Ch9: Pt1 DNA structure and analysis

Part I: How did people figure out that DNA is the molecule of heredity?

Search for the material of heredity

what was before mendel

define pangenes

-Before Mendel: Some substance passed from generation to generation contains information necessary for heredity of traits

-Pangenes: substances produced in all body parts, and transferred to germ-line cells

what are the necessary qualities of genetic substance

1.Must contain information regarding traits
2. Information must be expressable by the cell.
3. Must replicate accurately
4. Must be subject to occasional change

the search for genetic material

what did Friedrich Miescher discover in 1869

Discovers DNA in bandage pus

-Contains C, H, O, N and P: Not a protein
-Appears in nucleus of all cells: named nuclein

the search for genetic material

what happens in early 1900’s

-Early 1900’s: Cytological studies show that chromosomes are passed from generation to generation.
-Chromosomes contain both protein and nucleic acid

Proteins: 20 amino acids, diversity of side chains, and chemical structures

what are the 9 important amino acid diversity

-Glycine

-Alanin

-Valine

-Leucine

-Isoleucine

-Proline

-Methionine

-Phenylalanine

-Tryptophan

DNA: 4 nucleotides, Two chemical classes, (Purines and Pyrimidines)

what are the 6 nucleotide diversity

-adenine

-purines

-guanine

-cytosine

-pyrimidines

-thymine

If you were to design a new language (information
carrying system), which would be better? 

DNA or Proteins

DNA

At the outset of the 1920’s scientists were betting that
proteins carried genetic material.

what did frederick griffith show

Frederick Griffith (1928): Demonstrated that traits of
bacteria could change by exposure to dead bacteria.
-Died in his lab during a London air-raid in WWII

Define IIS (smooth) and IIR (rough) in Streptococcus pneumoniae that Frederick Griffith saw

what differs between type II and type III

Streptococcus pneumoniae:
IIIS (smooth): has polysaccharide coat, virulent
IIR (rough): lacks polysaccharide coat, non-virulent, mutant of IIS

Polysaccharide structure differs between type II and type III.

Describe the mice experiment Griffith conducted with Type IIR and Type IIIS of Streptococcus pneumoniae

1. Type IIR (living, nonvirulent) is injected into mice: Mice survives, no bacteria recovered

2. Bacteria w/ polysaccharide capsule. Type IIIS (living, virulent) is injected into mice: Mice dies, type IIIS viruent bacteria was recovered

3. Head applied to Type IIIS (heat killed, nonvirulent) is injected into mice: Mice survives, no bacteria recovered

4. Type IIR (living, nonvirulent) AND Type IIIS with applied heat (heat killed, nonvirulent) is injected into mice: Mice dies, type IIIS virulent bacteria is recovered

What was seen in Griffith experiment, what is it called

The killed IIIS strain passes something to the IIR strain that confers virulence AND type III coat.

transforming principle

Avery, McCary and MacLeod question:

→What material was passed from transforming to transformed bacteria

Polysaccharide wall? Protein? DNA? RNA?

what was the steps

1. Lysed IIIS bacteria, and removed cellular debris by centrifugation
2. Treat IIR with the lysate → transformation
3. Remove products selectively from the lysate and see if transforming principal remains.

Avery, McCary and MacLeod experiment to find what material was passed from transforming to transformed bacteria

what was it?

Active factor is DNA

Hershey and Martha Chase question:

DNA is transforming factor but is it the molecule of heredity?

what did they know DNA and Protein contained, what were their differences

T2 bacteriophage*: Known to consist only of DNA and protein
-DNA contains phosphorus not found in protein
-Protein contains sulfur, not found in DNA

Hershey and Martha Chase Experiment steps

what wins?

1. When grown with 32P, phages pass radiolabel to host DNA. Radiolabel detected in further generations of phages.
2. When grown with 35S, phages incorporate radiolabel into head. Radiolabel not detected in hosts or future generations.

DNA wins it!

Without overlooking RNA

Prokaryotes and eukaryotes use DNA as molecule of heredity, what is different with RNA

what do some viruses use as molecule of heredity

-All known prokaryotes and eukaryotes use DNA as the molecule of heredity
→Higher chemical stability than RNA
→Lower mutation rate (In RNA, cytosine residues may be converted to uracil)

-Some viruses use RNA as the molecule of heredity.
TMV (tobacco mosaic virus): RNA chromosome surrounded
by a protein coat

what did Gierer and Schramm (1956) see with tobacco

-Tobacco treated with purified TMV RNA develop lesions
-Tobacco treated with TMV proteins do not develop lesions

What is the structure of DNA

what was known in 1950

-In 1950, very little was known about the chemical structure of DNA: It was composed of nucleobases + deoxyribose.

What did Erwin Chargaff discover in 1950 (Chargaff’s rule)

-Chargaff hydrolyzed DNA (broke it into nucleotides) and examined the relative quantities of the 4 nucleotides

A:T= 1

C:G= 1

what did Linus pauling see

-Determined the structure of the peptide a-helix and was working to unravel the
structure of DNA.

What happened when Watson and Crick met Chargaff in 1952 at a conference

-immediate insight into structure of DNA

-hypothesize that A shares chemical bond with T, and C with G

History on “Where are the sugar molecules?” question

who worked on it originally, who found out the two strands/what that meant

Franklin makes x-ray crystallography, which Wilkin shares a secret copy to Watson

-Watson discerns two strands, nucleotides on the inside connected by outer backbones of sugar

Putting all the info together, who got a nobel prize?

Watson Crick and Wilkins