DNA Replication
• DNA Replication is a semiconservative process where the
new DNA is copied onto a parental (conserved) strand. It
takes place with surprising efficiency and speed copying
~10 billion base pairs in a few hours with little or no errors.
Origin of replication
• Site of initiation of replication
– bacteria have a single site while
Eukaryotes have multiple sites
• proteins (enzyme helicase) recognize site
and open up a replication bubble
– as replication begins a replication forks
form as replication proceeds in both
directions
• Nucleotides (ACTG) are added 1 at a time
by DNA polymerase (~50/sec) in the 5' to
3' direction (copied 3' to 5')
– replication forks eventually fuse
completing the newly formed strands
• leading strand - 3' to 5'
– an RNA primer is needed for
attachment of DNA pol
• RNA attached with the enzyme
primase
– DNA polymerase attaches to
the primer and adds
nucleotides one at a time in the
5' to 3' direction
• replication continues until
completion or meeting another
replication fork
– Replicated fragments joined
with enzyme ligase
Antiparallel
elongation
• since nucleotides can only be added to the 3'
end of the newly forming strand, different
mechanisms must be in place for the
antiparallel strand
– DNA pol attaches at the replication fork
and copies back to the growing strand in
the 5' to 3' direction
• small 100 to 200 nucleotide segments
called Okazaki fragments
– replication continues until DNA pol
reaches a primer
• Primer falls off
• DNA pol replaces the RNA primer with DNA
– Okazaki fragments are joined by DNA
ligase as DNA pol detaches
lagging strand - 5'
to 3'
Other proteins involved
– topoisomerase - relieves supercoiling caused by
helicase
– single-strand binding protein - stabilizes the DNA
strand that has been unwound until it is replicated
Telomeres
• Small sections of DNA at the 3' end of the DNA cannot be replicated
as the RNA primer occupies the space. As a result daughter
chromosomes are shorter that the parent chromosomes.
– telomeres are regions of DNA located at the ends of chromosomes
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contain 100 - 1000 repeating units (TTAGGG)
protect internal gene sequences from erosion
get shorter with each replication
associated with the aging process
– telomerase is an enzyme active in germ cells and restores the length
to the chromosomes
• is inactive in somatic cells
– may protect somatic cells from cancer