3.1.5.2 Semi-conservative replication Flashcards
What is the purpose of semi-conservative replication?
-before a parent cell divides by mitosis or meiosis, it needs to copy the DNA contained within it
-This is so that there will be genetic continuity between the generations of cells (each of the two daughter cells will receive a full copy of the DNA of the parent cell).
Why is it called semi-conservative replication?
Semi=half
-In each new DNA molecule, one of the polynucleotide strands is from the original DNA molecule, and the other polynucleotide strand has to be newly created by the cell
-this means the DNA molecule has conserved half of the original DNA, and used this to create a new strand.
At what phase in the cell cycle does semi-conservative replication occur?
During the S phase/interphase when the cell is not dividing. This is also known as stage 1 in the cell cycle.
What is the final step in semi-conservative replication?
-DNA polymerase, a second enzyme, joins the nucleotides together to form the sugar-phosphate backbone
-DNA polymerase catalyses the formation of these phosphodiester bonds (speeds up the rate of reaction)
-This occurs via condensation reactions
What is the first step in semi conservative replication?
-The enzyme DNA helicase breaks the hydrogen bonds between the base pairs of DNA
-this unwinds and separates the double helix into two single polynucleotide strands
-DNA helicase’s active site is complementary to the DNA substrate
What is the second step in semi-conservative replication?
-Each single polynucleotide strand then acts as a template for the formation of a new strand
-free nucleotides are attracted to the complementary bases on the strands
-complementary base pairing tasks place and the bases are held together by hydrogen bonds
What does semi-conservative replication produce?
Two genetically identical DNA molecules, that are also genetically identical to the DNA of the original molecule
Why is semi-conservative replication important?
-ensures genetic continuity between generations of cells
-cells are replaced regularly, and new cells need to be genetically identical to old ones in order to carry out the same function
-this process is also needed during growth
In which direction does DNA polymerase build the new strand?
-can only build in one direction, in the 5’ to the 3’ direction
What did Mesleson and Stahl do?
They proved the theory of semi-conservative replication of DNA correct
What were the 2 achievements made by Watson and Crick?
-confirmed the double helix structure of DNA
-came up with the theory of semi-conservative replication(later proved true by meselson and stahl)
Outline the process to prove the theory of semi-conservative replication
-bacteria are grown in growth medium containing heavy 15N isotope (nitrogen 15)
-DNA replicates and used nitrogen from the growth medium to make new nucleotides
-culture eventually contained only the heavier 15N isotope
-A sample of this DNA settled at the bottom of a tube when spun in a centrifuge
-This DNA was then added to a growth medium containing only 14N isotope which is lighter
-it was left enough time for one generation of replication to occur
-when spun in the centrifuge, the DNA settled in the centre of the tube, showing it contained both 15 and 14 Nitrogen isotopes
What were the 3 known facts before Meselson and Stahl’s work that were essential?
-all bases contain nitrogen
-nitrogen has 2 isotopes with different masses
-bacteria will incorporate nitrogen from their growth medium into any new DNA they make (they could trace new and original DNA based on the isotope of nitrogen it contained).
What were the 2 other theories of how DNA replicated?
-conservative replication: helix remains intact and a new helix is synthesised
-dispersive replication: hybrid DNA where each strand is a mixture of old and new DNA
