Unlocking the Mystery Unlocking the Mystery ofof HeredityHeredity

DNADNA

The Ghost of Science Past…

I will take you on a journey to unravel the molecular mysteries of

genetics!

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.

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?

DNA stores and transmits the

genetic information from one generation of

an organism to the next!!!

!

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?

DNA is the genetic material of the viruses, not the protein coat! That’s two

experiments that say DNA is

the genetic material!!!

So what is DNA, anyway? DNA = DeoxyriboNucleic Acid

Long molecule made up of units called nucleotides

3 Basic Parts of DNA:

1. 5-carbon sugar (deoxyribose)

2. A phosphate group

3. Nitrogenous base

The deoxyribose sugar and the phosphate group combine to make up the

sugar-phosphate backbone

Nitrogenous Bases

Adenine pairs with Thymine (A with T)

Cytosine pairs with Guanine (C with G)

The bases are held together by hydrogen

bonds.

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.

Who figured out the structure of DNA? A lot of different scientists made

discoveries that led to a model of DNA…

Chargraff’s Rule: A=T and C=G

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.

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.

Should we give some credit to

Ros? Nah. She’s just a

woman…

The deoxyribose sugar and the phosphate group combine to make up the

sugar-phosphate backbone

Nitrogenous Bases

Adenine pairs with Thymine (A with T)

Cytosine pairs with Guanine (C with G)

The bases are held together by hydrogen

bonds.

DNA DNA REPLICATIONREPLICATION

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.

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!

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.

helicasehelicaseprimaseprimase

DNA DNA polymerasepolymerase

helicasehelicaseprimaseprimase

DNA DNA polymerasepolymerase

DNA polymerase adds nucleotides to the strand, creating a double-stranded DNA molecule that is a

copy of the original.

Primase gets the strand ready, so that DNA polymerase can do its job…

It also proofreads its work, to avoid mistakes in replication.

Replication is semiconservative.

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

http://www.dnatube.com/video/365/DNA-Replication