DNA replication

Question 1

Outline DNA replication

Answer 1

• Double helix uncoils when DNA helicase breaks hydrogen bonds in polynucleotide strands
• Each strand acts as a template for the formation of two new complementary strands
• DNA nucleotides align next to complimentary exposed bases of each template strand, AT, GC
• DNA nucleotides of each strand are joined together by the enzyme DNA polymerase to from complementary base pairing to original DNA strands.
• The 2 strands of each DNA molecule are joined by hydrogen bonds, the strands are identical to one another and the original DNA.
• Each newly formed DNA molecule contains one of the original polypeptide strands, hence semi-conservative replication

Question 2

Describe the Meselson and Stahl Experiment

Answer 2

• Cells of bacterium E Coli were grown on a medium in which normal isotope was replaced with heavy isotope 15N.
• Cells divided until it was certain heavy nitrogen had been incorporated into entire DNA
• The bacteria were then transferred to a medium containing only the normal isotope 14N and allowed to divide.
• Samples of bacteria were then taken after each division (generation) and DNA was extracted and centrifuged
• DNA containing 15N is slightly heavier than DNA containing 14N.

Question 3

Describe the 1st generation

Answer 3

Each DNA molecule contains one strand of heavy 15N and one of normal 14N

Question 4

Describe the 2nd generation

Answer 4

• 50% of the cells contain DNA molecule with one strand of 15N and one strand of normal 14N
• 50% contain DNA molecule with both strands containing the normal 14N

Question 5

Describe the 3rd generation

Answer 5

• 25% of cells contain DNA molecules with one strand of heavy 15N and one strand of normal 14.
• 75% contain DNA molecule with both strands consisting of the normal isotope 14N