6.2 The Watson and Crick double helix model of DNA Flashcards
What did R.Franklin and M.Wilkins (1952) solve
They solved the X-ray diffraction pattern of DNA:
1. DNA is helical structure with 20A diameter
2. Spacing between repeating units is 3.4A
3.Helix undergoes a complete turn every 34A
What did E.Chargaff do
He determined the base composition of DNA from many organisms
What is the ratio of bases
A:T ratio is 1:1, G:C ratio is 1:1
Nucleotides are linked in a __________ chain
directional
Where do phosphodiester bonds always form?
they always form covalent linkage between 3’ carbon of one nucleotide and 5’ carbon of the next nucleotide . Note the 5’-3’ polarity
Base pairs consist of ___________
hydrogen bonds (weak electrostatic bonds) between a purine and a pyrimidine
The double helix structure of DNA
- Strands are antiparallel (one strand goes from the 5’-3’ while the other one goes from the 3’-5- direction)
- sugar - phosphate backbone on the outside
- Base pairs in the middle
- two chains held together by H bonds between A-T and G-C base pairs
What are the different shapes DNA can take
Linear, circular, double or single stranded
The majority of naturally occuring DNA molecules have the configuration known as _______ and __________
B-form DNA and Z-form DNA
Explain the structure of B-form DNA
It appears spiral to the right
Explain the structure of Z-form DNA
It has a helix spiral to the left and the backbone takes on a zigzag shape
How does DNA carry information
Via its base sequence
How is the information from DNA copied for transmission to future generations
DNA replication
What two mechanisms allow the genetic information to change
- Recombination
- Mutations
How does DNA-encoded information govern the expression of phenotype
Gene functions of DNA
