1. 7.1 DNA structure and replication
Applications:
Rosalind Franklin’s and Maurice Wilkins’ investigation of DNA structure by X-ray diffraction
Tandem repeats are used in DNA profiling
Use of nucleotides containing dideoxyribonucleic acid to stop DNA replication
Understanding:
DNA structure suggested a mechanism for DNA replication
Nucleosomes help to supercoil the DNA
DNA replication is continuous on the leading strand and discontinuous on the lagging strand
DNA replication is carried out by a complex system of enzymes
DNA polymerase can only add nucleotides to the 3’ end of the primer
Some regions of DNA do not code for proteins but have some other important function
Skills:
Analysis of results of the Hershey and Chase experiment providing evidence that DNA is the genetic material
Utilisation of molecular visualisation software to anaylyse the association between proteins and DNA within a nucleosome
Nature of science:
Making careful observations: Rosalind Franklin’s X-ray diffraction provided crucial evidence that DNA is a double helix

2. Hershey-Chase experiments
Is DNA the genetic material? Alfred Hershey and Martha Chase used radioisotopes to help prove this.

3. Radioisotopes
Forms of elements that decay over time at a predictable rate. Particles given off in this decay allow detection of the specific isotope used

4. Hershey-Chase experiments
Grew bacteriophage viruses in two different types of cultures.
Radioactive phosphorous-32
- Detectable phosphorous in DNA
Radioactive sulphur-32
Detectable sulphur found in outer coat of virus
Allowed to infect bacteria (E.coli)

5. Hershey-Chase experiments
As DNA contains phosphorous and not sulphur, they concluded that DNA was the genetic material not protein.

6. Hershey-Chase experiments
Describe the Hershey-Chase experiment:

7. DNA structure 3 components: Pentose sugar Phosphate Organic base
Stay the same
Changes
Contains nitrogen & carbon
Pentose sugar
(5 Carbon atoms)
3 components:
Pentose sugar
(deoxyribose in DNA)
Phosphate
Organic base
(always contains nitrogen)

8. Single strand of DNA
Backbone is made of alternating phosphate and deoxyribose sugar
They are held together by a phosphodiester bond (just ester at SL!)
Condensation reaction – producing water
This produces a chain of DNA (as this links the single nucleotides together)

9. Phosphodiester bond Where does the phosphodiester bonds occur?
Phosphate group reacts on the 5’ carbon
Hydroxyl group reacts on the 3’ carbon

10. Double strand of DNA
The strands of DNA run in opposite directions to each other
There is a 5’ carbon free to bond on this end…
…and a 3’ carbon free to bond on this end
There is a 3’ carbon free to bond on this end…
…and a 5’ carbon free to bond on this end

11. Each end is either the 5-prime or 3-prime end
Double strand of DNA
Each end is either the 5-prime or 3-prime end 5’ 3’ 3’ 5’

13. Label the phosphodiester bond, hydrogen bonds, 3’ end and 5’ end for both strands, bases, ribose sugar, phosphate group, 3’ carbon and 5’ carbon in ribose.

14. DNA packaging DNA molecules are paired with a protein called histone
Histones help to package DNA
Essential as DNA can be 4cm long, it must fit into a microscopic nucleus

15. DNA packaging Unfolded DNA looks like beads on a string
These beads are nucleosomes
8 histones (proteins) make up a nucleosome core
DNA wraps round these histones twice
(slight negative charge on DNA attracts to positive charge of histones)
Between the nucleosomes are strings of DNA

16. DNA sequences
From the Multinational Genome Project we have learnt that less than 2% of human DNA codes for proteins.
What does the other 98% do?
Regulate gene expression
Telomeres (protects DNA)
Introns (interruptions in coding region)

17. Types of DNA sequences Find out what the following are:
Protein-coding genes
Highly repetitive sequences
Structural DNA
Short tandem repeats

18. Protein-coding genes
Structural DNA
Highly repetitive sequences
Short-tandem repeats

19. Protein coding genes Genes that have coding functions
Provide base sequence to produce proteins
Genes will have interruptions of non-coding regions in between them then must be spliced out before proteins are made.
Coding regions = Exons
Non-coding regions = Introns

20. Highly repetitive sequences
3-500 base pairs long in a repetitive sequence
It could go up to 100,000 repeats of a single unit
If it is clustered together – satellite DNA
So far not discovered any coding function (not genes)
Transposable – can move from one location to another
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT

21. Structural DNA Highly coiled DNA with no coding function.
Near centromere and telomeres
Could have lost their function due to mutations involving a base sequence change

22. Short Tandem Repeats
Your DNA is almost identical to the person next to you
Specific regions are varied – polymorphisms
Analyse these using DNA profiling
Usually look at short tandem repeats
Short repeating sequences of DNA – 2-5 base pairs
T T T C C C T C A T C A T C A T C A T C C C G
A A A G G G A G T A G T A G T A G T A G G G C

23. DNA replication Replication starts as a bubble
Helicase breaks the hydrogen bonds between nucleotides
Two strands separate
At each end of the bubble there is a replication fork – where DNA strands open.
Bubbles enlarge in both directions – bidirectional
Bubble eventually fuse with one another to produce two identical daughter DNA molecules
DNA replication

24. DNA replication
Primer (short piece of RNA) is produced at the replication fork by primase
Primers match exposed DNA bases, marks the start of replication
DNA polymerase III allows addition of nucleotides in 5’ to 3’ direction
DNA polymerase I removes the primers from the 5’ end

25. Energy to create the bonds
Each nucleotide molecule is a deoxynucleoside triphosphate (dNTP) molecule
Contains
Deoxyribose
A base
Three phosphate groups
As the molecules are added together to form nucleotides, two phosphates are lost
This provides energy needed for the nucleotides to bind

26. DNA replication
When DNA is replicated – it assembles 5’ to 3’ due to the action of polymerase III. 5’ 3’ 3’ 5’

27. DNA replication
This means that there is a difference in the assembly of DNA from the templates
Leading strand – continuous and fast (5’ to 3’)
Lagging strand – slowly (3’ to 5’)

28. DNA replication
The leading strand assembled continuously in 5’ to 3’ direction
Lagging strand is assembled by fragments produced moving away from the replication fork – in the 5’ to 3’ direction
Fragments of the lagging strand called Okazaki fragments
Primer, primase and DNA polymerase III are needed to begin both the leading and lagging strand
Once Okazaki fragments assembled, DNA ligase enzyme attaches them together to make a single strand

29. Formation of the lagging strand1. 2. 3. 4. 5. 6.

30. Proteins in replication
What do they do?
Protein
Role
Helicase
Primase
DNA polymerase III
DNA polymerase I
DNA ligase

31. Proteins in replication
What do they do?
Protein
Role
Helicase
Uniwnds double helix
Breaks H bonds
Primase
Synthesises RNA primer
DNA polymerase III
Synthesises new strand by adding nucleotides onto the primer (5’ to 3’ direction)
DNA polymerase I
Removes primer
DNA ligase
Joins Okazaki fragments

32. Complete for your homework
DNA sequencing
What is DNA sequencing?
What is the process?
What is it used for?
DNA profiling
What is DNA profiling?
How is it different to DNA sequencing?