Structure Of DNA Flashcards
How are is the DNA coiled?
Pair of DNA strands coiled around common axis- right handed
What holds together and stabilises the strands?
Held together by intermolecular interactions
Stabilised by interactions in the chain- van der waals and hydrogen bonds
Which direction do the strands run?
The strands run in opposite directions- anti parallel
Describe the polarity of the chain
The polarity of the chain is opposite
5’-> 3’, 3’-> 5’
Describe the layout of the components in the chain
The sugar and phosphates are on the outside- backbone
Purine and pyrimidine bases are on inside
Plane of bases is perpendicular to helix axis and plane of sugars
What is the diameter of the helix?
Diameter of helix is 20A
How far are adjacent bases separated and rotated?
Separated by 3.4A
Rotated by 36 with regards to one another
What is one complete helical turn?
10 base pairs
What bonds form between which bases and how many?
Hydrogen bonds are formed between the bases
A-T has 2 hydrogen bonds
G-C has 3 hydrogen bonds
Base on one strand forms H bonds with base on another strand
How do bases pair together and why?
Bases pair together so that a purine goes with a pyrimidine
2 purines are too large to fit within the helical structure
2 pyrimidines are too small to hydrogen bond
Describe the restrictions on the sequence of the strands
No restrictions on sequence of first strand
This sequence determines the sequence of the second strand
The strands are complementary
Describe the grooves in the double helix and why are they like this?
The double helix has major and minor grooves
This is because the glycosidic bonds are not diametrically apart- at an angle that is neither parallel or at right angles
The point the sugars are attached at less than 180 apart- not diametrically opposite
Describe the function of DNA polymerase
Adds individual nucleotides to the 3’ OH group of a strand of DNA
Ensures accuracy of replication- ensures bases added are complementary
Mediates chemistry
How does DNA polymerase add bases?
Substrate is nucleotide triophosphate (base)
Activates the 3’ OH as a nucleophile- activated form of base
Nucleophile attacks the phosphate
Where does the energy come from to add bases?
Energy required to drive the reaction comes from cutting high energy phosphate bonds on the nucleotide triphosphates
Tightly coupled the free energy of nucleotide triphosphate hydrolysis (breaking of bonds) to phosphodiester bond formation
One is spontaneous, one isn’t- DNA polymerase runs two reactions simultaneously- the second reactions provides the energy required for the first reaction
