The History of DNAHonors Biology

Chapter 12 Section 1

1928 Fredrick Griffith studying how bacteria made people sick and how different types of bacteria caused pneumonia

Isolates 2 strains (types) of bacteria from mice. Disease causing (grows in smooth colonies) and non-disease causing (grows in rough colonies)

Injects mice with disease causing: mice get pneumonia and die

Injects mice with non-disease causing: mice survive

Griffith

Griffith wonders if disease causing bacteria produce a toxic poison???

Griffith takes disease causing bacteria and heats it to kill the bacteria. Injects the heat killed bacteria into mice: mice survive

Griffith mixes heat killed bacteria with the non-disease causing bacteria injects into mice: mice get pneumonia and die

Griffith

Somehow the heat killed bacteria had passed their disease causing ability to the harmless strain

Griffith called this process transformation because the non-disease causing bacteria had been changed to disease causing

Griffith hypothesized that when the 2 strains were mixed, one factor was transferred from one strain of bacteria to the other. That factor had the ability to be passed from 1 generation to the next, transforming factor might be a gene

Griffith

Video of Griffith Experiment

Chapter 12A.mpg

Griffith

1944 Scientists wanted to know which molecule of the heat killed bacteria was required for the transformation

Took an extract from the heat killed bacteria and added enzymes that destroyed proteins, transformation still occurred

Repeated procedures for lipids, carbs and RNA, transformation still occurred

Repeated procedures for DNA, transformation did not occur

Avery

CONCLUSION: DNA was the transmitting factor

Concluded that the nucleic acid DNA stores and transmits the genetic information from 1 generation to the next

Avery

1952 Studied viruses, non-living particles that infect living organisms. Studied bacteriophage which means “bacteria eater”. Consists of a DNA or RNA core and a protein coat.

When bacteriophage infects a bacterium, the virus attaches, injects it’s genetic info. The viral genes produce many bacteriophages until they destroy the bacterium. The cell splits open and 100’s of bacteriophages burst out

Hershey and Chase

Bacteriophage

Reasoned that if they could determine which part of the virus (protein coat or genetic info) infected the cell, they could determine whether genes were made from DNA or protein

Used isotopes of phosphorus 32 and sulfur 35

protein does not contain phosphorusDNA and RNA does not contain sulfur

Radioactive isotopes used as markers

Hershey and Chase

DNA and RNA marked with phosphorus Protein marked with sulfur Mixed viruses with the bacteria for

radioactivity All of the radioactivity in the bacteria was

from phosphorus 32 the marker found in DNA

Conclusion: the genetic material of the bacteriophage was DNA and not the protein coat

Hershey and Chase

Hershey and Chase

DNA is a long molecule made of units called nucleotides

Each nucleotide made of a 5 carbon sugar called deoxyribose, a phosphate group and a nitrogen base. Hence DNA stands for Deoxyribonucleic acid

There are 4 nitrogen bases: adenine, guanine, thymine and cytosine

Adenine and guanine belong to a group called the purines whose structure contains 2 rings

Cytosine and thymine belong to a group called the pyrimidines whose structure contains 1 ring

Components and Structure of DNA

The backbone of a DNA chain is made of the deoxyribose sugars and phosphate groups of each nucleotide

The nucleotides may be joined in any sequence

Although there is only 4, many combinations are possible (alphabet analogy)

Components and Structure of DNA

DNA Nucleotides

Discovered the following, known as chargaff’s rule

amount of cytosine (c) = amount of guanine (g)

amount of adenine (a) = amount of thymine (t)

Chargaff’s Rules

1952 used an xray beam to help determine structure of DNA. The xrays diffracted on film providing specific patterns for the shape of DNA

The x shaped pattern Suggested DNA was a double

helix, double stranded and nitrogen bases were at center of molecule

Died at age 37 from cancer

Rosalind Franklin

1953 James Watson and Francis Crick developed a 3D model of DNA from cardboard and wire

Watson shown a pic of Franklin’s x-ray “my mouth fell open and my pulse began to race”

Model was a double helix – 2 strands wound around each other

Later discovered that the forces that held the nitrogen bases together were hydrogen bonds

Hydrogen bonds can only form between certain base pairs, A and T & C and G

Came up with the idea of base pairing which explained chargaff’s rule: A bonds with T and C bonds with G

Watson and Crick

Watson and Crick

DNA Structure

Create a time line of the scientific findings that led us to the discovery of the structure and function of DNA

BE CREATIVE!!!!!!!

Due tomorrow

Assignment