Lecture 11 - DNA structure Flashcards
What is Chargaff’s discovery
There is significant DNA variation between species, thus DNA could be the genetic material
What are Chargaff’s two rules?
1) [A]=[T] and [G]=[C]
2) The composition of DNA varies between species
What is the structure of DNA
A double stranded helical molecule with particular features
What did the x-ray diffraction pattern of DNA show?
Helical structure
Bases perpendicular to the length of the DNA molecule
Double stranded
Describe the formation of a polynucleotide
Nucleotide monomers are joined together with phosphodiester bonds to form a polynucleotide (nucleic acid)
What is a phosphodiester bond?
Chemically strong covalent bond that holds nucleotides together
Describe the formation of the phosphodiester bond
The OH group on the 3rd carbon of one nucleotide reacts with the phosphate group attached to the 5th carbon on another nucleotide.
Why is DNA synthesised from the 5’ to 3’?
Because DNA synthesise doesn’t happen automatically, there is an enzyme that recognises and binds to DNA. When the nucleotides are floating around they have a 3 phosphate group attached which the enzyme will break which releases energy that will be used to catalyse the reaction.
How are the DNA strands organised in the DNA helix?
One DNA strand will have to be flipped in order to have the bases always pointing in the middle and to be able to have complementary base pairing. Because of this the DNA strands have to be organised in an anti parallel direction.
How many hydrogen bonds are between each complementary base pair?
A-T has two hydrogen bonds
G-C has three hydrogen bonds
What do hydrogen bonds do in DNA?
Hold the two strands together and stabilise the DNA molecule
Describe the features of the Watson-Crick model of DNA structure
DNA has a double stranded helical structure
The sugar phosphate backbone is on the outside
The bases are on the inside
Stabilised by hydrogen bonds
The two polynucleotide strands are oriented in opposite directions
What did the Watson-Crick model provide?
A stimulus for deciphering the genetic code
A possible mechanism for the replication of DNA
What type of replication is DNA replication?
Semi-conservative
What makes DNA replication semi-conservative?
Each DNA strand of the double helix is used as a template strand for the synthesis of two new strands
