Lecture 22 Flashcards
When did scientists fully accept DNA not protein was genetic material
1950s
Why did scientists used to think protein carried information
Because they thought DNA was too simplistic
Protein monomers had 20 amino acids while DNA monomers consisted of 4 nucleotides
Proteins have a much more varied 3D structure as compared to the relatively uniform structure and nucleotide composition of DNA
Chase and Hershey
1952
Alfred Hershey and Martha Chase used radioactively labeled T2 viral phases to infect cells of the bacterium E. coli
Virus has an outer protein coast enclosing a DNA molecule (labeled 32P) or a protein (labeled 35S)
Put them into a blender to take the shears off
Then inspect the bacteria to see where the radioactivity is
What do the phases do in Hershey and chase experiment
Attach to cell wall of E. coli
Inject DNA from inside the phage into the bacteria
Infected bacteria begins to produce new phages inside
Bacteria later rupture, releasing many newly formed phages into the external environment
Results of the Hershey chase experiment
Phages with the protein:
Radioactivity remained in the solution
There was no radioactivity in the bacteria
Phages with the DNA:
No radioactivity in the solution,
Radioactivity was inside bacterial cells
THIS confirmed DNA did enter bacteria and the protein coats didn’t enter the bacteria
If DNA was the genetic info. Only phages with radioactive dna should be able to produce the new phages with radioactive dna. This is true
Conclusions of Hershey chase experiment
Phage protein coats remained outside bacterial cells it was concluded that they couldn’t be genetic material
Since DNA did enter the cells it was concluded DNA was genetic material
2 macromolecules classified as nucleic acid
DNA
RNA
DNA full name. Atomic structure
Deoxyribonucleic acid
Has 5C sugar in each of its nucleotides deoxyribose sugar
Deoxyribose sugar has an H atom attached to its 2’C rather than hydroxyl (OH) group that is attached to the 2’C of ribose sugars found in RNA
DNA nucleotides only vary in
Which nitrogenous base they contain
Same sugar and same phosphate group in every DNA nucleotide
Polynucleotide chain has polarity
Nitrogenous bases in DNA nucleotides are of 2 types
Purine (two ringed)
Pyrimidines (1 ring)
Purine which types of
AGTC
A and G
Purimidines which types
AGTC
T and C
Watson and crick 1953
DNA structure
Nucleotides can join together into a polynucleotide chain
Backbone of polynucleotide chain is a sugar phosphate backbone
-each phosphate group forms a bridge between the 3’ carbon and one sugar and the 5 ‘carbon of the next sugar
Polynucleotide chains have polarity or directionality with a 5’ carbon at one end and a 3’ carbon at the other end
Phosphodiester bond
Entire linkage between nucleotides
Chargaffs rule
DNA from any cell of all organisms should have a 1:1 ratio of pyrimidine and purine bases
Why?
Watson and crick realize each rung had and pyrimidine and a purine
Amount of guanine is equal to amount of cytosine
Amount of adenine is equal to thymine
BASICALLY
A-T or C-G connections in DNA
Purine
Nitrogenous base built from a pair of fused ring ma of carbon and nitrogen atoms
Pyrimidines
Nitrogenous base built from a single carbon ring
X ray diffraction pictures of DNA obtained by Rosalind Franklin who worked in Maurice Wilkins lab
Pictures suggested a helical molecule relatively uniform in diameter
Diameter suggested it consisted of 2 rather than 1 polynucleotide strand. DOUBLE HELIX
diameter of the molecules suggested enough room to fit a purine and pyrimidine nitrogenous bases between the 2 polynucleotide strands
Used molecular model building techniques developed by Linus Pauling
Constructed DNA models until they got one that accounted for experimental data and in which all components fit together properly
Decided that nitrogenous base pairs formed rungs between 2 polynucleotide chains
Rungs are made of complimentary base pairs consisting of one purine and one pyrimidine
Which bonds between A and T
Which bonds between C and G
A and T forms 2 hydrogen bonds
C and G form 3 hydrogen bonds
2 polynucleotide chains are
Anti parallel
Running in opposite directions so they each chain has a 3’ and a 5’ end
3’ end (hydroxyl group)
5’ end (phosphate group)
Double helix is stabilized by 2 main forces
Hydrogen bonds between nucleotides
Base stacking interactions among the nitrogenous bases
