7.1 DNA Structure and Replication (HL) Flashcards
Who understood that it was DNA and not protein that is the genetic material of a cell?
Alfred Hershey and Martha Chase
How did we understand that DNA is the genetic material of cells?
One batch of bacteriophages were grown in a radiolabelled sulphur (present in proteins) medium and another was grown in radioactive phosphorous (present in DNA)
They then infected E. coli and found only the phosphorous was passed on by gene transmission and not the sulphur
How did Rosalind Franklin and Maurice Wilkins investigate the structure of DNA?
Using X-ray diffraction
Describe the process of X ray diffraction
DNA is purified and the fibres are stretched in a thin glass tube
This is then targeted with an X ray beam which is diffracted when it hits an atom
This creates a scatter pattern
What inferences can be made from DNA scatter patterns?
DNA is double stranded
Nitrogenous bases are closely packed together on the inside and phosphates form an outer backbone
The DNA twists at regular intervals to form a double helix
How often does the DNA molecule twist?
Every 34 Angstrom
Who proposed the current structure of the DNA model?
James Watson and Francis Crick
Which molecules in DNA are purine?
Adenine
Guanine
What molecules are pyrimidines?
Cytosine
Thymine
How do adenine and thymine bond?
2 H bonds
How do guanine and cytosine bond?
3 H bonds
What is the function of helicase?
Unwinds and separated the double stranded DNA (replication)
How does helicase complete it’s function?
Breaking hydrogen bonds between base pairs
What is an origin of replication?
A specific region which is replicated
What is a replication fork?
The name given to the two strands running in antiparallel direction after being unwound by Helicase
What is the function of DNA Gyrase?
Reduces the torsional strain from unwinding (from helicase)
How does DNA Gyrase complete it’s function?
Relaxing the supercoils (via negative supercoiling)
What are SSB proteins?
Single stranded binding proteins
What is the function of SSB proteins?
Prevent re-annealing
Prevent the single stranded DNA from being digested by nucleases
When do SSB proteins dislodge from the DNA?
When a complimentary strand is synthesised onto the single strand
What is the function of DNA Primase?
Generates a short RNA primer on each template strand
What is needed before any base pairs can be added to a new nucleotide strand?
Primers
How long are most RNA primers?
(10 to 15 nucleotides)