DNA Replication
When?
• S (synthesis) phase of cell cycle
• Creates copy of DNA and two copies are
held together by centromere.
• Thousands of times per second
throughout the body.
Where?
• Always in the nucleus of eukaryotic cells.
• All cells that are not in G0 phase (no
longer dividing).
Why?
• When cells divide through meiosis or
mitosis, new cells need their own DNA.
How?
• Two different mechanisms: lagging strand
and leading strand, but always in the 5’ to
3’ direction.
• Eukaryotic chromosomes average about
150 million nucleotides, so multiple
replication forks are needed to finish the
job.
• Semi-conservative
Key enzymes
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Topoisomerases unknot the DNA.
Helicase unzips the DNA
RNA primase adds RNA primer
DNA polymerase adds nucleotides, repairs
mismatch pairs, and removes the RNA
primer
• DNA ligase seals the okazaki fragments
together by creating phosphodiester
bonds.
Proofreading
• 1/1,000,000,000 nucleotides end up being
incorrectly copied, however the original
number is closer to 1/10,000.
• During replication, DNA Polymerase finds
mismatched pairs and fixes them.
• There are many other proteins that also
proofread the mistakes.
Telomeres
• The 5’ ends of daughter strands can never be fully
completed due to the 5’ to 3’ direction.
• Telomeres are DNA sections at the 5’ end of a
chromosome that do not carry genes.
• Telomerase is an enzyme with RNA that adds to
the telomeres as the wear down through many
rounds of replication.
• Telomerase is only found in gamete producing
cells as newborns need long telomeres.
• Limitations on the life span of cells is based on
the length of the telomeres.
Cancer
• Cancer cells divide very quickly and often,
so telomerase activity is need to keep the
telomeres from getting too small.
• Telomerase activity is found in about 90%
of all cancers.