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DNA Replication
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Copy me!Copy me!
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What is DNA REPLICATION?• DNA Replication:
process of copying and synthesizing new DNA.
• Occurs in nucleus before a cell divides so that each new cell
produced has a complete set of chromosomes.
Mitosis-prophase-metaphase-anaphase-telophase
G1 G2
Sphase
Interphase
DNA replication takesplace in the S phase.
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How does DNA Replication begin?
• Begins at Origins of Replication• Two strands open forming
Replication
Forks (Y-shaped region)• New strands grow at the forks
ReplicationFork
Parental DNA Molecule3’
5’3’
5’
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Enzymes in DNA replication
HELICASE unwinds parental double helix
DNA POLYMERASEbinds nucleotides to form new strands
LIGASE joins Okazaki fragments and seals other breaks in
sugar-phosphate backbone
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Replication Step 1:
HELICASE unzips DNA down the middle by breaking Hydrogen
bonds.
5’3’
5’
3’
3’5’
5’3’
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Replication Step 2:Before new DNA strands can form, there must
be RNA primers present to start the addition of new nucleotides
RNAPrimerNucleotide
5’
5’ 3’
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Step 2:DNA POLYMERASE moves along the 2 strands and base pairs
nucleotides to each original parent strand.
5’
5’Overall directionof replication
5’
3’
5’3’
3’
3’
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Step 3:Replication moves in only ONE direction. So, small DNA
segments called OKAZAKI FRAGMENTS are made on the other strand.
5’ 5’
5’3’5’3’
3’5’
3’Overall directionof replication
3’
Okazaki fragment
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Step 4:
DNA LIGASE seals up all of the gaps in the DNA molecule, like
the Okazaki fragments.
3’5’
3’
5’ 3’
5’3’
3’
5’9
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2 identical DNA molecules are formed from the original.
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Semi-conservative Model• Idea presented by Watson & Crick•
The two strands of the parental
molecule separate, and each acts as a template for a new
complementary strand
• New DNA consists of 1 PARENTAL (original) and 1 NEW strand of
DNA
Parental DNA
DNA Template
New DNA
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What are mutations?• DNA MUTATIONS can occur when
a base (A, T, C or G) is accidentally changed.
– DNA Polymerase proofreads new DNA molecules to catch most of
these mutations and fix them.OTHER CAUSESEnvironmentalAgents:• UV
rays• Chemicals• X-rays
EFFECTS• Genetic
disorders• Cancer
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Any Questions?
• Finish your Cornell Notes…
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DNA ReplicationWhat is DNA REPLICATION?How does DNA Replication
begin?Enzymes in DNA replicationReplication Step 1:Replication Step
2:Step 2:Step 3:Step 4:Slide Number 10Semi-conservative ModelWhat
are mutations?Any Questions?Slide Number 14
