Honors Biology 2006-2007 The History Of DNA Honors Biology 2006-2007.

Honors Biology 2006-2007

The History Of DNA


Quick Review Chromosomes are made up of DNA and Proteins.

Genes are a segment of the DNA molecule that carries the instructions for producing a specific traits.


DNA or Proteins? Prior to the 1950’s, scientists could not decide whether DNA or protein was the material responsible for heredity.

3 experiments would lead to the answer:

1. Frederick Griffith

2. Avery, McCarty & MacLeod

3. Hershey & Chase

1952)


Griffith’s Experiment Frederick Griffith was working to find a cure for Streptococcus pneumonia (bacteria) Test Subjects: lab mice

He used different strains (types) of bacterium in this experiment.

1928


Strains of Bacteria Used S Bacteria = Disease Causing (pathogenic) Have protective outer coat that “hides” the bacteria from the body’s immune defenses.

R Bacteria = Do not cause disease (non-pathogenic) Lacked the protective coat and thus the immune system can identify and destroy them.


The Experimental Trials

Transformation?something in heat-killed bacteria could still transmit disease-causing properties

live pathogenicstrain of bacteria

live non-pathogenicstrain of bacteria

mice die mice live

heat-killed pathogenic bacteria

mix heat-killed pathogenic & non-pathogenicbacteria

mice live mice die

A. B. C. D.


Griffith’s “Transforming Factor” Why did the mice die that were given a mixture of heat-killed pathogenic bacteria and live non-pathogenic bacteria?

A substance had been passed from the dead bacteria to live bacteria = “Transforming Factor”


Griffith’s Results After examining blood samples, he found that somehow the uncoated bacteria had TRANSFORMED into bacteria with coats.

Transformation is a change in the genetic makeup caused when cells take up foreign genetic material.

BUT, what was it? DNA or Protein??


What Next? Whatever the molecule, it had to have several properties in order to fit the bill: It had to be duplicated whenever a cell divided, so it could be passed on unchanged.

It had to be in the form of an informational code

It had to be (mostly) stable and resistant to change


Avery, McCarty & MacLeod

Oswald Avery

Maclyn McCarty

Colin MacLeod

1944


DNA causes Transformation Avery, McCarty & MacLeod

purified both DNA & proteins from Streptococcus pneumonia bacteria

which will transform non-pathogenic bacteria?

Experiment 1: injected protein into bacteria = no effect injected DNA into bacteria = transformation!

Experiment 2: added protein destroying enzymes to bacteria = transformation!

added DNA destroying enzymes to bacteria = no effect


Hershey & Chase

Alfred HersheyMartha Chase


Confirmation of DNA Hershey & Chase

“Blender” experiment using bacteriophages

viruses that infect bacteria Half of the phages had their proteins tagged with radioactive sulfur (35S)

The other half had their DNA tagged with radioactive phosphorus (32P)

All phage were then allowed to infect bacteria!

1952 | 1969


Protein coat labeledwith 35S

DNA labeled with 32P

bacteriophages infectbacterial cells

T2 bacteriophagesare labeled with

radioactive isotopesS vs. P

bacterial cells are agitatedto remove viral protein coats

35S radioactivityfound in the liquid

32P radioactivity foundin the bacterial cells

Which radioactive marker is found inside the cell?

Which molecule carries viral genetic info?

Hershey & Chase


Blender experiment Results Radioactive phage & bacteria

in blender 35S phage

radioactive proteins were in liquid therefore protein did NOT enter bacteria

32P phage radioactive DNA was in pellet therefore DNA did enter bacteriaTaaa-Daaa!


Hershey & Chase’s Conclusion

Hershey & Chase Confirmed DNA is “transforming factor”

Proved that DNA AND NOT protein is the hereditary material in cells (i.e., genes are made up of DNA)


Wilkin’s and Franklin’s Photographs In 1952, Maurice Wilkins and Rosalind Franklin, developed high quality X-ray diffraction photographs of strands of DNA.

These photos were key pieces of data used to determine the helical structure of DNA molecules.


Watson and Crick 1953 article in Nature


The Structure of DNA


Deoxyribonucleic Deoxyribonucleic AcidAcid DNA is the molecule of heredity.

DNA (Deoxyribonucleic Acid) is a double-stranded nucleic acid that determines an organism’s traits by controlling the production of proteins.

DNA holds the information for life!

Why do we study DNA??Why do we study DNA??


DNA’s Size DNA is an extremely long

molecule

DNA from one of your cells would measure 6 feet tall!

All of the DNA in your body All of the DNA in your body could stretch to the sun and could stretch to the sun and back 400 times!back 400 times!

5 million strands of 5 million strands of DNA can fit through DNA can fit through the head of one the head of one needle!needle!

The earth is 93 million miles from the sun!


DNA’s Helical Structure Watson & Crick discovered DNA is made up of 2 chains of nucleotides joined together by their nitrogen bases. The 2 strands are twisted together, forming a Double-Helix.

Like a spiral staircase, twisted ladder, or zipper


Subunits of DNASubunits of DNA When we group the

deoxyribose sugar, phosphate group and nitrogen base together into a subunit it is called a Nucleotide.

One strand of DNA has many millions of nucleotides.

nucleotide


Nucleotides Continued

Phosphate

C

C

O

O

CC

O -P OO

O Nitrogenous base

DeoxyriboseC

Sugar = Deoxyribose. The Phosphate Group = Phosphorus atom surrounded by 4 oxygen atoms.

The Nitrogen Bases are carbon ring structures that

contain one or more atoms of nitrogen.


What is DNA made of?What is DNA made of? The backbone of the molecule is alternating phosphate groups and deoxyribose sugars.

The “rungs” of the DNA “ladder” are nitrogenous bases.

phosphate

deoxyribose

bases


Four nitrogenous basesFour nitrogenous bases DNA has four different nitrogenous bases: Adenine (A) Thymine (T) Cytosine (C) Guanine (G)


Two Classes of DNA Bases Pyrimidines are Pyrimidines are single ring bases.single ring bases. ThymineThymine CytosineCytosine

(Pyrimidins have (Pyrimidins have a “y”)a “y”)

Purines are double Purines are double ring bases.ring bases. AdenineAdenine GuanineGuanine

C

C

C

C

N

N

OO

N

CC

CC

N

N

N

N

N

C


Chargaff’s Rule The amount of Adenine is always equal to the amount of Thymine.

Adenine and Thymine have a 1:1 ratio

The amount of Guanine is always equal to the amount of Cytosine.

Guanine and Cytosine have a 1:1 ratio


Base pairing in DNA Pairing

Adenine and Thymine always join together

Two Hydrogen Bonds

Cytosine and Guanine always join together

3 Hydrogen Bonds

Adenine-Thymine and Guanine-Cytosine are called Complimentary Base Pairs.


How do the strands How do the strands stick?stick? Hydrogen Bonds form

between the nitrogenous bases.

Weak, but there are millions and millions of them in a single molecule of DNA.

Remember hydrogen bonds can form between hydrogen and any electronegative atom!

hydrogenbonds


Nucleotide Sequence Nucleotide Sequence is extremely important.

Differences in organisms are due to the difference in sequence of the nucleotides along a strand of DNA.

Nucleotide sequence forms the unique genetic information of an organism.


DNA ReplicationQuickTime™ and a

TIFF (Uncompressed) decompressorare needed to see this picture.


Copying DNA Each cell in an organism has an exact copy of the DNA that was in the fertilized egg (Mitosis)

The DNA in the chromosome is copied in a process called DNA Replication.

If DNA wasn’t copied before cell division, new cells would only have half the DNA of their parents.


DNA Replication Base pairing allows each strand to serve as a pattern (template) for a new strand

Semi-Conservatice Model 2 molecules are formed. Each has an original strand and one new strand.


let’s meetthe team…

DNA Replication Large team of enzymes coordinates replication:

1. DNA Helicase - unwinds the DNA so that it can be copied

2. DNA Polymerase(s) - Brings in the nucleotide building blocks to make the complimentary strand AND “proofreads” the finished strands.

3. DNA Ligase - Acts as “glue” cementing new pieces of DNA together to form a continuous strand.


1st step: Unwinding

single-stranded binding proteins

Unwind & unzip DNA helicase enzyme

unwinds DNA helix stabilized by single-stranded binding proteins

The areas where the double helix separate are called replication forks.


Replication fork

direction of replication


DNA Polymerases bring in new nucleotides that match up to template strands

Now, that’s a compliment!

2nd step - Adding Bases


Results of DNA Replication DNA Replication results in the formation of 2 DNA molecules, each identical to the original DNA molecule.

Each new molecule has one strand from the original DNA molecule and one new strand. Semi-conservative!


Checking for Errors Sometimes errors occur when the wrong nucleotide is added to the new strand.

Luckily, DNA Polymerase has a “proofreading function.”

DNA Polymerase can backtrack and remove incorrect nucleotides.


Fast & accurate! It takes E. coli <1 hour to copy 5 million base pairs in its single chromosome divide to form 2 identical daughter cells

Human cell copies its 6 billion bases & divide into daughter cells in only few hours remarkably accurate only ~1 error per 100 million bases ~30 errors per cell cycle

Honors Biology 2006-2007 The History Of DNA Honors Biology 2006-2007.

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