DNA

Question 1

Who was Erwin Chargaff

Answer 1

• found that in all cellular DNA A=T and G=C

Question 2

Who was Rosalind franklin and Maurice Wilkins

Answer 2

Used X-ray crystallography to identify the secondary structure of DNA - chromosomes are very long molecules and tend to shear and fragment during isolation - each fragment is similar but not identical, it is difficult to form well ordered crystals with them - X-ray diffraction image of DNA showed that molecules are helical with two periodicities along the axis: a primary one of 3.4, and a secondary of 34

Question 3

Who were Francis Crick and James Watson

Answer 3

came up with the model for the secondary structure of DNA

Question 4

What is the geometry of the double helix

Answer 4

• two helical DNA chains wind around a single axis forming a right handed double helix
• the hydrophilic sugar phosphate backbone is on the outside of the helix facing the water
• the hydrophobic bases are stacked inside the double helix perpendicular to the helix axis
• each base is baired in the same plane with a base on other stand
• two stands are antiparalelle
• 3/ and 5/ run in opposite direction
• vertically stacked bp are 3.4 A apart
• each turn consists of 10 base pairs (34 A)
• the two stands are wrapped around eachother (pletonemically coiled, cannot be separated except by unwinding from an end
• DNA undergoes supercoiling

Question 5

What is Self complementarity

Answer 5

• allows each pre-existing stand of a double helix to serve as a template to guide synthesis of new daughter strands
• explains how a call can reproduce and repair damaged DNA

Question 6

How are Base pairs held together

Answer 6

• via hydrogen bonds
• 3 bonds between C and G
• 2 bonds between A and T

*higher the ratio of GC the more difficult it is to separate DNA strands

**how many H bonds ic __ CG and ___ AT explain why it is easier to separate and break apart

Question 7

What are the grooves of DNA

Answer 7

• glycosylic bonds of a base pair are not colinar, they are at an angle
• there are short angle (minor groove) and a large angle (magor groove between
• when two stands wind around each other, leafing to a wider gap between the backbone on one side and a narrower gap on the other
• as helic winds up, major and minor grooves alternate on surface
• each groove is lined by potential h bond donor and acceptor atoms
• the large the size of the major grove makes it more accessible for interactions with proteins that recognize specific DNA sequences

Question 8

What forces stabliize the double helix

Answer 8

• Hydrophobic effect:
• hydrophobic bases hidden in the core
• h bonding between base pairs
• van der waals stacking of bases

* secondary structure is largely independent of sequence bc base pairs ahve similar shpaes and properties

Question 9

the composition of one stand of a double helical DNA molecule is [A]=30% and [G]=24% for this stand what can be said?

Answer 9

• [C] +[T] = 46 %

[A] DOES NOT EQUAL [T] BECAUSE IT IS FOR ONE STAND