1. Unlocking the Mystery of Heredity
DNA
Unlocking the Mystery of Heredity

2. The Ghost of Science Past…
I will take you on a journey to unravel the molecular mysteries of genetics!

3. The year is 1928.
We are in the lab of Frederick Griffith as he tries to determine why certain bacteria give people pneumonia.
Little does he know, he is about to discover something even more important.
Something was able to be passed from harmful bacteria to harmless ones, making them DEADLY.
Discovery:
Genetic information could be transferred from one bacteria to another. This is called transformation.
Help the students realize that if the info was transferred, it must have been encoded in a molecule… something that can be passed between bacteria, but that doesn’t disappear when a bacteria dies.

4. The year is 1944.
We are in the lab of Oswald Avery in New York.
Avery is repeating the experiments that Griffith did.
But he is trying to figure out which molecule was transforming the harmless bacteria into killers.
He uses enzymes to break down different molecules.
When he breaks down DNA, the harmless bacteria stay harmless.
What does this tell us about DNA?

5. DNA stores and transmits the genetic information from one generation of an organism to the next!!! !

6. It is now 1952.
Alfred Hershey and Martha Chase are busy in their laboratory studying viruses called bacteriophages. (they infect bacteria!)
The scientists knew that viruses infect cells by injecting their own genetic material inside, but they weren’t sure what this genetic material actually was.
Hershey and Chase radioactively marked the viral DNA.
When the viruses infected bacteria, they saw that the bacteria now contained radioactive material.
What could Hershey and Chase deduce from these results?

8. That’s two experiments that say DNA is the genetic material!!!
DNA is the genetic material of the viruses, not the protein coat!
That’s two experiments that say DNA is the genetic material!!!

9. DNA = DeoxyriboNucleic Acid
So what is DNA, anyway?
DNA = DeoxyriboNucleic Acid
Long molecule made up of units called nucleotides
3 Basic Parts of DNA:
5-carbon sugar (deoxyribose)
A phosphate group
Nitrogenous base

10. The bases are held together by hydrogen bonds.
Nitrogenous Bases
Adenine pairs with Thymine (A with T)
Cytosine pairs with Guanine (C with G)
The bases are held together by hydrogen bonds.
The deoxyribose sugar and the phosphate group combine to make up the sugar-phosphate backbone

11. Who figured out the structure of DNA?
A lot of different scientists made discoveries that led to a model of DNA…
To find out more about the discoveries about DNA we’ll visit Erwin Chargraff’s lab. Erwin was a biochemist and discovered that the percentage of adenine present in DNA is the same as the percentage of thymine. Also, the percentages of cytosine and guanine are equal to one another.
Chargraff’s Rule: A=T and C=G

12. It’s 1952 and Rosalind Franklin is using X-rays to study the structure of DNA.
Her X-ray pictures show a pattern that suggests two strands of DNA coiled around one another in a helix.
The pictures also suggest that the nitrogenous bases are near the center of the molecule.

13. Should we give some credit to Ros?
Meanwhile, Francis Crick and James Watson are working in their lab building 3D models of DNA using wire and cardboard. They can’t figure anything out until they are shown a copy of Rosalind Franklin’s X-ray picture.
They immediately figure out that the DNA molecule has to be a double-helix with two strands of DNA coiled around one another, nitrogenous bases facing inward, paired according to Chargraff’s Rule.
Nah. She’s just a woman…

14. The bases are held together by hydrogen bonds.
Nitrogenous Bases
Adenine pairs with Thymine (A with T)
Cytosine pairs with Guanine (C with G)
The bases are held together by hydrogen bonds.
The deoxyribose sugar and the phosphate group combine to make up the sugar-phosphate backbone

15. DNA REPLICATION

16. DNA is located in the nucleus.
Remember that…
DNA is located in the nucleus.
DNA is tightly coiled up around histone proteins so that it can all fit in the nucleus. The coiled DNA is called chromatin.
During mitosis, the DNA is duplicated and the chromatin condenses into chromosomes.

17. This is why replication can occur!
DNA is made up of two complementary strands.
Remember, the base pairing rules!
Due to specific base pairing, if you know the sequence of one strand, you can figure out the sequence of the other!
This is why replication can occur!

18. helicase primase DNA Replication occurs a replication forks.
The DNA is “unzipped” by an enzyme called helicase.
Helicase breaks the hydrogen bonds that hold the bases together.
helicase
primase
DNA polymerase

19. Primase gets the strand ready, so that DNA polymerase can do its job…
DNA polymerase adds nucleotides to the strand, creating a double-stranded DNA molecule that is a copy of the original.
It also proofreads its work, to avoid mistakes in replication.
helicase
primase
DNA polymerase

20. Replication is semiconservative.
This means that each new strand of DNA created is half parental or “old” DNA and half new DNA.