1. If you can’t see it how can you tell what it is made of?
Discovering DNA
If you can’t see it how can you tell what it is made of?

2. DNA Ground Zero: What we knew to start with
1) Parents pass genetic traits on to their offspring
(inheritance)
2) Chromosomes in a cell’s nucleus carry the traits
3) Chromosomes are made of DNA and Proteins
(histones)
So we can conclude that…..
Genetic info is carried by either DNA or protein

3. Many scientists contributed to discovering the structure of DNA
1) Griffith – bacteria give genetic traits to other bacteria.
the trait they passed on was ability to secrete a capsule
Didn’t know if DNA or protein was being traded

4. 2) Avery – 1st to show DNA is the genetic material.
Showed that only DNA could transfer a trait from one bacteria to another….
give rough bacteria the ability to make a capsule

5. 3) Hershey and Chase – conclusive evidence that DNA is genetic material
Background:
1) Viruses are only made of DNA and Protein
2) Viruses transfer their genes to other cells
3) DNA has phosphate, but Protein does not
4) Bacteriophage Viruses inject genetic material into bacteria

6. Hershey and Chase Experiment:
1) create viruses with radioactive DNA
2) Let viruses put their genetic material into a bacteria
3) If the bacteria is then radioactive, the genetic material
is DNA
4) If the bacteria is NOT radioactive, the genetic material
is Protein

8. 4) Chargaff -
Showed that there was always the same amount of A nucleotides as T nucleotides
and
There were always the same amount of G as C
Chargaff’s Rule: A=T G=C
Clue this gave Watson and Crick…..
A is linked to T and G is linked to C

9. 5) Franklin- Background:
DNA molecules are too small to see with a microscope
Visible light’s wavelenth is too long & goes around it
X – rays have shorter wavelenth and bend around DNA
Bending light is called diffraction
The pattern of bending can be used to identify the shape of the object that bends it

10. Franklin’s Experiment –
Took X-ray diffraction photos of DNA
Measured the diffraction patterns and concluded
DNA was a helix and calculated the diameter of the DNA molecule

11. 6) Watson & Crick
Based on the fact that DNA contains the carbon sugar Deoxyribose.
Based on Chargaff’s Rule : DNA contains the nitrogenous bases A, T, G and C
Based on Franklin’s photo showing a helical shape

12. Watson & Crick Constructed the first working Model of DNA’s structure

13. The Players:
1) Griffith – bacteria give genetic traits to other bacteria.
2) Avery – 1st to show that DNA is the genetic material.
3) Hershey & Chase – Showed conclusively that DNA is the genetic material
4) Chargaff – A = T and G = C
5) Franklin – x-ray diffraction
6) Watson & Crick – model of DNA

14. DNA structure DNA is made of subunits called Nucleotides
Nucleotides are made of 3 parts
1) one 5 carbon sugar
2) phosphate
3) nitrogen containing base

15. Nucleotides are bonded together in 2 chains
covalent bonds hold one nucleotide to the next
The strong bonds between phosphates and sugars forms the backbone of the double helix

16. 2 chains of nucleotides are held together by weak
Hydrogen bonds between their bases

17. The two joined strands are twisted into a double helix.
The cell stores DNA wrapped around proteins called histones to form a bundle called a nucleosome
Further wrapping of DNA forms the X-shape
Seen in mitosis

19. Semi conservative Replication
One double helix is replicated so there are 2 double helixes.
1 molecule of DNA becomes 2 molecules of DNA
Each molecule retains one of the original chains of nucleotides

20. Replication origins and replication Forks
In eukaryotes replication takes place at multiple replication origins at the same time.
The area where the 2 strands of parental DNA are being separated is a
Replication fork
Daughter strands are
Fused together where
They meet

21. Parent vs Daughter Strands
The original DNA strands (chains of nucleotides) are called Parent strands.
The strands formed of new nucleotides are called daughter strands
Daughter strands are complimentary to parent strands
The two strands are antiparallel (5’ -> 3’ and 3’ -> 5’)

22. Helicase Topoisomerase Primase DNA polymerase DNA ligase Nuclease
Enzymes
Helicase
Topoisomerase
Primase
DNA polymerase
DNA ligase
Nuclease
Telomerase

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24. In order to help overcome these types of topological problems caused by the double helix, topoisomerases bind to either single-stranded or double-stranded DNA and cut the phosphate backbone of the DNA. This intermediate break allows the DNA to be untangled or unwound, and, at the end of these processes, the DNA backbone is resealed again. Since the overall chemical composition and connectivity of the DNA does not change, the tangled and untangled DNAs are chemical isomers, differing only in their global topology, thus their name. Topoisomerases are isomerase enzymes that act on the topology of DNA.[1]