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Chapter 11

BIOA01H3 Chapter Notes - Chapter 11: Arthur Kornberg, Bacteriophage, Complementary Dna

Biological Sciences
Course Code
Mark Fitzpatrick

of 9
Biology Notes – Chapter 11 October 27,
11.1 What is the Evidence that the Gene is DNA?
- the presence of genes were associated with chromosomes by the 1920’s
- chromosomes were made up of DNA and proteins
- a new dye was developed that could bind specifically to DNA and turned red
in direct proportion to the amount of DNA present in a cell
- DNA was the genetic material because
oIt was in the right place – DNA was an impt component of the nucleus
and chromosomes
oVaried among species – own specific amount of DNA
oIt was present in the right amounts – amount of DNA in somatic cells
was twice that in sex cells
DNA from one type of bacterium genetically transforms another type
- Frederick Griffith was studying the bacterium Streptococcus pneumonia
(causes pneumonia in humans) trying to determine a vaccine against it
oCells of the S strain produce colonies that looked smooth protected
from attack by a host’s immune system cells cause pneumonia
when injected into the mice (virulent)
oCells of the R strain produced colonies that looked rough lacked
protective capsule and did not cause pneumonia
oCome mice with heat killed S pneumocci did not produce infection but
the ones with a mixture of R bacteria and heat killed S bacteria, the
mice died with pneumonia
When he looked at blood from these mice he found living
bacteria with characteristics of virulent S strain
Some of the R strain had been transformed into the virulent S
strain, because they were in the presence of the dead bacteria
Therefore when you put R strain and heat killed S strained
together it yields the same transformation
A chemical transforming principle from the dead S strain can
cause a heritable change in the affected R cells
The transforming principle is DNA
- Oswald Avery was the one that found the transforming principle
- Treated the samples to contain this transforming principle in ways to destroy
different molecules like proteins, nucleic acids, carbs and lipids
oIf DNA is destroyed there is loss in the transforming activity
destroying any other molecules did not result in the same effect
oWas not always believed
DNA was not as genetically variant like protein could be
Did not know if bacteria even contained DNA
Viral replication experiments confirmed that DNA is the genetic material
- Hershey – Chase experiment
oUsed to determine whether DNA or proteins were the genetic material
oBacteriophage virus consists of a DNA core packed inside a protein
oWhen this virus attacks a bacteria and enters the bacterial cell
oThe cell then burst 20 mins later releasing dozens of viruses the
virus is able to replicate itself in the bacteria
oWanted to figure whether the DNA component or the protein
component of the virus enters the cell
- Proteins contain more sulphur not present in DNA grew bacteriophage in
the presence sulphur
- DNA is rich with phosphorus not present in DNA grew another batch of
bacteriophage in the presence of phosphorus
- The P- bacteriophage infected one bacteria, while the S-bacteriophage
infected another bacteria
- The then agitated both bacteria’s in a kitchen blender, to get rid of parts of
the virus that had not penetrated the bacteria
- They separated the bacteria from the rest of the material in a centrifuge
causes solutes or particles to separate and form a gradient according to their
- The lighter remains of the virus ( the ones not penetrated by the bacteria)
were captured in the fluid while the heavier bacterial cells fell to the bottom
- The fluid contained more sulphur whereas the phosphorus had stayed with
the bacteria
oSuggests that DNA had been transferred to the bacteria and was
responsible for redirecting the genetic program of the bacterial cell
Eukaryotic cells can be genetically transformed by DNA
- Wanted to see whether or DNA was the genetic material in more complex
- Can be seen using transfection
oUse a gene marker, a gene whose presence in the recipient cell
confers an observable phenotype
oResearches usually use a nutritional or antibiotic resistance marker
that permits the growth of genes in transformed cells but not in
nontransformed cells any cell can be transfected results in a
transgenic cell
oThymidine example in textbook
11.2 What is the Structure of DNA?
The chemical composition of DNA was known
- DNA was a polymer of nucleotides
- Each nucleotide consists of a sugar deoxyribose, a phosphate group and
nitrogen containing base
- The difference between the nucleotides are the bases
oAdenine, cytosine, guanine and thymine
- Erwin Chargoff found that DNA from different species exhibit certain
- The amount of A always equals the amount of T
- The amount of G always equals the amount of C
- The total abundance of (A+G) = the total abundance of (T + C)
Watson and Crick described the double helix
- Watson and Crick tried to combine all that they learned so far about DNA
structure into a single coherent model
- They were convinced that DNA had a helical structure , measurements
suggested that there are two polynucleotide chains in the molecule
- In 1953, built a model out of tin that established the general structure of DNA
Four Key features define DNA structure
- It is a double stranded helix of uniform diameter
- It is right handed
- It is antiparallel (two strands run in opposite direction)
- The outer edges of the nitrogenous bases are exposed in the major and minor
The Helix
- The sugar-phosphate backbone are the outside of the helix and the
nitrogenous bases point toward the centre
- The two chains are held together by hydrogen bonds
- Adenine pairs with thymine forming two hydrogen bonds
- Guanine pairs with cytosine by forming three hydrogen bonds
- Complementary base pairing = each pair is made up of one purine and one
- The stacking of the base pairs in the centre is stabilized by hydrophobic
interactions = more stability
Antiparallel Strands
- What does it mean by antiparallel?
- The phosphate group connect to the 3 carbon of one deoxyribose molecule
and the 5 carbon of the next = linking successive sugars together
- At one end of the chain is a free 5 phosphate group and at the other end is a
free 3 phosphate group
- In a double helix, the 5 end of one strand is paired with the 3 end of the other
Base exposure at the grooves
- There are major and minor grooves in the double helix, the outer edges of the
flat hydrogen-bonded base pairs are accessible for potential hydrogen
oEx. A protein could come and bind on to it
The double helical structure of DNA is essential to its function
- The genetic material stores an organisms genetic info
- The genetic material is susceptible to mutation
- The genetic material is precisely replicated in the cell divison cycle
- The genetic material is expressed as the phenotype
oProvide the information required to make proteins, and proteins
provide much of the phenotype for an organism