DNA Replication

Facts:

DNA replicates before a cell divides

Occurs during the S or synthesis phase of mitosis

Replication creates identical copies of DNA strands

Takes place in the nucleus

3

NitrogenousNitrogenousBase (A,T,G or C)Base (A,T,G or C)

““Rungs of ladder”Rungs of ladder”

““Legs of ladder”Legs of ladder”

Phosphate &Phosphate &Sugar BackboneSugar Backbone

DNA Double HelixDNA Double Helix

Replication:

Begins atBegins at Origin of ReplicationOrigin of Replication Two strands open forming a Two strands open forming a

Replication Forks (Y-shaped Replication Forks (Y-shaped region)region)

New strands grow at the forksNew strands grow at the forks

ReplicationReplicationForkFork

Parental DNA MoleculeParental DNA Molecule

3’

5’

3’

5’

Enzymes

The Enzyme, The Enzyme, Helicase,Helicase, unwinds and separates the unwinds and separates the 2 DNA strands by breaking 2 DNA strands by breaking the weak hydrogen bondsthe weak hydrogen bonds

DNA polymeraseDNA polymerase adds adds nucleotides complimentary nucleotides complimentary to the parental strandto the parental strand

Mutations

What do you think a mutation is?

Proofreading

- Replication occurs at multiple sites along the DNA

- DNA polymerase is very accurate- However mutations do occur- Proofreading occurs continuously

by DNA polymerase

More practice

Write the complimentary strand:

T A C G G A C T A A C G C

More practice

Write the complimentary strand:

T A C G G A C T A A C G CA T G C C T G A T T G C G

Review:

If there is 20% Thymine bases on a DNA strand what are the percentages of the other three bases?

Answer:

20% Thymine = 20% Adenine20% + 20% = 40 % 100% - 40% = 60% 60% / 2 = 30%30% Cytosine and 30% Guanine

Review

When and where does DNA replication occur?

What is the relationship between adenine and thymine called?

What are the two enzymes involved in DNA replication?

Activity

In pairs you are going to create your own DNA models.

Pieces

Sugar - Black pentagon Phosphate - White tube Nitrogenous bases -

– Adenine - Orange (5)– Guanine - Yellow (4)– Cytosine - Blue (4)– Thymine - Green (5)

Hydrogen bonds - White rods

Procedure

Create 18 nucleotidesPut together a 9 rung ladder of DNABe sure to have correct base pairingAfter having a complete DNA molecule

create 18 more nucleotides

DNA replication

Begin to “unzip” DNA molecule from one end

Add complimentary nucleotides to create two DNA molecules