Lecture 7 - The structure of DNA Flashcards
Who was Phoebus Levene and what theory did he propose? (1909)
Tetranucleotide theory
Showed each building block of DNA is a nucleotide:
Phosphate group linked to a deoxyribose sugar, linked to nitrogenous base
What sugar is used in DNA and RNA?
DNA: pentose (5C) deoxyribose
RNA: pentose (5C) ribose
What is the difference in structure between ribose and deoxyribose molecular structure?
Hydroxyl group in ribose on 2’
Hydrogen in deoxyribose on 2’
What bases are purines?
Adenine
Guanine
What bases are pyrimidine?
Cytosine
Thymine
Uracil
What are nucleosides and where/what is the bond?
Sugar and base
Glyosidic bond: between C-1’ and N-9 (purine) or N-1 (pyrimidine)
What are nucleotides and where/what is the bond?
Phosphorylated nucleosides
Ester links: between sugar C-5’ group and the phosphate
What forms do nucleotides come in?
All come in mono, di and tri phosphate form
What kind of molecule is DNA?
Polynucleotide with polarity
What is the polarity associated with DNA?
Phosphodiester bond links the 3’C of one nucleotide to 5’ of the next
Base sequence is written and read 5’ to 3’
Polarity at the 5’ phosphate end –> 3’ hydroxyl end
How did Levene ‘tetranucletoide model’ not support Avery et al?
Showed how DNA was simple and repetitive and could not be the genetic material
Not believed to be transforming principle
Who was Erwin Chargaff and what did he propose?
Nucleotides in DNA should be present in equal proportions
(If Levene was correct)
%T = %A = %G = %C
How did Chargaff show Levene to be incorrect?
Measured concentrations of each of 4 nitrogenous bases in different organisms
Different organisms had different DNA constitutions
Reality: %T = %A and %G = %C
What did Linus Pauling say about DNA? (1951)
Described the alpha helix
Basic structure present in many proteins
Used X-ray crystallography - so shape must be helical
How does an X-ray crystallography indicate the helical structure of DNA?
Crystalline target molecule diffracts X-rays
Causes exposed patches on photographic film
Resulting diffraction pattern is unique
What did Huygens show with the ‘Wave theory of light’?
When light passes through a small opening, a wave front is propagated on the other side, single spot appears on a screen
Wider slit, all points across the slit act as point source - results in single slit diffraction pattern on screen
2 slits causes interference - causes double slit interference pattern on screen
With a diffraction grating the pattern becomes much sharper
What did Augustin Fresnel extend the ‘Wave theory of Light’ concept?
Showed diffraction occurred around a solid object with the same width as a slit
Replacing slit with object of same size causes the same results
What are the spatial relationships with the light theory?
Features that are close produce widely separated reflections
Features that are distant produce closely separated reflections
Vertical grids = horizontal spots
Grid = cross
What did Maurice Wilkins do?
Stretched DNA and air dried it
Allowing for stretching into long fibres and mounting in front of X-ray source
What was Maurice Wilkins produce?
1950 - Produced X-ray diffraction of dried DNA
High resolution
Without clear model in mind - too difficult to interpret
What was Gosling and Franklin B model of DNA?
1952 - Used hydrated DNA after 100 h of exposure
Cross is unmistakable - DNA must be helical
Who proposed the triple helical model of DNA?
Linus Pauling
Triple helix 3-strand model, with bases pointing outwards
(supported by Watson and Crick)
What did Watson, Crick and Wilkins propose?
Double helix 2-strand model, with the bases pointing inwards
What was each turn of the DNA helix measured to be?
3.4nm
(Spots far apart give features that are close together)
How many bases were there thought to be per turn of the DNA helix?
10 bases per turn
(More distant the spots, smaller the actual distance in the target)
What was the measured diameter of DNA to be?
2nm
Calculated using degrees of rise within the ‘X’
Why did Pauling model of DNA fail?
(triple helix)
The negative charges of the stacked phosphate groups would repel each other and destabilise the molecule
- Sugar phosphate backbone must be compressed together
How did Watson’s deduction support Chargaff rule?
2 purines = too wide for DNA
2 pyrimidines = too narrow for DNA
= A must pair with T
= G must pair with C
What was Watson and Crick’s model in 1953?
Made DNA with metal scraps, almost 2m tall
Watson: specific A/T and G/C pairing scheme
Crick: idea of antiparallel strands
Everything clicked into place beautifully
What are 6 key features of Watson-Crick model?
- Right-handed (clockwise) double helix
- The strands are anti-parallel 5’ –> 3’ and 3’–> 5’
- Sugar-phosphate backbones are on the outside of the helix, bases oriented towards the central axis
- Complementary base-pairing - bound by weak hydrogen bonds
- Base pair distance (10.5 bp per turn, helix turn = 3.6nm
- major and minor grooves - backbone not equalling spaced causing grooves
How many bonds does A-T have?
2 hydrogen bonds
What are the other structural variants of DNA?
A-DNA: occurs in low hydration conditions (uncertain whether this occurs)
B-DNA: most structurally stable
Z-DNA: taken up physiologically by stretches of alternating pyrimidines and purines
