Download Notes_DNA Replication_teacher

DNA Replication Notes
Purpose: To make an identical copy of DNA for cell division
Process:
Background:
 DNA must always be
protected inside the
nucleus except during cell
division (mitosis); it will get
destroyed by enzymes in
the cytoplasm.
 enzymes and freenucleotides are available
for replication in the
nucleus.
Part 1 Initiation:
 an enzyme called helicase
attaches to DNA and
breaks the Hydrogen
bonds to unwind and
separate the
complementary strands.
 This creates a replication
fork
Part 2 Elongation:
 Another enzyme called
DNA polymerase
attaches to one strand.
 It reads the DNA code,
and attaches
complementary
nucleotides to the original
exposed strand.
 After it attaches each
complementary nucleotide,
it proofreads for
mistakes.
Replication is Quick:
 More Helicase and DNA
Polymerase enzymes can
attach at other sites on the
strands to speed up the
process.
 This creates the
appearance of bubbles
along the strand called
replication bubbles.
DNA molecule is antiparallel: Complementary strands run in opposite directions. Scientists
label the ends 3’ and 5’.
Leading and Lagging Strands: DNA polymerase can only attach new nucleotides to the 3’ end of
the new DNA strand. This means that it must constantly back track to copy parts of the strand newly
exposed by helicase. Since this process takes longer than the other strand, it is called the lagging
strand. The Strand that does not need to do this is called the leading strand.
The fragments that are created in the lagging strand are called Okazaki fragments.
A third enzyme, Ligase, fuses the sugar-phosphate backbone of the Okazaki fragments together.
Part 3 Termination:
 When complete, the result
should be two perfect
copies of the original.
 Though 3.3 billion base
pairs of the human
genome can be copied in a
matter of hours, few
mistakes (mutations)
occur.
 This process is semiconservative: Each new
molecule has one original
strand and one new
strand.
** Note: Make sure you understand the roles of the three enzymes, what semi-conservative
means, and where in the cell this occurs and why.