Lecture 23 Flashcards
DNA replication
Process in cells that produces 2 identical copies of DNA from one original DNA molecule
Plays an essential part in biological inheritance
Cells divide so dna replication is important
Where does dna replication take place
And what phase of mitosis
Nucleus in eukaryotes
Cytoplasm in prokaryotes
S phase of interphase in eukaryotes
Model 1: Conservative mechanism for DNA replication model
AFTER 1st replication 2 DNA molecules produced :
1 retains old parental strands
The other is made up of 2 newly synthesized replicated strands
White- W- parental DNA
Red-R- replicated DNA
WW -> WW and RR
(First replication)
WW-> WW AND RR (second replication)
RR-> RE AND RR (second replication)
Model 2: semi conservative mechanism
DNA replication
Involves complete separation of the 2 parental template strands
This allows each parental strand to act as a template directing synthesis of new complementary replicated strands
WW-> WR AND WR (1st)
WR-> WR AND RR(2nd)
WR-> WR AND RR(2nd)
Model 3 dispersive mechanism
First replication results in both strands of the 2 new DNA molecules having interspersed segments of both parental and replicated DNA
both replications are mixtures of the previous generation of DNA
Matthew Medellin and frank stahl did what
1958 confirmed SNA replication was semi conservative
How did Medellin and stahl distinguish parental DNA from newly synthesized DNA
Used non radioactive isotopes of 15N that is heavier than more usual form of 14N (heavier bc and extra neutron in its nucleus)
Meselson and stahl experiment
Grew E coli on heavy isotopes of nitrogen 15 for many generations
Gradually 15N would replace more common. Lighter 14N atoms in nitrogenous bases of all the nucleotides in bacterial DNA
Bacteria were then switched to medium containing 14N
Bacteria could use only 14N to create new nitrogenous bases for replicated strands
Now any replicated parts would have 14N
What did meselson and stahl do to find the conclusion
Dna was mixed with césium chloride and the sample was centrifuged
CsCl established a density gradient during centrifugation
DNA molecules moved to positions where their density equaled that of CsCl forming a band of DNA there
In a test tube:
Top: 14N-14N
Middle: 15N-14N
Bottom: 15N-15N (heavy)
What would the centrifuge vial look like in the meselson stahl experiment if it was
Conservative, semi conservative, dispersive mechanism
And which is the right one
Conservative:
All 15N-15N (band at the bottom)
Semi conservative mechanism:
All 15N-14N (band at the middle)
Dispersive mechanism:
14-14 and 15-14 (top and middle band)
Semi conservative is right
Replication in eukaryotes
Separation of parental strands begin at several origins of replication in each chromosome
Replication bubble appears at each origin
At each end of an origin there is a replication fork
Replication occurs at both forks. Proceeding in opposite directions away from the origin
This replication is BIDIRECTIONAL
Bubbles keep elongating until all neighbours are joined, creating 2 identical DNA chromatids linked together at the centromere region
Note. Each chromatids consists of one parental and one daughter strand
Separation in eukaryote replication
Separation has to happen so 2 parental strands can act as templates
Separation allows dinucleotide triphosphates to form the new strands by directing the synthesis of new complementary daughter strands
DNA polymerase
Emzyme
Synthesizes new DNA strands
Can only add nucleotides onto the 3’ end of a new daughter strand
Leading strand
At each replication fork , one strand is synthesized as a continuous strand TOWARDS the replication fork alongside the leading strand template. This strand is called the leading strand
What’s the direction of synthesis
Goes from 5’ to 3’ ends
Sliding clamp
Holds DNA polymerase onto the template strand
Okazaki fragments and discontinuous synthesis
The strand that’s not the leading strand is synthesized AWAY from the replication fork alongside the lagging strand template in fragments called Okazaki fragments
In a process called discontinuous synthesis
Okazaki fragments are joined together to form a continuous lagging strand.
