Upload
colin-nicholson
View
220
Download
0
Tags:
Embed Size (px)
Citation preview
1
Aim #16: How does a DNA molecule replicate
itself?
1.
2
3
CELL DIVISION
ENZYMES
4
2. Replication Facts
• DNA has to be copied before a cell divides
• New cells will need identical DNA strands
• DNA serves only as a template.
• Enzymes and other proteins do the actual work of replication.
copyright cmassengale 5
• DNA REPLICATION ANIMATION
6
7
3. Starting Replication
a) Begins at Origins of Replicationb) Two strands open forming
Replication Forks (Y-shaped region)
ReplicationFork
Parental DNA Molecule
3’
5’
3’
5’
8
3. Starting Replication
c. Enzyme DNA Helicase “unzips” and separates the 2 DNA strands by breaking hydrogen bonds
d.Single-Strand Binding Proteins attach and keep the 2 DNA strands separated and untwisted
9
Each unzipped strand is a template, or pattern, for a new molecule of DNA to
form
10
11
4. RNA Primase• RNA primers: before new DNA
strands can form, there must be RNA primers present to start the addition of new nucleotides (by DNA Polymerase).
• RNA Primase: enzyme that synthesizes the RNA primer.
12copyright cmassengale
13
5. DNA Polymerase• DNA Polymerase: with a RNA
primer in place, DNA Polymerase (enzyme) adds new nucleotides to the DNA template in the 5’ to 3’ direction– DNA polymerase also “proofreads” as it
adds nucleotides to make sure it is copied correctly
RNAPrimerDNA Polymerase
Nucleotide
5’
5’ 3’
Direction of Replication
6. Leading and Lagging Strands
14
15
Remember the Strands are Antiparallel
P
P
P
O
O
O
1
23
4
5
5
3
3
5
P
P
PO
O
O
1
2 3
4
5
5
3
5
3
G C
T A
16
Leading and Lagging Strands
• The Leading Strand is synthesized as a single strand from the point of origin in a 5’ to 3’ direction
RNAPrimerDNA PolymeraseNucleotides
3’5’
5’
17
Leading and Lagging Strands
• Lagging Strand: also synthesized in the 5’ to 3’ direction, but
discontinuously against overall direction of replication
• This strand is made in MANY short segments
RNA Primer
Leading Strand
DNA Polymerase
5’
5’
3’
3’
Lagging Strand
5’
5’
3’
3’
18
7. Okazaki Fragments• Okazaki Fragments - series
of short segments on the lagging strand
• Must be joined together by an enzyme
Lagging Strand
RNAPrimer
DNAPolymerase
3’
3’
5’
5’
Okazaki Fragment
19
Joining of Okazaki Fragments
• The enzyme Ligase joins the Okazaki fragments together to make one strand
Lagging Strand
Okazaki Fragment 2
DNA ligase
Okazaki Fragment 1
5’
5’
3’
3’
20
Leading and Lagging Strands
Replication Fork
Point of Origin
8. Ligase• Joins Okazaki Fragments• Replaces RNA primer with DNA
nucleotides
copyright cmassengale 21
22
9. Proofreading New DNA
• DNA polymerase initially makes about 1 in 10,000 base pairing errors
• Enzymes proofread and correct these mistakes
• The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors
23
10. Semiconservative Model of Replication
• 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
24
25
11. Prokaryote vs Eukaryote
26
Begins at one point-continues in two directions until chromosome is completely copied
Begins at multiple points-continues in both directions until chromosome is completely copied (replication bubbles)