Download 2.7 DNA Replication - LaPazColegio2014-2015

Topic 2.7
DNA Replication
Why does DNA replicate?
• All cells must replicate DNA before mitosis/meiosis
• All daughter cells receive the correct chromosomal
(genetic) information
When does DNA replicate?
• DNA Synthesis (SPhase) Replication
• All cells must
replicate DNA before
mitosis or meiosis
How does DNA replicate?
• Three possible modes of DNA replication
are possible:
• conservative
• semiconservative
• dispersive
Meselson-Stahl experiment
• DNA replication is
semiconservative
• each new DNA molecule
consists of one old strand and
one newly synthesized strand
Meselson-Stahl experiment
• DNA replication is semiconservative
• each new DNA molecule consists of one old
strand and one newly synthesized strand
Semiconservative Replication
• The complementarity
of DNA strands allows
each strand to serve
as a template for
synthesis of the other
Where does DNA replication start?
• begins at the origin of
replication
• bidirectional
• replication fork
DNA Synthesis in Bacteria
• DNA polymerase catalyzes DNA
synthesis and requires a DNA template
and all four dNTPs
DNA Synthesis
• Chain elongation occurs in the 5' to 3' direction
by addition of one nucleotide at a time to the 3'
end
• As the nucleotide is added, the two terminal
phosphates are cleaved off
What Occurs During DNA Replication?
• There are seven key issues that must be
resolved during DNA replication:
• unwinding of the helix
• reducing increased coiling generated during
unwinding
• synthesis of a primer for initiation
• discontinuous synthesis of the second strand
• removal of the RNA primers
• joining of the gap-filling DNA to the adjacent
strand
• proofreading
What Occurs During DNA Replication?
Primer for polymerization
• To elongate a polynucleotide chain, DNA
polymerase requires a primer
• The enzyme primase synthesizes an RNA
primer
replication fork
• leading strand
• continuous DNA
synthesis
• lagging strand
• discontinuous DNA
synthesis
discontinuous DNA synthesis
• lagging strand
• RNA primer - Okazaki fragments
• primers removed - DNA polymerase I
• fragments joined - DNA ligase