1. DNA Replication

2. DNA Replication DNA is a self-replicating molecule
This is vital as DNA is the genetic material of all cells
DNA will duplicate itself before cellular reproduction

3. Why does DNA Replicate?
When cells go through Mitosis, they will split into two cells
Mitosis makes any identical copy of an original cell
In order for the cell to be identical it must copy the DNA exactly.
Each cell must have the exact same DNA and same number of chromosomes for the cell to be viable.

4. When does this occur
DNA is replicated during the S (Synthesis) stage of the cell cycle.
This process if complicated and involves numerous proteins to aid in the replication of DNA

5. Where does this occur?
DNA replication takes place in the nucleus or eukaryotes
Prokaryotes do this in the cytoplasm
As the DNA carries the instructions for the cell, it is extremely important
To protect the DNA this takes place in the nucleus

6. Overview of Replication
The DNA will “split” at the nitrogenous bases
Complementary nucleotides will be added to the ‘unzipped’ strands
This will make 2 new strands of DNA

7. Semi-Conservative Replication
This process is called semi-conservative replication
There is always half of the DNA that is preserved the same from the original cell
This way each new cell gets a half of the DNA from the old cell

8. DNA Replication Details
Each strand contains a complete and complimentary copy of the information found on the other strand.
Eg - A is on one side T must be on the other side
If the Hydrogen-bonds are broken between base pairs, the one side has all the information necessary to make a new strand. All that has to be done is to make sure that the bases match up.
The strand that is read to make a complimentary copy is called the template.

9. Strands One strand is called the leading strand
The leading strand is 3` to 5`
Leading strands are replicated continuously
One strand is called the lagging strand
This is 5` to 3`
Lagging strands are replicated in sections

10. Replication Enzymes Replication is a chemical process
Since this will involve biological chemical reactions, it will involve enzymes to speed up the rate of these reactions.
Each reaction has its own enzyme
i.e. – each enzyme has its own specific job

11. Replication Enzymes
Helicase enzyme- breaks the Hydrogen bonds between the nitrogen bases.
Topoisomerase – helps with the uncoiling of the double helix
DNA primase – makes RNA primers that attach to ‘unzipped’ DNA
Helps DNA Polymerase by adding primers

12. Replication enzymes
DNA polymerase – adds nucleotides to an existing DNA strand
Works 5` to 3`
Cannot make DNA from scratch. They require a primer
DNA Ligase – Joins the original parental strand with the new strand of DNA.

14. DNA Replication
DNA replication begins at an origin of replication site
DNA Helicase begins to separate the hydrogen bonds while topoisomerase begins to help unwind the strands
Each separated strand will serve as a template during replication

15. DNA Replication The leading Strand
DNA Primase adds and RNA primer to the start of the DNA strand
DNA Polymerase (III) attaches at the site of the RNA primer and continuously attaches the appropriate nucleotides to match the leading strand’s bases
DNA polymerase only works from 3` to 5`

16. DNA Replication The Leading Strand
DNA Polymerase (I) removes the RNA primers and fills in the gaps with DNA
DNA ligase bonds the unjoined sections of the newly made strand

17. DNA Replication The Leading Strand
The leading strand continues to work forward as it replicates
This process is easy for DNA Polymerase as it is moving the same direction as Helicase

18. DNA Replication The Lagging Strand
The lagging strand is much more complicated for DNA to replicate
This is because DNA Polymerase can only work in one direction
This happens using a discontinuous process
Small sections are made and then joined together

19. DNA Replication The Lagging Strand
Helicase splits the strands creating a replication fork
DNA Primase adds an RNA primer
DNA Polymerase (III) attaches to the primer and adds complementary nucleotides
This happens in a small section called an Okazaki Fragment
DNA Polymerase is actually moving AWAY from the direction of the replication fork

20. DNA Replication The Lagging Strand
Closer to the replication fork, DNA primase has added another primer
DNA Polymerase (III) attaches and adds another Okazaki Fragment
This process continues and adds more DNA along the lagging strand

21. DNA Replication The Lagging Strand
DNA Polymerase (I) removes the RNA primers and fills in the gaps with DNA
DNA ligase bonds the unjoined sections of the newly made strand

23. Accuracy DNA replication is a highly accurate process
DNA replication in bacterial makes about one mistake every 100 million nucleotides
DNA polymerase is not only highly accurate but also works to proof-read the new DNA strand