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Chapter 6

BIO206H5 Chapter Notes - Chapter 6: Dna Polymerase, Dna Replication, Okazaki Fragments

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George S Espie

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Chapter 6 – DNA Replication, Repair, and Recombination
The ability of a cell to survive and proliferate in a chaotic environment depends on the accurate
duplication of the vast quantity of genetic information carried in its DNA
oThis duplication process, called DNA replication, must occur before a cell can divide to produce
two genetically identical daughter cells
Maintaining order in a cell also requires the continual surveillance and repair of it genetic information, as
DNA is subjected to unavoidable damage by chemicals and radiation in the environment and by reactive
molecules that are generated inside the cell
Despite systems for protecting a cell’s DNA from copying errors and accidental damage, permanent
changes – or mutations – sometimes do occur
Mutations can be silent, detrimental, beneficial, or can produce small variations that underlie the
differences between individuals of the same species
Mutations are much more likely to be detrimental than beneficial
After each cell division, a cell must copy its genome with extraordinary accuracy
Base-Pairing Enables DNA Replication
Each strand of a double helix contains a sequence of nucleotides that is exactly complementary to the
nucleotide sequence of its partner strand
Each strand can serve as a template, or mold, for the synthesis of a new complementary strand
The ability of each strand of a DNA molecule to act as a template for producing a complementary strand
enables a cell to copy, or replicate, its genes before passing them on it its descendants
The copying must be carried out with incredible speed and accuracy
This impressive feat is performed by a cluster of proteins that together form a replication machine
DNA replication produces two complete double helices from the original DNA molecule, with each new
DNA helix being identical (except for rare copying errors) in nucleotide sequence to the original DNA
double helix
Because each parental strand serves as a template for one new strand, each of the daughter DNA double
helices ends up with one of the original (old) strands plus one strand that is completely new; this style of
replication is said to be semiconservative
DNA Synthesis Begins at Replication Origins
The process of DNA synthesis is begun by initiator proteins that bind to specific DNA sequences called
replication origins
oHere, the initiator proteins pry the two DNA strands apart, breaking the hydrogen bonds between
the bases
Separating a short length of DNA a few base pairs at a time does not require a large energy input
because individual hydrogen bonds are weak, and the initiator proteins can readily unzip the double helix
at normal temperatures
A-T rich stretches of AND are typically found at replication origins
A bacterial genome, which is typically contained in a circular DNA molecule of several million nucleotide
pairs, has a single replication origin
The human genome, which is very much larger, has approximately 10,000 such origins an average of
220 origins per chromosome
oBeginning DNA replication at many places at once greatly shortens the time a cell needs to copy
its entire genome
Once and initiator protein binds to DNA at a replication origin and locally opens up the double helix, it
attracts a group of proteins that carry out DNA replication
oThese proteins form a replication machine, in which each protein carries out a specific function
Two Replication Forks Form at Each Replication Origin
DNA molecules in the process of being replicated contain Y-shaped junctions called replication forks
Two replication forks are formed at each replication origin
At each fork, a replication machine moves along the DNA, opening up the two strands of the double helix
and using each strand as a template to make a new daughter strand
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