12 3 DNA Replication How Does DNA Copy

12. 3 DNA Replication How Does DNA Copy Itself?

DNA Replication – What and Why l Replication = DNA making copies of itself – – DNA must be copied before a cell can divide Each new cell will have a complete set of DNA

History of Replication l Watson & Crick – realized that complementary base pairing provided a way for DNA to copy itself - base pairing could allow a new strand to be built on an old strand - 3 possible models of DNA replication

3 Models of Replication: Semi-conservative Replication l l Each strand in a DNA molecule is used as a template to build a new strand using complementary base pairing Results in new molecule with one original DNA strand one new strand

3 Models of Replication: Conservative Replication l Leaves the original DNA intact and produces another molecule of DNA identical to the first

3 Models of Replication: Dispersive Replication l Creates two molecules that are a mix of new and old DNA interspersed along each strand of the molecule

The Question is Answered l 1957 Matthew Meselson & Franklin Stahl -conducted an experiment proving replication is semiconservative - each DNA molecule has one new strand one old strand

DNA Replication: Process l @Replication begins when the enzyme DNA helicase opens the DNA forming replication bubbles@

DNA Replication: Process l Multiple replication bubbles are opened simultaneously allows the molecule to be replicated quickly

DNA Replication: Process l The nitrogen bases on the original DNA strands are exposed in the replication bubbles. l They serve as a template to build new DNA strands

DNA Replication l The ends of the replication bubbles known as the replication fork is where replication begins

DNA Replication – DNA Polymerase l @The enzyme DNA polymerase brings new nucleotides to the replication fork@ - it pairs them according to base pairing rules A pairs with T C pairs with G

DNA Replication – Leading Strand l l The process of replication proceeds from 5’ to 3’ leading strand- replication is continuous – it is built toward the replication fork

DNA Replication – Lagging Strand l lagging strand- replication occurs in short segments - called Okazaki fragments - the new DNA strand grows away from the replication fork

DNA Replication- The Big Picture Each Bubble has 2 Forks – each fork has a leading and lagging strand

DNA Replication l The process continues until 2 complete copies of the DNA are produced l @Each copy of the DNA has 1 strand of DNA from the original DNA and one new strand that was produced by replication@ l Known as semi-conservative replication

DNA Replication

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Telomeres – The tips of chromosomes are known as telomeres. – Over time, DNA may actually be lost from telomeres each time a chromosome is replicated. – @An enzyme, telomerase, solves this problem by adding short, repeated DNA sequences to telomeres@

Prokaryotes vs. Eukaryotes l Prokaryotes- starts from a single point and goes in two directions until the entire chromosome is copied. l Eukaryotes- replication begins at dozens or even hundreds of spots on the DNA, going in both directions until each chromosome is completely copied.

Which enzyme is used to separate the DNA? A. B. C. D. Helicase Ligase Polymerase Telomerase

Which enzyme adds nucleotides to the strands? A. B. C. D. Helicase Ligase Polymerase Telomerase

Which enzyme prevents DNA from being lost during replication? A. B. C. D. Helicase Ligase Polymerase Telomerase

How is Prokaryotic DNA different from Eukaryotic? l l Prokaryotic DNA is circular and replication begins at 1 point. Eukaryotic DNA is linear and replication begins at many points.