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Transcript of DNA Replication
of the replication
fork caused by the untwisting
of the double
helix DNA cannot initiate polynucleotide synthesis, only add on nucleotides. Primer is a short RNA nucleotide to initiate the synthesis of DNA Primase is the enzyme that synthesizes primer, and starts an RNA chain from a single nucleotide, primer. It also synthesizes the nucleotides that are added onto the RNA strand, using the parental DNA strand as a template. This is the incorporation of a nucleotide into a DNA strand. Because of the antiparallel arrangement in the double helix structure, DNA polymerase can add nucleotides only to the free 3' end or growing DNA strand. Each nucleotide added comes from a nucleoside triphosphate, like ATP, using 2 phosphates to supply energy to add on the nucleoside Making the Leading Strand 1. After RNA primer is made, DNA pol III starts to synthesize the leading strand. 2. The leading strand is elongated continuously in the 5' -> 3' direction as the fork progresses. Making the Lagging Strand Primase interacts with other proteins at the fork and slows down the progress. It organizes the primers and rates of replication of the leading and lagging strands to simplify the productivity of the process. DNA moves through the structure during replication. Numerous copies of the complex in eukaryotic cells stay at the nuclear matrix, which is an arrangement of fibers going across the inside of the nucleus. Studies show that two DNA polymerase molecules take the parental DNA and force out new daughter DNA molecules. The lagging strand is taken back all the way through the complex. 1. Primase joins RNA nucleotides into a Primer. 2. DNA pol III adds DNA nucleotides to the primer forming Okazaki fragment 1. 3. After reaching the next RNA primer to the right, DNA pol III detaches. 4. Fragment 2 is primed. Then
DNA pol III adds DNA nucleotides,
detaching when it reaches the
fragment 1 primer. 5. DNA pol I replaces the RNA with DNA, adding
the 3' end of fragment 2. 6. DNA ligase forms a bond
between the newest DNA and the DNA
of fragment 1. 7. The lagging strand in this region is
now complete. To elongate the other new strand of DNA in the 5' -> 3' direction, DNA pol III must work along the other template strand in the direction away from the replication fork. That DNA elongating in this direction is the lagging strand. Antiparallel here means that if you have a
in one direction,
moving in the opposite
direction. DNA is made up of nucleotides, but what are nucleotides made of?
Nucleotides are made of :
phosphate + sugar + base What's our Goal? The goal is to add base pairs and make up a sequence so we can recreate a daughter strand. Basics : Basics : Since it's antiparallel and you can only build on a 5' -> 3' direction, the leading strand is going to have a 5' and go left towards the helicase. This means it's being continuously built to the left, non stop. Basically the daughter strand is
going to be built antiparallel.
-Because it's being built continuously and is going in the 5' -> 3' direction, that is going to be called our leading strand. -DNA can only be made in
a 5' -> 3' direction while 3' it is being added to the
DNA strand. -As the helix is opening up, sequences are added going right, towards the 5'.
-Because the helix contiuously opens up, it has to keep on adding sequences. This is discontiuous synthesis. So this will be our lagging strand.
(You can not add it one after the other.) -These little sequences/gaps are called Okazaki fragments. 3' 5' 5' 3' You can't add segments one after another. Because as is keeps on opening you'd have to come back and add more.