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DNA Replication

DNA Replication. Watson and Crick…again. After releasing their paper on the structure of DNA, they released a followup paper on replication In it, they stated that each chain of the DNA would serve as a template for copying, yielding two identical molecules

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DNA Replication

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  1. DNA Replication

  2. Watson and Crick…again • After releasing their paper on the structure of DNA, they released a followup paper on replication • In it, they stated that each chain of the DNA would serve as a template for copying, yielding two identical molecules • Through their idea, each daughter stand would contain a chain of the original • This is the semiconservative model

  3. However, other models had to be ruled out • In a conservative model, the new strand of DNA contains no chain from the parent • In this, one is entirely new, other is entirely the original • The third possibility is the dispersive model • Here, each strand contains part of the original and the copy on each chain

  4. An experiment at Cal Tech was performed to ensure the semiconservative model was correct • A culture of bacteria was grown in a medium containing 15N, then transferred into a medium containing only 14N • The samples of DNA were then extracted and centrifuged

  5. Replication Specifics • Starts at a sequence of nucleotides called origins of replication • Length and location varies • Proteins attach to this sequence and open the DNA, forming a bubble • Replication proceeds in both directions • Y-shaped region at either end of bubble is called replication fork • At fork, helicase unwinds DNA by breaking the hydrogen bonds

  6. Ahead of the site, topoisomerase breaks and swivels strands, relieving strain • When strands are separated, single strand binding proteins (SSBPs) bind to regions of unpaired DNA, stabilizing it • Primase enzyme moves in and base-pairs a short (5-10) nucleotide region of RNA • This is called the primer • DNA polymerase III (DNA pol III) adds nucleotides to existing chain in the 5’->3’ direction (attaches new nucleotides to 3’ end)

  7. The uninterrupted addition of nucleotides on is called the leading strand • However, there is strand on the opposite side • Must be added in the 5’->3’ direction, which is the opposite direction of what is supposed to happen • Is this case, the primase has to move upstream and make another primer • DNA pol III then adds DNA nucleotides until it reaches the first primer • This partial region is called an Okazaki fragment

  8. DNA pol I then replaces the RNA primer with DNA nucleotides • The individual strands are linked by ligase (think of it as a gluing enzyme) • DNA Replication • DNA Replication Model

  9. If any errors are made, it is corrected by DNA pol II

  10. Telomers • When the replication process takes places at the end of DNA, the primer will be removed, but not replaced when going from the 5’ -> 3’ direction • This means the daughter strand is going to have a strand with a missing segment • The hundreds of the hexanucleotide repeats will be slowly eaten up division after division

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