DNA structure - Lecture 7

Question 1

Nucleotide consists of?

Answer 1

Nucleoside + phosphate:
- Nucleoside = sugar + 1 of 4 bases
- Phosphate group

Question 2

Nucleoside consists of?

Answer 2

Sugar + 1 of 4 bases

Question 3

Nitrogenous bases

Answer 3

Adenine
Guanine
Cytosine
Thymine

Question 4

Two types of nitrogenous bases?

Answer 4

Purines and Pyrimidines

Question 5

Purines - features

Answer 5

• A + G
• Numbering from top left N
• Number counterclockwise

Question 6

Pyrimidines - features

Answer 6

• T, C + U
• Convention is backwards vs purines

Question 7

Nucleoside formation

Answer 7

• N-1 of pyrimidine/ N-9 of purine attaches to C-1
• Glycosidic bonds

Question 8

Nucleotide formation

Answer 8

• Nucleoside + 1 or more phosphoric groups from phosphate
• Ester linkage
• Phosphate attaches to 5’ carbon

Question 9

DNA features and bond

Answer 9

• polynucleotide
• ALWAYS 5’ -> 3’
• Phosphodiester bond btwn 3’C of nucleotide and 5’ of next

Question 10

Polarity

Answer 10

5’ -> 3’
5’ = Phosphate group
3’ = Hydroxyl group

Question 11

Levene’s model and limitations including ratio etc

Answer 11

• Tetranucleotides w bases pointed outwards
• Would mean A:T:G:C would = 1:1:1:1 (couldn’t be correct)

Question 12

Chargaff’s test

Answer 12

• Tested 4 organisms
• Worked out ratios of bases

Question 13

Chargaff’s rule

Answer 13

• %T=%A and %G=%C
• %G and C = different/organism

Question 14

X-ray crystallography

Answer 14

• Shoot x-rays at crystallised molecule
• X - rays are diffracted into photographic film
• Unique diffraction patters occur

Question 15

(x-ray) What happens if there is a wider slit?

Answer 15

• Overlapping wavefronts

Question 16

(x-ray) What happens if there are 2 slits?

Answer 16

Diffraction patterns will interfere

Question 17

(x-ray) More than 2 slits?

Answer 17

More diffraction resulting in tighter patterns.

Question 18

How does x-ray crystallography prove helical structure of DNA?

Answer 18

• Helices cause cross in diffraction grid
• Proven via photo ‘51’: wet “B” DNA formed cross. (Franklin and PHD student’s work)

Question 19

Pauling’s model of DNA

Answer 19

• Nitrogenous bases outwards
• Triple Helix
• Sugar-phosphate backbone inwards (wrong -> -ve charged phosphate would repel)

Question 20

Second DNA model - Watson + Crick w access to photo 51

Answer 20

• 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

Question 21

How did Watson and Crick use Chargaff’s rule?

Answer 21

• A would have to pair w T
• G would have to pair w C

Question 22

RH double helix features

Answer 22

• “B” DNA = physiological form
• Clockwise whilst descending

Question 23

Antiparallel strands features

Answer 23

• 2 strands:
  One = 5’ -> 3’
  Other = 3’ -> 5’

Question 24

Base-Phosphate orientation features

Answer 24

• Bases in
• Phosphate backbone on outside

Question 25

Complimentary base pairing features

Answer 25

• 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.

Question 26

Base distances features

Answer 26

• 10.5 base pairs / turn
• 1 Turn = 3.6 nM