DNA REPLICATION AND REPAIR

before cell division occurs the DNA molecule unwinds and is replicated
DNA Replicates Semiconservatively
 each daughter cell receives one strand from the parent molecule and one strand is newly
synthesized
 see Fig. 1 on pg. 217
 Matthew Meselson and Franklin Stahl devised an experiment to
prove this in 1958
The Process of DNA Replication
Separating the DNA Strands
 replication begins when a protein binds to a specific site on the DNA molecule called the
replication origin
 the linear DNA of eukaryotes have more than one replication origin, while
the DNA of prokaryotes have only one
 an enzyme (DNA helicase) is responsible for breaking the
hydrogen bonds between the nitrogenous base pairs causing the helix to unwind
 single stranded binding proteins binds to the exposed bases and prevent hydrogen bonding
from occurring
 during bacterial replication an enzyme called DNA gyrase cuts
the strands to relieve the tension from unwinding then reseals
them later
 DNA replication will begin at the origins and move out in both
directions
 replication forks will exist where the two strands are still joined
 one strand moves towards the fork while the other moves away
 eukaryotes contain many forks since they have multiple sites
or origin
 a replication bubble forms between two closely situated forks
 see Fig. 4 on pg. 220
Building the Complementary Strand
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DNA polymerase III is the enzyme that builds the complementary strand

an RNA primer (a sequence of 10 – 60 RNA bases) sticks to the
3’ end with the help of an enzyme called primase

it recognizes the 3’ end and adds a deoxyribonucleoside
triphosphate to the end
DNA polymerase III is now free to add free
deoxyribonucleotide triphosphates
 the enzyme uses the energy from breaking the bond between the first and second phosphate
group so carry out the condensation reaction that is responsible for adding the
complementary nucleotide to the strand
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the two remaining phosphate are recycled to make more
nucleoside triphosphates
the 3’ to 5’ strand is the template used as a guide to create the leading strand (it moves
towards the fork), and builds continuously
the 5’ to 3’ strand is the template used as a guide to create the lagging strand (it moves away
from the fork), and is built discontinuously
DNA is built in short segments (Okazaki fragments) by Polymerase III
DNA Polymerase I removes the RNA primers in both the leading and lagging strands and
replace them with the appropriate deoxyribonucleotides.
DNA ligase joins the Okazaki fragments on the lagging strands by creating a phosphodiester
bond
See Fig. 7 on pg. 221
resulting in two double-stranded DNA molecules that automatically form a helix.
DNA Repair
 DNA polymerase III and I act as quality control checkers by proofreading the new strands
 if there is a mistake the enzyme can function as an
exonuclease; an enzyme that is free to cut out nucleotides that are incorrectly paired, and
add new ones.
Seatwork
Pg. 223 # 1, 2, 4 – 7, 8, 10