DNA structure - Lecture 7 Flashcards
Nucleotide consists of?
Nucleoside + phosphate:
- Nucleoside = sugar + 1 of 4 bases
- Phosphate group
Nucleoside consists of?
Sugar + 1 of 4 bases
Nitrogenous bases
Adenine
Guanine
Cytosine
Thymine
Two types of nitrogenous bases?
Purines and Pyrimidines
Purines - features
- A + G
- Numbering from top left N
- Number counterclockwise
Pyrimidines - features
- T, C + U
- Convention is backwards vs purines
Nucleoside formation
- N-1 of pyrimidine/ N-9 of purine attaches to C-1
- Glycosidic bonds
Nucleotide formation
- Nucleoside + 1 or more phosphoric groups from phosphate
- Ester linkage
- Phosphate attaches to 5’ carbon
DNA features and bond
- polynucleotide
- ALWAYS 5’ -> 3’
- Phosphodiester bond btwn 3’C of nucleotide and 5’ of next
Polarity
5’ -> 3’
5’ = Phosphate group
3’ = Hydroxyl group
Levene’s model and limitations including ratio etc
- Tetranucleotides w bases pointed outwards
- Would mean A:T:G:C would = 1:1:1:1 (couldn’t be correct)
Chargaff’s test
- Tested 4 organisms
- Worked out ratios of bases
Chargaff’s rule
- %T=%A and %G=%C
- %G and C = different/organism
X-ray crystallography
- Shoot x-rays at crystallised molecule
- X - rays are diffracted into photographic film
- Unique diffraction patters occur
(x-ray) What happens if there is a wider slit?
- Overlapping wavefronts
(x-ray) What happens if there are 2 slits?
Diffraction patterns will interfere
(x-ray) More than 2 slits?
More diffraction resulting in tighter patterns.
How does x-ray crystallography prove helical structure of DNA?
- Helices cause cross in diffraction grid
- Proven via photo ‘51’: wet “B” DNA formed cross. (Franklin and PHD student’s work)
Pauling’s model of DNA
- Nitrogenous bases outwards
- Triple Helix
- Sugar-phosphate backbone inwards (wrong -> -ve charged phosphate would repel)
Second DNA model - Watson + Crick w access to photo 51
- Worked out that one purine and one pyrimidine would fit (base pairing) in the helix.
- Watson = worked out data consistent w double helix and base pairs.
- Crick = idea of antiparallel strands
How did Watson and Crick use Chargaff’s rule?
- A would have to pair w T
- G would have to pair w C
RH double helix features
- “B” DNA = physiological form
- Clockwise whilst descending
Antiparallel strands features
- 2 strands:
One = 5’ -> 3’
Other = 3’ -> 5’
Base-Phosphate orientation features
- Bases in
- Phosphate backbone on outside
Complimentary base pairing features
- opposite strands bind together via weak H-bonds.
- A->T bind via 2 H-bonds
- G-> C bind via 3 H-bonds
- G-C is therefore stronger.
Base distances features
- 10.5 base pairs / turn
- 1 Turn = 3.6 nM
