three major enzymes that are involved • These three enzymes allow DNA replication to proceed DNA Replication Enzymes • DNA Helicase is the enzyme unzips the DNA strand • Its job is to separate the DNA strand and make sure it does not reattach • It takes away the hydrogen bonds that hold together the bases DNA Replication Enzymes • DNA Polymerase is the principal enzyme involved with DNA Replication • It has 2 main functions. • It “polymerizes” or creates a complex compound by linking together nucleotides • It also proofreads and corrects any mistakes that happen in the DNA strands DNA Replication Enzymes • DNA ligase pieces together the segments of DNA to make one complete molecule • It fits together fragments that allow DNA to come together completely DNA Replication • DNA Replication starts on sections of DNA called origins of replication • These are specific sections of DNA that code in a specific way where enzymes can attach and separate the strands • There can be hundreds of origins of replication on any given strand of DNA DNA Replication • This means that DNA replication happens many different places on any given strand of DNA • This creates many different replication bubbles that will eventually meet and merge DNA Replication • When completed the replication bubbles will create four strands of DNA • One original strand (daughter strand) will be attached to an older strand (parental strand) • This means that DNA conserved through the process of DNA replication One Way Street • DNA Replication actually happens two different ways on any given strand • This is because DNA replication can only happen in the 5’ – 3’ direction • Since both strands of DNA are not going in the 5’ – 3’ directions there are actually two different methods for completing DNA replication One Way Street • Lets start with the strand that is already in the 5’ – 3’ orientation • This strand is the simplest to replicate • This strand uses a very linear strategy to replicate • This strand is called the leading strand One Way Street • First DNA Helicase will break hydrogen bonds at an origin of replication • The Helicase separates the strands and creates a replication fork • This is a site where both DNA strands are separated and have unattached nucleotides One Way Street • After the DNA Helicase breaks the hydrogen bonds DNA polymerase follows and attaches the appropriate nucleotides to create a stable new strand • DNA polymerase also “proofreads” the nucleotides to ensure the stability of the DNA 3’ – 5’ • The daughter strand that is oriented in the 3’ – 5’ direction has to be done differently • This direction is synthesized in segments that are fitted together • It is a slightly more involved process but it has the same general idea • This strand is called the lagging strand 3’ – 5’ • The process starts the same way • DNA helicase unzips the DNA at a replication fork • However DNA polymerase has to fit together the bases in small segments called Okazaki fragments • These are small sections of nucleotides that are attached to the 3’ – 5’ strand 3’ – 5’ • After the Okazaki fragments are fitted DNA ligase is used to attach all of the fragments together • Once this is completed you have a daughter strand to the 3’ – 5’ parent strand Video Animation Animation 2 Routine Maintenance • During the process of DNA replication several other steps are happening to ensure the health and well being of DNA • During replication DNA ligase and polymerase are fixing any routine damage that could have happened to DNA Routine Maintenance • DNA can have routine damage from any variety of sources • X Rays, ultraviolet radiation, toxic chemicals and tobacco are common chemicals that damage DNA