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Chapter 12-1 Scientists & DNA Structure Notes
DNA: The Molecule of Heredity
I. DNA’s “Experiment” History
A. Griffith
In 1928 an army medical officer named Frederick Griffith was trying to find a vaccine against streptococcus pneumoniae (the bacteria that causes pneumonia, a lung disease), but instead made a breakthrough in the world of heredity. He did four experiments in which he injected strands of bacteria into mice, one strand that was harmless (R) and one that was harmful (S).
Griffith worked with two strains of streptococcus pneumoniae (the bacteria that causes pneumonia, a lung disease) –one encircled by a carbohydrate capsule that was deadly, and another that lacked a capsule and was not deadly (harmless).
1. How do certain types of bacteria cause pneumonia?
2. The experiment that tested this question led to new knowledge.
3. Genetic information could be transformedfrom one bacterium to another.
Encapsulated
Nonencapsulated
Genetic Info.
Transferred
Protein coat
No protein coat
Griffith found from this experiment that even though he had killed the S cells, he hadn’t destroyed their hereditary material, which was the one part that caused the disease! When more experiments had been done, it was discovered that the harmless R cells had used the information from the hereditary material of the dead S cells and became harmful; he called this hereditary transformation.
(Oh and by the way… he never did find the vaccine.)
B. Avery & DNA
1. Avery and a group of scientists decided to repeat Griffith’s experiment (1932)
2. To determine which molecule in the heat killed bacteria was needed for transformation
3. Made an extract (juice) from heat killedbacteria and added enzymes that destroy proteins, carbohydrates, & lipids
a. Transformation still occurred! = DNA is transforming agent!
4. Repeated the experiment but added enzymesthat destroy DNA
a. Transformation DID NOT occur!!
In 1944, he and his colleagues reported that the transforming substances—the genetic material of the cell—was DNA. This result was met initially with skepticism, as many scientists believed that proteins would prove to be the genetic information.
C. Hershey-Chase Experiment
1. Studied viruses; specifically bacteriophages
a. Bacterio = bacteria Phage = eaterBacteria eater
2. Viruses are made up of DNA and a proteincoat
3. Research Question: Which part of the virus enters the infected bacteria cell?
a. Grew viruses in the presence of radioactive markers 32P and 35S.
b. Sulfur is found in protein coat
c. Phosphorus is found in DNA
**Label missing parts
D. What DNA Looks Like
1. Rosalind Franklin
a. Used X-Ray diffraction (x-ray crystallography) patterns to discover that DNA twisted around each other like a helix
2. Watson & Crick
a. While trying to build a 3-D model of DNA, Watson was shown Franklin’s photograph(without her permission, leaked by Wilkins, her partner) and within weeks, figured out the structure of DNA
3. Chargaff
a. Base-pairing rules
b. A pairs with T, C pairs with G
c. Found equal ratios of Adenine and Thymineand Cytosine and Guanine in DNA of multiple species
II. What we’ve learned so far…
A. Cells make proteins
B. Genetic information is passed on through chromosomes
1. Mitosis = Cell Division
2. Meiosis = Sexual reproduction (new offspring)
C. Genes are located on chromosomes(specifically on the DNA)
DNA (contain genes)
Chromatin
Chromosome (cell division)
Protein (amino acid sequence)
Trait (like eye color)
D. Let’s Review:
1. Chromatin (interphase) = granular material within the nucleus (loose DNA)
2. Chromosome = visible during mitosis and can be clearly seen under the microscope as an “X” (two sister chromatids) consisting of DNA tightly coiled around histone proteins.
Prokaroyotes –single chromosome
III. DNA = deoxyribonucleic acid
A. Genetic material of organisms
1. Molecule of heredity (passes genes from 1
generation to the next)
2. Stores information in cells in the form of a
genetic code
a. This info is used to form all enzymes and
structural proteins
http://anthro.palomar.edu/biobasis/bio_4.htm
3. The more closely related two organisms,
the more alike their DNA will be
Three phylogenetic trees were
reconstructed based on the DNA
sequences of:
(a) 4700 bp of mitochondrial
DNA,
(b) the testis specific protein on
the Y chromosome,
(c) noncoding regions of the -
globin gene.
4.Can be copied
5.Condenses into chromosomes
6.Exists inside the nucleus
IV. DNA Structure
A. DNA is a polymer made up of smaller
units called nucleotides (monomer)
B. Each nucleotide consists of:
1. Simple 5-carbon sugar (deoxyribose)
2. Phosphate group
3. Nitrogen base
a. Four nitrogen bases exist:
i. Adenine A
ii. Thymine T
iii. Cytosine C
iv. Guanine G
Drag-and-Drop DNA Model
http://www.execulink.com/~ekimmel/mend
el1a.htm
C. These nucleotides combine to form two
long chains called strands (joined by hydrogen
bonds) that are twisted = double helix
1. The Sugar and phosphate make up the back bone. (sides of the ladder held by covalent bonds)
2. The Nitrogen bases make up the center(steps of the ladder –hydrogen bonds; any base sequence is possible)
3. The 2 strands are antiparallel, one right side up and one upside down.
D. Chargaff’s RulesIn DNA, Adenine (A) pairs with
Thymine (T); and Cytosine (C) pairs with
Guanine (G).
1. Purines: Adenine & Guanine (2 rings)
2. Pyrimidines: Thymine & Cytosine (1 ring)
3. A & T are said to be complimentary to each other, so are C and G
4. A purine must always bind with a pyrimidine because only 3 rings can fit in the center of DNA
E. Eukaryotic chromosomes contain DNA
(double helix) wrapped around proteins
called histones.
1. Strands of nucleosomes are tightly
coiled and supercoiled to form
chromosomes
http://science.howstuffworks.com/environmental/life/cellular-microscopic/cell1.htm
Mitosis
a. What phase of the cell cycle do
chromosomes appear?
End of Interphase beginning of Mitosis
