Honors Biology 2006-2007
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.
Honors Biology 2006-2007
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)
Honors Biology 2006-2007
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
Honors Biology 2006-2007
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.
Honors Biology 2006-2007
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.
Honors Biology 2006-2007
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”
Honors Biology 2006-2007
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??
Honors Biology 2006-2007
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
Honors Biology 2006-2007
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
Honors Biology 2006-2007
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
Honors Biology 2006-2007
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
Honors Biology 2006-2007
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!
Honors Biology 2006-2007
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)
Honors Biology 2006-2007
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.
Honors Biology 2006-2007
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??
Honors Biology 2006-2007
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!
Honors Biology 2006-2007
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
Honors Biology 2006-2007
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
Honors Biology 2006-2007
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.
Honors Biology 2006-2007
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
Honors Biology 2006-2007
Four nitrogenous basesFour nitrogenous bases DNA has four different nitrogenous bases: Adenine (A) Thymine (T) Cytosine (C) Guanine (G)
Honors Biology 2006-2007
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
Honors Biology 2006-2007
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
Honors Biology 2006-2007
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.
Honors Biology 2006-2007
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
Honors Biology 2006-2007
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.
Honors Biology 2006-2007
DNA ReplicationQuickTime™ and a
TIFF (Uncompressed) decompressorare needed to see this picture.
Honors Biology 2006-2007
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.
Honors Biology 2006-2007
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.
Honors Biology 2006-2007
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.
Honors Biology 2006-2007
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.
Honors Biology 2006-2007
DNA Polymerases bring in new nucleotides that match up to template strands
Now, that’s a compliment!
2nd step - Adding Bases
Honors Biology 2006-2007
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!
Honors Biology 2006-2007
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.
Honors Biology 2006-2007
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