2. Cells make exact copies of themselves by the process of MITOSIS

3. To allow MITOSIS to commence the cell has to synthesise new ORGANELLES
To allow MITOSIS to commence the cell has to synthesise new ORGANELLES. This cell growth is sometimes referred to as the CELL CYCLE.
Mitosis produces 2 cells with the exactly the same GENOTYPE.

4. DNA Replication!

5. DNA Replication is Semi Conservative!
DNA must be replicated for cell growth, repair and the formation of gametes
There are lots of different suggestions about how DNA, which is very complex, is able to make accurate copies.
Semi conservative replication is the most universally accepted.
During semi conservative replication, each strand retains half of the original DNA material, and half the new material so some of it is conserved.

6. For semi conservative replication to occur there must be:
The four types of nucleotide, each with their bases present
Both strands of DNA must act as a template
DNA polymerase and DNA helicase must be present
A source of chemical energy must be present to drive the reaction

7. Semi Conservative Replication…
DNA helicase (enzymes that break hydrogen bonds) breaks the hydrogen bonds linking the two strands of DNA.
DNA Helicase

8. Semi Conservative Replication…
2. The double helix unwinds and separates into its two strands

9. Semi Conservative Replication…
3. Each polynucleotide strand acts as a template. Free nucleotides are activated (with energy!) and attracted to their complimentary bases to line up

10. Semi Conservative Replication…
4. DNA polymerase (an enzyme to join bases together) joins the nucleotides together to form a complete polynucleotide chain. In this way two new strands of DNA are formed.
New Strands
Original Strand
Original Strand

11. Another Possible model for DNA replication
Conservative Model – parental DNA remained intact and that a separate daughter DNA copy was built up from the new molecules of deoxyribose, phosphate and organic bases. Of the two strands present one strand would be made of entirely new material and one would be the original material.

12. Semi Conservative Replication…
Each strand retains half of the original DNA material, and half the new material so it is semi conservative.

13. Enzymes to remember
Helicases – break hydrogen bonds between complimentary pairs of bases in a DNA molecule.
DNA polymerases –catalyse formation of a new polynucleotide chain by complimentary base pairing.
DNA ligases – join together small sections of polynucleotide

14. Meselsohn and Stahl
Carried out an experiment to prove semi conservative replication

15. Meselsohn and Stahl
Bacteria were grown on a medium containing the heavy isotope of Nitrogen 15N.
The nitrogen was made into nitrogenous bases in both strands of their DNA
A sample of bacteria was fractionated and centrifuged, their DNA settled.
The remaining bacteria were grown on a medium containing the normal nitrogen isotope 14N.
After one generation the bacteria were fractioned and centrifuged and their DNA settled.
After two generations the bacteria were fractionated and centrifuged.

16. Meselson and Stahl 2nd Generation 50% Medium Weight DNA
One strand is original 15N and one new light strand of 14N nucleotides
50% Light Weight DNA
One strand of 14N and one new strand of 14N
Original Bacteria
ALL bacteria contained 15N
Heavy weight DNA
1st Generation
Medium Weight DNA
1 strand made from original 15N and one new strand of light 14N