Week 9 Recall Questions Flashcards
A: DNA is the genetic material
What is heredity?
1866 Gregor Mendel first described the rules of heredity (genetics)
- The transmission of traits from one generation to the next
• Nature of the genetic material, it needs to be able to:
Store information, replicate and change over time (= properties)
A: DNA is the genetic material
What were some characteristics of peas that made them good experimental organisms?
A classic “model organism”
• Easy to grow
• Short generation time
• Have distinct traits that are easily observable: Flower color, seed shape, seed colour, pod shape. …
• Usually self-fertilize
A: DNA is the genetic material
Why is it important to have a good model experimental system?
A: DNA is the genetic material
What is ‘nuclein’?
- Substance in the nucleus that is acidic and high content of phosphorus.
—> nucleic acids (phosphodiester bonds)
Johann Friedrich Miescher in 1868 looked at the composition of the cell nucleus and found nuclein
- Looked at white blood cells most easily found in pus from discarded bandages at the local hospital
- Found an acidic substance high in phosphorous
it is nucleic acids
A: DNA is the genetic material
How did T.H. Morgan characterize a ‘gene’.
1911 T. H. Morgan established that traits are transferred as discrete units called genes, which were associated with
chromosomes
- Gene: Unit of hereditary information
- Chromosome: A structure in cells (that is a combination of DNA and protein)
- Identified mutant Drosophila
No agreement on what was the genetic material, but agreement on the general properties of the material:
• Ability to store genetic information,
• Ability to replicate
• Ability to change over time
A: DNA is the genetic material
How did the two strains of Streptococcus pneumonieae used by Griffith differ?
Virulent: (s-cells)
—> smooth cell capsule, a polysaccharide layer
—> living s-cells
—> killed ppl & mice = pathogenic
Rough: (r-cells)
—> no capsule
—> living r-cells (control—> expect to happen)
—> mice and & humans stay health = non-pathogenic
A: DNA is the genetic material
What was the transformation Griffith observed in the bacterium Streptococcus pneumonieae?
Question:
— Can a genetic trait be transferred between different bacterial strains?
— specifically, if there is something in the s cells containing information for polysaccharide later and by extent could it be transferred.
Hypothesis:
— Bacterial cells have a substance that contains genetic material that transfers information from one strain to the next and a non-pathogenic strain can turn pathogenic when exposed to this material (= prediction)
Mixture with heat killed s cells and living r cells = mice dies and s cells found in the blood.
—> non pathogenic to pathogenic.
- Trait info transferred between cells.
A: DNA is the genetic material
To demonstrate the presence of a “transformative molecule” present in Streptococcus pneumonieae,
Griffith infected mice with harmful and harmless Streptococcus.
What were the controls in this experiment and what did each of the controls show?
What were the independent and dependent variables?
Living S cells (control) —> mice death
Living R cells (control) —> mice lived
Heat killed S cells (control) —> mice lived
Mixture of heat killed S cells and living R cells —> mice death
• Dead S cells could not infect mice.
— Control sample - tests assumption that dead cells are not infectious.
• Test mouse health (dependent variable) in response to these normal bacterial strains.
— or the virulence of the Strains
• Mixture of heat killed S cells and living R cells = independent variable.
A: DNA is the genetic material
Based on Griffith’s experiment what properties does the genetic material need to have?
Conclusions:
• S cells have an “information” component not destroyed by killing/heat
• This component transfers information to non-pathogenic R cells, transforming them into pathogenic S cells
• Established the concept of transformation
- Transfer of trait information between cells
- Modern definition: Change due to uptake of external DNA by a cell
A: DNA is the genetic material
What is contained in chromosomes?
- The genetic material that’s transferred.
- RNA
Proteins
- DNA
A: DNA is the genetic material
What was the question Avery, MacLeod and McCarty tried to answer with their follow-up experiment to Griffith?
Wondered how to prevent transformation and therefore allow mice to live.
• Wanted to know what was the transforming principal from Griffith’s experiment.
I.e. what specific macromolecule
A: DNA is the genetic material
What were the controls in this experiment and what did each of the controls show?
What were the independent and dependent variables?
• Control sample: R cells + heat-killed S cells - tests that R ) transformation is possible with macromolecules from S cells.
Mice dies:
— No components destroyed
— Polysaccharides destroyed
— Lipids destroyed
— RNA destroyed
— Protein destroyed
= live s cells recovered.
Mice lives:
— DNA destroyed = No live S strain
recovered
• Took samples of heat- killed S cells and removed different components (DNA, RNA or protein) using enzymes = independent variable.
• Mixed each sample with live R cells, and observed the ability to infect and kill mice = dependent variable.
A: DNA is the genetic material
Avery, MacLeod and McCarty followed up on Griffith’s experiments by digesting extracts of heat-killed smooth cells with various enzymes, and looking for transformation of rough cells.
How did these experiments show that DNA is the transforming molecule?
Conclusion:
• Transformation cannot occur unless DNA is present, therefore DNA must be the transforming principle, i.e. the hereditary material.
A: DNA is the genetic material
Why did Avery, MacLeod and McCarty add the different enzymes (protease, DNAase and RNAase)?
• to destroy proteins, DNA, and RNA respectively.
• Transformation (of s cell info to r cells) occurs with proteins and RNA destroyed. But not DNA.
• used a bunch of macromolecules to test whether a cells were recovered or not.
A: DNA is the genetic material
Hershey and Chase used E. coli (bacteria) and T2 phage (virus) to demonstrate that DNA, not protein, is the hereditary molecule.
What about the structure and life cycle of T2 phage made it so useful for this experiment?
A: DNA is the genetic material
Given what you know about the structure of DNA and the structure of proteins (from topic 3), what
element(s) is/are found in DNA that are not found in protein?
What about elements in protein that are not found in DNA?
• Phosphorus
• sulfur
A: DNA is the genetic material
What is a bacteriophage?
Why did Hershey and Chase choose to use a bacteriophage?
Bacteriophage:
- A virus that infects bacteria
- Takes over bacterial cell and turns it into a virus production factory
- Composed of a protein coat and DNA
A: DNA is the genetic material
Why did they use radioactive isotopes as labels?
Radioactive Isotopes Can be used to label organic molecules:
- An unstable form of an element that emits detectable energy (radiation).
• easy to detect on photo films
A: DNA is the genetic material
Which part of the bacteriophage was radioactively labelled when using 35S in the growth medium?
Why?
- Protein is specifically labeled with 35S (sulfur)
— Methionine
— Cysteine - b/c sulfur isn’t in DNA
- radioactive material didn’t enter cell, stayed in bacteriophage = Next generation of Phages not radioactive
A: DNA is the genetic material
Which part of the bacteriophage was radioactively labelled when using 32P in the growth medium?
Why?
- DNA is specifically labeled with 32P (phosphorous)
-b/c phosphate isn’t in proteins.
- radioactive material entered cell, left bacteriophage = Next generation of Phages radioactive.
A: DNA is the genetic material
Which part of the bacteriophage entered the E. coli cells?
The radioactive DNA (32P)
A: DNA is the genetic material
Why was it important that the genetic material entered the E. coli cells?
It showed the passing of information from 1 gen to the next.
If answered the question:
Is the genetic material DNA or protein?
— it’s DNA
A: DNA is the genetic material
Which treatment resulted in radioactive labelled bacteriophages in the next generation?
What did this imply?
The radioactive 32P in DNA.
- Viral DNA enters bacterial cells and provides instructions to reprogram bacteria, not protein
- DNA stores the genetic information and is the hereditary material
B: Structure of DNA
What is meant by the “polarity” of DNA?
DNA is a polymer of nucleotides
• Deoxyribose
• Phosphate group
• Nitrogenous base (4)
- Nucleotides within a strand are joined by phosphodiester bonds (covalent)
- Sugar and phosphate backbone faces out, bases face in
B: Structure of DNA
What do we mean when we say that DNA strands in a double helix are antiparallel?
Two strands are antiparallel
(Different ends: 5’ with P and 3’ with OH group)
B: Structure of DNA
How is base pairing specified? (what bond type)
What are the complementary base pairings?
Strands held together in the middle by H-bonds (2 or 3).
Fixed width (2 nm) of double helix dictates that a purine must pair with a pyrimidine, always “3 rings”, bases are flat: “base stacking”
— Purine + purine: too wide
— Pyrimidine + pyrimidine: too narrow
— Purine + pyrimidine: width consistent with X-ray data
B: Structure of DNA
What did Chargaff observe? (1947)
- All DNA made of the same 4 bases
- Each species has a specific and unique amount of each base
- A species’ base composition is neither random, nor 1:1:1:1
Hypothesis: If Avery et al. are correct, the diversity of life (genetic differences I must be reflected in the chemical differences of DNA
Experiment: Examined the DNA in a variety of species = Indep. Var.
Measured the amount of each nitrogenous base (A, C, G, and T) = Dep. Var.
B: Structure of DNA
What do Chargaff’s Rules express?
• Chargaff’s Rules:
- Amount of adenine = Amount of thymine
- Amount of cytosine = Amount of guanine
- Amount of purines = Amount of pyrimidines
- A pyrimidine is paired with a purine
• This means that with knowing the amount of one base (%) you can calculate the amount of others
B: Structure of DNA
What are the ratios that Chargaff found?
- e.g. E. coli DNA - 26% A, 26% T, 24% G, 24% C
- e.g. Human DNA - 30% A, 30% T, 20% G, 20% C
B: Structure of DNA
What were Rosalind Franklin’s observations?
• Franklin and Wilkins (1951)
• Question: What is the structure of DNA?
• Experiment: Rosalind Franklin used X-ray crystallography to produce a diffraction image of DNA:
B: Structure of DNA
What was the evidence that allowed Watson and Crick to build a model for the structure of DNA? (6 ‘observations’)
• Watson and Crick (1953)
• Built a structural model to interpret Franklin’s results:
- DNA is a helix made of two antiparallel nucleotide strands (= double helix).
- Fixed width of 2 nm.
• Developed the idea of complementary base-pairing
— Fixed width (2 nm) of double helix dictates that a purine must pair with a pyrimidine, always “3 rings”, bases are flat: “base stacking”
