Download DNA Replication

DNA, RNA and Protein Synthesis
Bell work
Discovery of DNA
Objectives
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Describe the methods and experiments that were used by scientists in their search for
the hereditary molecule.
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How did scientific experiments lead the conclusion that DNA and NOT protein is the
hereditary molecule.
Griffith’s Experiments
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Griffith was studying the bacteria that caused pneumonia to find out how it made people
sick.
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He was also trying to come up with a vaccine against the VIRULENT strain of the
bacteria.
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S type bacteria are protected from our bodies’ immune system by a sugar that
surrounds them
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R Type are not
Griffith Explained
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First he did what we call a control for his experiment – he injected one group of mice with
one type of bacteria and another group with the other to see what would happen.
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The group injected with the R type lived. 
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The group injected with the S type died. 
Griffith Continued
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Next he killed the S type bacteria by heating it up to 60 degrees centigrade
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This is hot enough to change the shape of proteins but not hot enough to do the
same to DNA!
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When the shape of the protein is changed it will not function the same way!!!
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After he killed the S type bacteria he injected it into a third group of mice
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They lived 
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Griffith Continued
Finally in a fourth test he mix the heat killed (h-k) S type and the normal R type and injected
the mixture in to a fourth group of mice.
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They died. 
Griffith Conclusion
Griffith figured that when he killed the S type whatever causes characteristics to be passed
from parent to offspring was still alive and got into the R type bacteria
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Because this was not parent to offspring delivery of this information we call it
TRANSFORMATION
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Latin: trans – to cross; forma – a form; Transforma – to change form
The hereditary molecule was now outside the h-k S type bacteria and made its way
into the R type bacteria which allowed the R type to transform into the S type
bacteria!
Griffith’s Experiment
Avery’s Experiments
1940’s America
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Wanted to find out if it was DNA, RNA or protein that caused Griffith’s transformation
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Used three different enzymes to destroy these molecules in three different batches
of h-k S type bacteria
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Group 1: H-k S type w/ Protease (destroys protein)
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Group 2: H-k S type w/ RNase (destroys RNA)
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Group 3: H-k S type w/ DNase (destroys DNA)
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Then mixed the heat killed S types with the normal R type and injected three
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different groups of mice
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Group 1 and 2 lived  Group 3 died
Avery’s Conclusion
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The groups with the destroyed protein and RNA were still able to pass on hereditary
information that caused the mice to die
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The group with the destroyed DNA did not kill the mice
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So it MUST be the DNA that is the hereditary molecule.
Hershey-Chase Experiment
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1952 America
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Martha Chase and Alfred Hershey wanted to confirm that DNA was the hereditary
molecule
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Used bacteria and bacteriophages (phages)
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Group 1: radioactive phosphorus used to identify DNA injected into phages
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Group 2: radioactive sulfur used to identify protein injected into phages
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Each group was allowed to infect a group of bacteria
Hershey-Chase Conclusion
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When the bacteria was examined after the infection, it was found that ALL of the viral DNA
and only a little protein made it into the bacteria
QUIZ
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Griffith’s experiment with pneumonia bacteria in mice showed that harmless bacteria
could turn virulent when mixed with h-k bacteria that cause disease. True or False?
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Avery’s experiments clearly demonstrated that the genetic material is composed of
DNA. True or False?
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The experiments of Hershey and Chase cast doubt on whether DNA was the hereditary
material. True or False?
DNA STRUCTURE
Objectives
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Describe the discovery of DNA’s double helix structure.
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Describe the structure of DNA and nucleotides.
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Explain base pairing.
DNA Double Helix
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By the 1950’s it was accepted that DNA was the hereditary molecule but we still didn’t
know what it looked like or how it worked
DNA Double Helix
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1953 American James Watson and Englishman Francis Crick complete their work on the
structure of DNA
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For years they came close but couldn’t quite get the right structure without
convincing Maurice Wilkins to steal Rosalind Franklin’s X-rays to give to them.
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They concluded that the structure was a double helix which also help explained
how the molecule could replicate itself.
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In 1962 Watson, Crick and Wilkins won the Nobel Prize in Physiology or Medicine.
DNA Nucleotides
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DNA is made of repeating sub units called NUCLEOTIDES.
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Each NUCLEOTIDE has three parts
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Deoxyribose (five carbon sugar; yellow part of our model)
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Phosphate group (white part of our model)
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Phosphorus and oxygen covalently bonded together
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Nitrogenous base (blue, orange, red and green parts of our model)
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Made of nitrogen and carbon atoms
DNA: How it’s held together
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Deoxyribose and phosphate alternate on the sides
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These are held together by a covalent bond
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The Nitrogenous bases (bases) face the center and join the two strands in the middle
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They are held together by hydrogen bonds
Nitrogenous Bases
Four different kinds
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Adenine (A)
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Thymine (T)
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Guanine (G)
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Cytosine (C)
If they have a double ring they are PURINES (A and G)
If they have a single ring they are PYRIMIDINES (T and C)
Complementary Bases
1949 American Erwin Chargaff
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Observed that in a many different organisms the amount of Adenine matched the
amount of Thymine and that the amount of Cytosine matched the amount of
Guanine
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This gave us the BASE-PAIRING RULES and helped us even more understand
what DNA looked like.
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One complementary base pair contains one purine and one pyrimidine (A-T,
G-C)
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If one strand of DNA is GATTACA then the other is CTAATGT
Complementary Bases
Bases are held together by hydrogen bonds which are not very strong this allows for the
DNA to split and one side of the DNA to act as a template for creating a new
complementary strand.
DNA Models
We have many different ways to model the structure of DNA
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One you have been staring at during this entire power point!
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Another you made and is hanging from the ceiling!
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A third is just using the letters of the base pairs: ATTCTC
TAAGAG
Quiz
What does the abbreviation DNA stand for?
Distinguish between purines and pyrimidines.
What was the significance of Franklin and Wilkins’s X-ray diffraction photographs
regarding DNA structure?
Answers
DNA =
Purines are
Pyrimidines
Their photographs suggested
DNA Replication
Objectives:
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Describe the steps in DNA replication
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Describe the differences between prokaryotic and eukaryotic DNA replication
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Explain the accuracy of replication and how errors are corrected.
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Relate mistakes in DNA replication to cancer
How DNA Replication Occurs
Before we get started you need to know this: anything that ends in –ase is an enzyme, used to
either break something apart or put something together
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HELICASE brakes the hydrogen bonds holding the base pairs together creating a “Y”
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shape to the DNA, this is called a REPLICATION FORK
How DNA Replication Occurs
DNA polymerases add complementary nucleotides to each of the original strands
The DNA polymerases release from the DNA and two new and identical strands of DNA
are left ready for cell division
In each new double helix there is one original strand and one new one – SEMICONSERVATIVE REPLICATION
Action at the Replication Fork
Replication happens in opposite directions for each strand
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One side of the DNA will be copied and follow the direction of the replication fork
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The other strand will be pieced together and seemed by an enzyme called DNA
ligase
Prokaryotic vs Eukaryotic Replication
Recall the shape of prokaryotic DNA and that of eukaryotic DNA
In prokaryotes there are ONLY TWO replication forks and they move in opposite directions
until they meet
Prokaryotic vs Eukaryotic Replication
In eukaryotes DNA polymerase adds new nucleotides at 50/second
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if there were only two spots where nucleotides were being added it would take 53
days for our DNA to copy!!!
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Replication begins at thousands of locations on a eukaryotic DNA molecule
DNA Errors in Replication
Errors occur once for every BILLION paired nucleotides!
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This is AMAZINGLY ACCURATE!!!
DNA polymerase not only adds nucleotides to the growing strand it ALSO proofreads for
errors!
When an error does happen we call this a MUTATION
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This has potential to change or harm the cell’s function
DNA Errors in Replication
Some errors do not get fixed some can be caused by UV light or chemicals
Some mutations can lead to cancer
DNA Replication and Cancer
If DNA is not replicated correctly a number of things can happen
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Nothing at all
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A mutation that allows the organism to survive and reproduce better
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This mutation will usually become more common in a population
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The mutation could be harmful
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One type of harmful mutation is cancer. If the genes for controlling
replication are changed a cell could begin to reproduce uncontrollably,
creating a tumor
Quiz
How is the exact replication of DNA ensured?
What are replication forks?
Protein Synthesis
Objectives:
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Outline the flow of genetic information in cells from DNA to protein
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Compare the structure of RNA with that of DNA
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Summarize the process of transcription
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Describe the importance of the genetic code
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Compare the role of mRNA, rRNA and tRNA in translation
Identify the importance of learning about the human genome