Transcript Transformation - Net Start Class

DNA: The Genetic Material
Chapter 14
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Hammerling Experiment
•
Hammerling Experiment
 Cells of green alga cut into pieces and
observed to see which were able to
express heredity information.
- Discovered heredity information stored
in the foot of the cell (nucleus location)
because the foot was needed to
regenerate the cap and stalk.
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Fig. 14.2
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Transformation Experiments
•
Mendel discovered chromosomes are
composed of proteins and DNA. But it took
several experiments to conclusively determine
specifically which substance made up genes.
 Griffith Experiment
- Documented movement of genes from
one organism to another (transformation).
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Griffith Experiment
1.
2.
3.
Used a strain of strep bacteria
The polysaccharide coat on this bacteria
makes it virulent.
The DNA specifying the toxic coat had
passed from the dead virulent bacteria to
the live, coat-less bacteria, making them
virulent (transformation).
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Fig. 14.4
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Transformation
The receiving bacteria must be in a state of
“competence” (high cell density and or
nutritional limitation). Some cells release
their DNA upon death to be taken up by
other cells.
Transformation can occur by:
1.
Conjugation : transfer of genetic material
between two bacterial cells in direct contact
2.
Transduction: injection of foreign DNA by a
bacteriophage virus into the host bacterium
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Avery and Hershey-Chase Experiments
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Avery Experiment
 Removed almost all protein from bacteria, and
found no reduction in transforming activity.
Concluded that DNA is the hereditary material,
not the proteins.
Hershey-Chase
 Used radioactive isotopes to label DNA and
protein. Found genes used to specify new
generations of viruses were made of DNA
because radioactive DNA was found in the
bacteria (from bacteriophage injection) and
protein with tracer was not in the bacteria.
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Fig. 14.5
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DNA
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Discovered by Fredrich Miescher in 1869
Originally called nuclein (nucleus) that he
extracted from human cells and fish sperm
The nitrogen and phosphorous proportions
were different enough from any other
substance in a cell and the DNA is slightly
acidic, so name became nucleic acid
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Chemical Nature of Nucleic Acids
•
DNA made up of nucleic acids.
 Five carbon sugar, phosphate group, and an
organic base.
- Purines - Large bases
 Adenine and Guanine
- Pyrimidines - Small bases
 Cytosine and Thymine
 Chargaff’s Rule
- A =T and G=C
Nucleotides are distinguished by the bases
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Fig. 14.6
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Numbering Of Nucleotide Carbons
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To ID the various chemical groups in DNA it
is customary to number the carbon atoms of
the sugar and then refer to any chemical
group attached to a carbon atom by that
number (1-5).
Carbons are numbered clockwise from the
oxygen.
The primary symbol(‘) means that it refers to
a carbon in sugar, not in a base.
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Fig. 14.7
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Structure of a Nucleotide
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The phosphate group is attached to the 5’
carbon of the sugar.
The base is attached to the 1’ carbon of the
sugar.
A free hydroxyl group is (OH-) is attached to
the 3’ carbon.
Remember…..the hydroxyl group is a
functional group that makes C-H cores of
molecules (like nucleotides) polar for bonding
and solubility
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Chemical Nature of Nucleic Acids
Nucleotide made up of a sugar attached to a
phosphate and a base.
• The chemical reaction of the 5’ phosphate of one
nucleotide and the 3’ hydroxyl group of another
allows DNA and RNA to form long chains of
nucleotides through dehydration synthesis. This
reaction results in a Phosphodiester Bond.
- Each time a nucleotide is added, the free 5’
phosphate and the free 3’ hydroxyl group at the
other end remain so thousands of bonds are
possible.
- We list nitrogen bases in a molecule of DNA from 5’
to 3’ and nucleotides can only be added to a
template strand in a 5’ to 3’ direction.
•
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Fig. 14.8
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Fig. 14.9a,b
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Fig. 14.9c1
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Fig. 14.10b
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Three-Dimensional Structure of DNA
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X-ray diffraction suggested DNA had helical
shape with a diameter of about 2 nanometers.
Watson and Crick deduced DNA is a double
helix with bases of two strands pointing inward
forming base-pairs.
 Purines pairing with pyrimidines.
- Constant 2 nanometer diameter.
 Strands are antiparallel with one running 5’
to 3’ (leading strand) and the other running
3’ to 5’ (lagging strand)
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Semi-Conservative Replication
•
Each chain in the helix is a complimentary
mirror image of the other.
 Double helix unzips and undergoes semiconservative replication.
- Each strand of the original duplex
becomes one strand of another duplex.
- One strand is conserved but the original
DUPLEX is not.
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Replication Process
•
Replication of DNA begins at one or more
sites on the DNA molecule where there is a
specific sequence of nucleotides called a
replication origin.
 DNA replicating enzyme DNA polymerase
III and other enzymes add nucleotides to
the growing complementary DNA strands.
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Fig. 14.13
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Replication Process
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DNA polymerase cannot link the first nucleotides in a
newly synthesized strand.
 RNA polymerase (primase enzyme) constructs an
RNA primer of about 10 nucleotides
DNA polymerase adds nucleotides to 3’ end.
 Leading strand replicates toward replication fork.
 Lagging strand elongates from replication fork (so
continually has to “backtrack” and add chunks of
nucleotides – Okazaki Fragments)
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DNA Replication Fork
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Fig. 14.17
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Replication Process
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DNA ligase attaches fragments to lagging
strand.
Because synthesis of the leading strand is
continuous, while the lagging strand is
discontinuous, the overall replication of DNA is
referred to as semidiscontinuous.
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Replication Process
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Opening DNA Double Helix
 Initiating replication
 Unwinding duplex (helicase enzyme)
 Stabilizing single strands (single strand binding
proteins)
Building a Primer (RNA Primer called Primase)
Assembling Complementary Strands (DNA
Polymerase III)
Removing the Primer (DNA Polymerase I) and add
regular nucleotides
Joining Okazaki Fragments (Ligase)
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Eukaryotic Replication
•
In Eukaryotic cells, DNA is packaged in
nucleosomes (wrapped around histone
proteins) within chromosomes.
 Each individual zone replicates as a
discrete section called a replication unit or
a replicon.
- Each replication unit has its own origin of
replication.
 Fast Replication (approx 8 hours to
replicate a chromosome in humans)
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One-Gene/One-Polypeptide Hypothesis
•
Genes produce their effects by specifying the
structure of enzymes.
 Each gene encodes the structure of one
enzyme.
 Enzymes are responsible for catalyzing the
synthesis of nucleic acids, proteins, carbs,
and lipids.
 By encoding the structure of enzymes,
DNA specifies the structure of the
organism itself.
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Enzyme Deficiencies
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British physician Archibald Garrod
concluded that patients suffering from certain
diseases lacked the required enzymes to
make them normal (Ex. Dystrophyn for
muscular dystrophy).
Stanford Geneticists Beadle and Tatum
came up with the 1-Gene/1-Polypeptide
Hypothesis based on their research that built
on that of Garrod.
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Fig. 14.21
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Central Dogma
All organisms use the same basic mechanism of
reading and expressing genes. This is referred
to as the CENTRAL DOGMA of biology.
• Information passes from the genes (DNA)
-> an RNA copy of the gene (mRNA) -> sequential
assembly of a chain of amino acids ( work of tRNA,
rRNA, and ribosomes)
General: DNA to RNA to PROTEINS
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Hardy-Weinberg Law
HW Law: describes the conditions for genetic
equilibrium and states that the proportions of
dominant and recessive genes tend to
remain constant in a randomly mating
population unless there are outside
influences.
p(squared) + 2pq + q(squared) = 1; p+q=1
(homo dom) (hetero) (homo rec)
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Hardy-Weinberg Principle
To maintain Hardy-Weinberg Equilibrium we
must assume:
 Population is very large.
 Random mating
 No mutation
 No gene input from external sources.
 No selection occurring
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Hardy-Weinberg Ratio
frequency: proportion of individuals falling within a certain
category in relation to the total number under consideration
HW Ratio: ratio of genotype frequencies that evolve when
mating is random and neither relocation or drift (random
fluctuation in allele frequencies over time due to chance) are
operating. Ex. Population of 100 cats Dd=48(black)
DD=36(black) dd=16(white)
(Dd X Dd) 48X48 = 2304 = 4 (constant)
(DD X dd) 36X16
576
(ratio)
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Fig. 20.4
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Microevolution
Microevolution: small changes in populations from
generation to generation
- Two primary causes of microevolution are genetic drift and
natural selection
•
Two main causes of genetic drift:
Bottleneck effect: most individuals of the pop. die off,
leaving behind an overrepresentaion of alleles
Founder Effect: a few individuals leave the population to
create a new population
BOTH REDUCE THE POPULATION NUMBER SO THAT
GENETIC DRIFT IS SIGNIFICANT
•
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Five Agents of Evolutionary Change
1. Mutation
 Ultimate source of genetic variation and
makes evolution possible
2. Gene Flow
 Movement of alleles from one population to
another.
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Five Agents of Evolutionary Change
3. Genetic Drift
 Frequencies of particular alleles may
change by chance alone.
4. Nonrandom Mating
Individuals with certain genotypes
sometimes mate with one another more
commonly than would be expected on a
random basis and increases the
expression of recessive alleleles
(Inbreeding)
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Five Agents of Evolutionary Change
5. Selection
 Artificial - Breeders exert selection.
 Natural - Nature exerts selection.
.
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Forms of Selection
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Disruptive Selection
 Selection eliminates intermediate types.
Directional Selection
 Selection eliminates one extreme from a
phenotypic array.
Stabilizing Selection
 Selection acts to eliminate both extremes.
- Fitness - Number of surviving offspring
passed to the next generation.
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Types of Natural Selection
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