DNA Replication - Mr. Steckle's SciencePagestecklescience.weebly.com/.../dna_replication_pp.pdf · 2018-10-10 · • DNA still copies from the 5’ to 3’ end, but this will occur

DNA Replication

DNA Nucleotide

DNA vs RNA

Numbering The Carbons

Double Stranded DNA Has a 5’ and 3’ End

The 5’ and 3’ Refer To The Carbons On The Sugar Ring

The 2 Strands Are Held Together By Hydrogen Bonds

DNA Can Only Be Added To The 3’ End

Adding Nucleotides Using Dehydration Synthesis To The 3’ End

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DNA Replication Happens In Several Steps Using Several Enzymes.

1. Unwind The Double Stranded Molecule Using TOPOSIOMERASE

2. Two Strands Are Separated Using DNA Helicase

3. Keep The Two Strands From Reannealing Using Single Stranded Binding Proteins (SSB’s)

Leading And Lagging Strands

• The DNA will separate and need to establish a leading and a lagging strand.

• Remember that DNA is added to the 3’ side, so the copied DNA works from 5’ to 3’.

• That means the leading strand will end at the 3’ end and the copying will be continuous. That means that the NEW DNA strand will continue to grow as the DNA unwinds.

• The lagging strand is a little more complicated.

• DNA still copies from the 5’ to 3’ end, but this will occur in discontinuous pieces called Okasaki Fragments.

• The piece of RNA primer needs to be replaced with DNA and the pieces need to be fused together.

• This will occur over and over again working away from the replication fork.

4. RNA Primase Comes In And Lays Down An RNA Primer.

5. DNA Polymerase III Begins To Copy The New Strand Of DNA.

First Okasaki Fragment

The Process Continues. The Leading Strand Continues To Grow. At The Same Time We Start A

Second Okasaki Fragment

The first Okasaki Fragment is now

farther from the replication fork

Second Okasaki

Fragment starts

(Needs RNA primer)

Leading strand continues to grow

6. DNA Polymerase I Will Come In And Swap The RNA Primer With DNA

7. DNA Ligase Fuses The Two OkasakiFragments Together

Two New Strands Are Formed. Since Each Strand Contains One NEW and One OLD, This Is

Considered SEMI-CONSERVATIVE

The Two Strands Will Begin To Twist Back Into A Double Helix. The Two New DNA Molecules

Should Be Identical