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)