6.4 DNA replication Flashcards
What does unwinding of the DNA double helix do
It exposes the bases on each strand
Each strand can act as a ________ for the synthesis of new strands
template
New strand forms by __________ of complementary base pair
insertion
1 DNA molecule yields ___________ at the end of DNA replication
Two identical daughter double stranded DNA molecules
What are the three possible models of DNA replication
Semiconservative, Conservative, Dispersive
Who discovered the semiconservative model
Watson and Crick
Explain the conservative model
The parental double helix remians intact, and both strands of each of the two daughter helices are newly synthesised.
Explain the dispersive model
Both strands of both daughter helices contain original and newly synthesised DNA
What is the aim of the Meselson-Stahl experiment
to determine which of three models accurately describes DNA replication.
Explain the Semicoservative model
This model proposed that each DNA strand serves as a template for a new strand, resulting in daughter DNA molecules containing one original and one newly synthesized strand.
Where does the energy for DNA replication come from?
It comes from high-energy phosphate bonds associated with dNTPs
What is the function of DNA polymerase
It catalyses new phosphodiester bonds
What are the two steps of DNA replciation
Initiation and Elongation
What occurs during Initiation
Proteins open up the double helix and prepare it for complementary base pairing
What occurs during elongation
proteins connects the correct sequence of nulceotides on newly formed DNA strands (correctly copied from the template strand)
Which direction does DNA replication proceed
In the 5’ to 3’ direction
DNA polymerase adds nucleotides to the ____of the new strand
3’-OH
What are the three requirements for DNA polymerase action
- four dNTPs for: incorporation into chain and energy
- Single-stranded template: may be unwound by other proteins
- Primer with exposed 3’ hydroxyl
What do the enzymes/proteins do during initiation
initiator protein: binds to the origin of replication
DNA helicase: unwinds the helix
single-stranded binding proteins: Keep the DNA helix open and prevent it from winding up again
Primase: it synthesises RNA primers
primers are…….
complementary and antiparallel to each template strand.
What do the proteins/enzymes do during elongation
DNA polymerase III: catalyses phosphodiester bond formation between adjacent nucleotides (polymerization)
DNA polymerase I: fills in the gaps between the newly synthesised okazaki fragments on the laggin strand
differentiate between the leading strand, lagging strand and okazaki fragments
leading strand- it has continuous synthesis
laggind strand- has discontinous synthesis
Okazaki fragments- short DNA fragments on the lagging strand
The DNA replication of a circular bacterial chromosome is:
bidirectional
Outline the steps of circular bacterial chromsome DNA replication
1.There is only 1 origin of replication
2.Two forms moving in opposite directions, meeting at a designated termination region
3.Unwinding distorts the DNA molecule creating supercoiled DNA ahead of the replication fork
4.DNA topoisomerase relaxes supercoils by breaking, unwindING and suturing the DNA
5.DNA topoisomerase cuts DNA
6.Cut strands rotate to unwind
7.Cut ends are re-joined by ligase
8.Replication is complete when replication forks meet at the termination region
8.Topoisomerases separate entwined daughter chromosomes
What are the three ways to ensure fidelity of DNA information
1.Redundancy- either strand of the double helix can specify the sequence of the other strand
2.Precision of cellualr replication machinery- DNA polymerase I and III have proofreading ability
3.DNA repair enzymes
