Molecular Biology Flashcards
significance of Chargaff’s work and 2 significant findings
He disproved Levene’s tetranucleotide hypothesis
- [A] = [T] and [C] = [G]
- the composition of DNA varies between species
Chargaff’s conclusion (based on his 2nd rule)
As there is significant variation between species, DNA is feasible to be the genetic material
What are the 2 strains of Streptococcus pneumoniae (bacterium that affects mammals) that Griffith identified for his experiment? Name and explain each
S-strain: smooth cells + pathogenic/virulent. Has protective capsule to prevent host cell from recognising it therefore it cannot be destroyed. Can multiply and kill the host
R-strain: rough cells + non-pathogenic/non-virulent. Lacks protective capsule therefore can be recognised by host cell and killed
Outline Griffith’s experiment (4)
- Living S-strain (virulent) injected into mouse. Mouse dies and S stain found living in heart
- Living R-strain (nonvirulent) injected into mouse. Mouse healthy and R strain not found
- S-strain (virulent) killed by heating and injected into mouse. Mouse healthy and S-strain not found
- Dead S strain cells + living R strain cells injected into mouse. Mouse dies and S-strain found living in heart
Conclusion of Griffith’s experiment
‘The Transforming Principle’: The information that determines a bacteria’s strain and virulence must be encoded in a nonliving chemical, as this information can be transferred from dead to living bacteria (suggesting that DNA may be the genetic material
Outline Oswald Avery’s experiment (3) - how Avery identified Griffith’s Transforming Principle
S strain (virulent) is heat killed with certain components destroyed and injected into living R-strain cells
- Control experiment: RNase is used to destroy S strain RNA and is then added to living R strain bacteria. Virulent S strain and R strain bacteria are present
- Protease used to destroy S strain proteins and is then added to living R strain bacteria. Virulent S strain and R strain bacteria are present
- DNase used to destroy s S strain DNA and is then added to living R strain bacteria. Only nonvirulent R strain bacteria are present
Oswald Avery’s conclusion
DNA is the chemical substance that causes transformation. It is the genetic material
Key figures in elucidation of the structure of DNA
4
Maurice Wilkins
Roslind Franklin
James Watson
Francis Crick
What did X-ray crystallography reveal about the DNA (3)
- DNA had a helical structure
- Bases were perpendicular to the length of DNA molecules
- Constant diameter
What did the constant diameter revealed by X-ray crystallography lead to (with help from Chargarff’s rule)
The base paring rule
A (purine) - T (pyrimidine)
G (purine) - C (pyrimidine)
DNA vs. RNA - sugar group structure
1st carbon = H 2nd carbon = H (DNA), OH (RNA) 3rd carbon = OH (3 prime OH group) 4th carbon = H 5th carbon = phosphate group (5 prime phosphate group)
Formation of phosphodiester bond (true for DNA and RNA)
The OH group on the 3rd carbon of one nucleotide (RNA)/deoxynucleotides (DNA) reacts with the phosphate group on the 5th carbon of another nucleotide/deoxynucleotide
Components of a nucleotide
phosphate, sugar (ribose or deoxyribose), base
The Watson-Crick model of DNA structure (5)
- The DNA has a double stranded helical structure
- The sugar phosphate backbone is on the outside
- The bases are on the inside
- Bases are stabilised by hydrogen bonds (3 in G-C and 2 in A-T)
- The 2 polynucleotide strands are orientated in opposite directions
What did the Watson-Crick model provide (2)
- A stimulus for deciphering the genetic code
- A possible mechanism for the replication of DNA
