Lecture 17: DNA Discovery and Replication Flashcards
How can something be purified?
- Separate a mixture into its components
- Assay each component to see which has the desired property
What is an assay?
-A way of measuring something
-Can measure a substance
-Or can measure a phenomenon such as memory
Cis chromosome?
-Chromosome contains both dominant or both recessive alleles
Trans chromosome?
-Chromosome contains one dominant and one recessive allele
Semi-dominant trait?
Blending of two alleles that results in a third phenotype that is different from the parents(heterozygous is different)
ex. RR(white), rr(red), Rr(pink)
Alleric Series
-More than two alleles per gene(R, r, r’)
Multigenic/Polygenic
Multiple genes affect one phenotype
Penetrance
Percentage of individuals of a given genotype that actually show the phenotype
Expressivity
Degree to which a phenotype is expressed(mild symptoms vs severe)
What three factors contribute to the continuous spectrum?
- Alleric Series
- Multigenic
- Environmental effects(penetrance/exppresivity)
How can DNA be purified?
- Grind up fav organism
- Extract the lipids and proteins of the organism with solvent
- Precipitate with ethanol
How was it determined that genetic material was actually genetic material?
It was thought that if you dip another organism in another organisms DNA that the organism should come out as the other organism
For what organisms does dipping into DNA work?
- Viruses
- Bacteria
What was Frederick Griffiths Experiment?
- Injected S Strain bacteria into mice they would die.
- Injected R strain bacteria into mice, they would live
- Heated up the S strain bacteria and killed it and when he injected it into the mice they survived
- Mixed dead S strain bacteria with living R strain bacteria the mice died
What did Federico Griffith’s experiment prove?
-A chemical component of one cell is capable of genetically transforming another cell
- R strain bacteria inherited the S strains DNA
How do phage infect bacteria?
-Phage bind to bacteria and inject them with their DNA
-The bacteria then replicates this DNA , producing phages that burst out of the bacteria and infect others
What are phages made up of?
-Proteins
-DNA
What was Hershey-chase experiment 1?
- Labelled DNA of phages with radioactive phosphorus
- Mixed the phages with the bacteria (allowed them to inject)
- Left with a pellet full of radioactive phosphorus
What did experiment 1 of hershey-chase conclude?
The DNA of phages was transferred into bacteria
What was Hershey-chase experiment 2?
- Labelled protein of phages with radioactive sulphur
2.Mixed the phages with the bacteria - Radioactive sulphur/protein was on top of the bacteria (in the supernatant)
What did experiment two of the Hershey-Chase experiment prove?
Phages protein does not enter bacteria
What would have happened if Hershey-Chase labelled their phage with 14C?
Both the pellet and supernatant would be radioactive because proteins(amino acids) and DNA(deoxyribose) are made up of carbon
Is there an equal amount of each nucleotide base in DNA?
No
- Varies amongst different organisms
What is chargaff’s rule?
-The amount of nucleotide base A is equal to the amount of nucleotide base T
-The amount of nucleotide base C is equal to the amount of nucleotide base G
How does crystallography work for DNA?
- Shine light through purified DNA
- The x-rays will deflect off of the atoms of the material
- Deflection pattern is telling of the structure of the molecule your shining the light through
What insight did crystallography give to the structure of DNA?(4)
- DNA is made up of two strands
- DNA is a double helix
- There are phosphates on the outside
- The strands run antiparallel
What was Watson and Cricks structure for DNA?
- Phosphates negatively charged, hydrophilic want to interact with water
-Nucleotide bases want to be in the middle as they are relatively hydrophobic
-Chargaff’s rules: A pairs with T and G pairs with C
-Proposed a correct structure!!
What does it mean when we say DNA strands are reverse complements?
Reverse: Run anti-parallel to each other (read in opposite directions going 5’ to 3’)
Complement: A on one strand corresponds to T on the other strand(same for G and C)
What direction is DNA transcribed?
5’ to 3’
Why do G-C pair and A-T pair?
This combination forms the most H-bonds resulting in the most stable structure
-Also the distance within the DNA double helix is constant since a purine( double ring) is always paired with a pyrimidine(single ring)
How many H-bonds do
G-C
A-T
A-C
G-T
G-C: 3 H-Bonds
A-T: 2 H-bonds
A-C: 0 H-Bonds
G-T: 1 H-Bond
How many H-bonds does A-U form?
2 H-bonds principals are the same for when RNA creates a complementary strand
Is DNA constrained to one sequence of letters?
NO
What about DNA suggests that it is replicated?
The fact that the two strands encode for the same information in complementary form
Three hypothesis for how DNA is replicated?
- Semi-conservative replication
2.Conservative replication - Dispersive replication
What is the semi-conservative hypothesis?
- DNA double helix separates into two strands
- Each strand is then replicated into a complementary new strand
- Creates 2 double helix DNA each with one original strand and one new strand
What is the conservative hypothesis?
- You take the original DNA and use it to create two brand new strands
- You then have 1 DNA molecule with both old strands and one with both new strands
What is the dispersive hypothesis?
The DNA makes little pieces of new DNA and then recombines them. Now you have strands of DNA that are mixtures of old and new DNA.
What experiment proved that DNA is replicated via semi-conservative replication?(Meselson and Stahl)
- Scientists fed bacteria heavy nitrogen which was incorporated into their DNA
- They spun the DNA in a centrifuge where heavy DNA would go to the bottom of the tube and light DNA would go to the top of the tube.
- Heavy strand produced two strands intermediate in weight(one old, one new)
- The intermediate strands then produced even lighter DNA (one old intermediate, one new)
If replication were conservative, what would have been the result of the Meselson and Stahl experiment after two rounds of replication in light nitrogen?
-3/4 light, 1/4 heavy
What did Friedrich Miescher do?
Figured out a way to isolate cell components
What was the transforming principle from Frederick Griffiths muse experiemnt?
It was the substance from the S-strain that transformed the R-Strain(DNA)
