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DNA Replication - Madison Public Schools

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					Unit 14: Quest Number 2: DNA Replication, Mutations and Cancer
                                Honors Biology 2011
                                        Monkemeier
   The process of DNA replication is
    fundamentally similar for prokaryotes and
    eukaryotes.
   DNA replication is a semi-conservative
    process – this means that when the double
    helix is unwound, each original strand serves
    as a template or pattern for the newly
    synthesized strand.
   The replisome is a       Major Components
    complex molecular
    machine that              ◦ DNA Helicase
    carries out
                              ◦ DNA Primase
    replication of DNA.
   The replisome             ◦ DNA Polymerases
    contains all the
    necessary enzymes         ◦ DNA ligases
    for replication.          ◦ DNA Binding Proteins
   The process of              Write these steps down:
    replication can be          INITIATION: formation of
    thought of as having a
                                 replication fork
    beginning where the
    process starts; a           ELONGATION: a number of
    middle where the             enzymes work together to
    complementary                accomplish the task of
    nucleotides are added
                                 assembling a new strand
    to the exposed
    parental strands; and       TERMINATION: An end
    an end where the             point to the replication
    process is terminated
    or stopped.
   DNA helicases
    unwind and unzip
    the DNA molecule
    forming a
    replication fork.
   A replication fork is
    a Y-shaped region
    where the new
    strands of DNA are
    elongating.
   The elongation process is semidiscontinuous.
   The leading strand of the DNA molecule
    (exposed 3’ end) is synthesized continuously
    in the direction of the opening replication
    fork.
   The lagging strand of the DNA molecule
    (exposed 5’ end) is synthesized
    discontinuously in the direction opposite to
    the opening of the replication fork.
   Remember - The
    two strands of the
    DNA double helix
    are antiparallel.
    This means that
    their sugar –
    phosphate
    backbones run in
    opposite
    directions!
 DNA polymerases add nucleotides
 only to the FREE 3’ end of a
 growing DNA strand, NEVER to
 the 5’ end.

 DNApolymerases synthesize
 ONLY in the direction 5’ to 3’.
   DNA Primase synthesizes a short
    complementary segment to the exposed 3’
    end of the DNA strand.
   DNA polymerases extend the complementary
    segment toward the replication fork as it
    opens.
   The ability of the DNA polymerase to remain
    attached to the template (leading strand) is
    known as PROCESSIVITY.
 The synthesis of the lagging strand is
  described as discontinuous since it is
  synthesized in pieces that get glued together.
 DNA primase synthesizes a short segment of
  complementary DNA to the exposed 5’ end.
 DNA polymerases add complementary
  nucleotides in the direction away from the
  opening replication fork.
 DNA ligases glue the Okazaki Fragments
  together forming the new strand.
   Exposed 3’ end of the        Exposed 5’ end of the
    original DNA molecule         original DNA molecule
   Synthesized                  Synthesized
    continuously toward           discontinuously away
    the opening replication       from the opening
    fork.
                                  replication fork.
   The ability of a DNA         The Okazaki fragments
    polymerase to remain
    attached to the leading       are glued together by
    strand is known as            DNA Ligases.
    processivity.

Leading Strand                Lagging Strand
   Prokaryotes have circular DNA so replication
    ends when the process comes around to the
    origin again.
   In eukaryotes, end points for each
    chromosome are indicated by telomeres.
   Telomeres are specialized structures on the
    ends of eukaryotic chromosomes that are
    composed of specific repeated DNA
    sequences.
   Telomeres are special nucleotide sequences
    on the ends of the linear eukaryotic
    chromosomes.
   The typical repeating unit in human
    telomeres is TTAGGG. The number of
    repetitions in a telomere varies between 100
    and 1000.
   The telometric DNA protects an organism’s
    genes from being eroded through successive
    rounds of DNA replications.
   Remember that DNA polymerases can only
    add nucleotides in the 5’ to 3’ direction.
   The exposed 5’ end represents the lagging
    strand.
   The replisome does not contain the
    machinery to complete the 5’ ends of the
    daughter strands of the DNA. As a result,
    repeated replication produces shorter and
    shorter DNA molecules.
   Briefly describe the functions of the following
    enzymes during DNA replication: DNA
    helicases, DNA polymerases, and DNA
    ligases.
   Briefly provide an overview of DNA replication
    using the terms: initiation, elongation,
    termination.
   Distinguish between the leading strand and
    lagging strand during DNA replication?

				
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