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Advantages of bi-directional replication
- Less stress so bonds don�t break
- Energetically favorable
- Enzyme machinery
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A free 3� OH is needed to add on DNA
- Pairs with phosphate
- Needs template DNA
- DNTPs � phosphate groups � new nucleotides � phosphodiester bonds
- Mg++
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Polymeases have elongagation function
Also 5 to 3 and 3 to 5 exonuclease activity � cuts from the end to repair faulty DNA
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DNA Polymerase III and I do what?
Primarily with DNA replication
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DNA polymerase II, IV and V do what?
DNA repair � repair nucleotide excision..
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RNA primers initiate
DNA synthesis
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Primer
- Short strands to build upon
- Provides a free 3� OH
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Single Strand Binding protein
Keeps the helices unwound
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Toposomerase I and DNA gyrase
Produces breaks in DNA
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Telomers � end sequences of DNA at the end of chromosomes
Junk DNA
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Telomerase
Adds sequence of DNA and provide a 3� OH to add DNA to ends
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Telomerase has to be shut off because?
There needs to be a limit to the amount of replication
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In Vivo DNA replication occurs
semiconservatively, is initiated at unique origins, and usually proceeds bidirectionally from each origin of replication.
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DNA Replication is Semiconservative
- � Each strand serves as a template
- � Complementary base pairing determines the sequence of the new strand
- � Each strand of the parental helix is conserved
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DNA replicates by a semiconservative mechanism:
As the two complementary strands of a parental double helix unwind and separate, each serves as a template for the synthesis of a new complementary strand.
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The hydrogen-bonding potentials of the bases in the template strands specify complementary base sequences in the nascent DNA strands.
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Replication is initiated at unique origins and usually proceeds
bidirectionally from each origin.
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DNA Polymerases and DNA Synthesis In Vitro Much of what we know about DNA synthesis was deduced from in vitro studies.
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Requirements of DNA Polymerases
- � ?Primer DNA with free 3'-OH
- � ?Template DNA to specify the sequence of the new strand
- � ?Substrates: dNTPs
- � ?Mg2+
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Polymerases in E. coli
- � �DNA Replication: DNA Polymerases III and I
- � �DNA Repair: DNA Polymerases II, IV, and V
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Polymerases in Eukaryotes
- � �Replication of Nuclear DNA: Polymerase alpha, sigma and/or epislon
- � �Replication of Mitochondrial DNA: Polymerase gamma
- � �DNA Repair: Polymerases beta, epsilon�
- � All of these enzymes synthesize DNA 5' to 3' and require a free 3'-OH at the end of a primer
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DNA synthesis is catalyzed by enzymes called
DNA polymerases.
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All DNA polymerases require a _____strand, which is extended, and a ____strand, which is copied.
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All DNA polymerases have an absolute requirement for a free ____on the primer strand, and all DNA synthesis occurs in the 5' to 3' direction.
3'-OH
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The 3' to 5' exonuclease activities of DNA polymerases proofread nascent strands as they are synthesized, removing any mispaired nucleotides at the 3' termini of primer strands.
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DNA replication is a complex process, requiring the concerted action of a large number of proteins.
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Synthesis of the leading strandis
continuous.
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Synthesis of the lagging strandis
discontinuous.
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The new DNA is synthesized in short segments called ___that are later joined together.
(Okazaki fragment)
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DNA Ligase Covalently
Closes Nicks in DNA
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RNA Primers are Used to
Initiate DNA Synthesis
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DNA Helicase
Unwinds the Parental Double Helix
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Supercoiling of Unwound DNA
- DNA Topoisomerase I Produces Single-Strand Breaks in DNA
- DNA Topoisomerase II Produces Double-Strand Breaks in DNA
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DNA replication is complex, requiring the participation of a large number of proteins.
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DNA synthesis is continuous on the progeny strand that is being extended in the overall 5' to 3' direction, but is ______on the strand growing in the overall 3' to 5' direction.
Discontinuous
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New DNA chains are initiated by short RNA primers synthesized by
DNA primase.
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The enzymes and DNA-binding proteins involved in replication assembled into a ______at each replication fork and act in concert as the fork moves along the parental DNA molecule.
replisome
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DNA Replication in Eukaryotes
- � Shorter RNA primers and Okazaki fragments
- � DNA replication only during S phase
- � Multiple origins of replication
- � Nucleosomes
- � Telomeres
- � Bidirectional Replication from Multiple Origins in Eukaryotes
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Eukaryotic Replication Proteins
- ?DNA polymerase alpha-DNA primase�initiation; priming of Okazaki fragments
- ?DNA polymerase sigma�processive DNA synthesis
- ?DNA polymerase epislon�DNA replication and repair in vivo
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PCNA (proliferating cell nuclear antigen)�sliding clamp
- Replication factor-C Rf-C)�loading of PCNA
- Ribonuclease H1 and Ribonuclease FEN-1�removal of RNA primers
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Telomere Length and Aging
- � Most human somatic cells lack telomerase activity.
- � Shorter telomeres are associated with cellular senescence and death.
- � Diseases causing premature aging are associated with short telomeres.
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The large DNA molecules in eukaryotic chromosome replicate bidirectionally from multiple origins.
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Two or three DNA polymerases (alpha, sigma and/or epsilon) are present at each replication fork in eukaryotes.
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Telomeres, the unique sequences at the ends of chromosomes, are added to chromosome by a unique enzyme called
telomerase.
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