TOPIC 1-DNA replication Flashcards
Why does DNA replicate?
DNA copies itself before cell division so that each new cell has the full amount of DNA
What is the method of DNA replication called?
Semi-conservative replication
Why is it called this?
Only half the strands in each new DNA molecule are from the original DNA molecule which means that there is genetic continuity between generations of cells
What is step 1 in DNA replication?
-The enzyme DNA helicase breaks the hydrogen bonds between bases on the two polynucleotide DNA strands
-This makes the helix unwind to form two single strands
What is step 2 in DNA replication?
-Each original single strand acts as a template for a new strand
-Complementary base pairing means that free-floating DNA nucleotides are attracted to their complementary exposed bases on each original template strand (A+T and G+C)
What is step 3 in DNA replication?
-Condensation reactions join the nucleotides of the new strand together- catalysed by the enzyme DNA polymerase
-Hydrogen bonds form between the bases on the original and new strands
-Each new DNA molecule contains one strand from the original DNA molecule and one new strand
What is the difference between the 3’(prime) and 5’ (prime) strand?
-At the 3’ end is a hydroxyl group (OH) that is attached to the pentose sugar of the nucleotide
-At the 5’ end there is a phosphate group
Why can DNA polymerase move down the template strand in a 3 to 5 direction?
-During DNA replication the active site of DNA polymerase is only complementary to the 3’ end of the newly forming strand
-This means the enzyme can only add nucleotides to the new strand at the 3’ end
What happens because the double helix is antiparallel?
-Because DNA polymerase works on one template strand it moves in the opposite direction the the DNA polymerase works on the other strand
Who proved Watson’s and Crick’s theories?
Meselson and stahls experiment
What did Watson’s and Crick’s theories prove?
That DNA replication is semi-conservative
What is an isotope?
Different forms of the same element
What two isotopes did they use in their experiments?
-Heavy nitrogen 15N
-Light nitrogen 14N
Why did they use nitrogen?
DNA contains nitrogen
What is the first step in their experiment?
-Two samples of bacteria were grown for many generations
-One in a nutrient broth containing light nitrogen and one broth with heavy nitrogen
-As the bacteria reproduced, they took up nitrogen from the broth to help make nucleotides for new DNA
-So the nitrogen gradually became part of the bacteria’s DNA
What is the second step in their experiment?
-A sample of DNA was taken from each batch of bacteria and spun in a centrifuge
-The DNA from the heavy nitrogen bacteria settled lower down the centrifuge tube than the lighter nitrogen
What is the third step in their experiment?
-The bacteria grown in heavy nitrogen broth were taken out and put in broth containing only light nitrogen
-The bacteria were left for one round of DNA replication and then another sample was spun in the centrifuge
What would happen if replication was conservative?
-If replication was conservative the original heavy DNA which would still be together would settle at the bottom and the new light DNA would settle at the top
What would happen if replication was semi-conservative
-The new bacterial DNA molecules would contain one strand of the old DNA containing heavy nitrogen and one new strand of light nitrogen
-The DNA would settle out between the light nitrogen DNA and the heavy nitrogen DNA
In conclusion, why is DNA semi-conservative?
The DNA settled out in the middle which showed that DNA contained a mixture of heavy and light nitrogen
-The bacterial DNA had replicated semi conservatively in the light nitrogen
