DNA replication

Question 1

What ensures genetic continuity between generations of cells?

Answer 1

Semi conservative replication

Question 2

Describe the process of semi conservitive replication

Answer 2

1) Unwinding of the double helix > breakage of hydrogen bonds between complementary bases in the polynucleotide strands > via DNA helicase

2) Exposed bases are attracted to new DNA nucleotides on template strand > complementary base pairing

3) Condensation reaction occurs > joins adjacent nucleotides > DNA polymerase is the enzyme that reforms H bonds

Question 3

When does semi conservative replication occur in the cell cycle?

Answer 3

Before nuclear division

Question 4

Describe the conservative model

Answer 4

• Original DNA molecule remains intact
• Separate daughter DNA copy is built up from new molecules > deoxyribose, phosphate, and organic bases
• Of two molecules produced > both would be made entirely of a new material

Question 5

Describe the semi-conservitve model

Answer 5

• Original DNA molecule splits into 2 separate strands
• Each molecule then replicated it’s mirror image
• Each of of the 2 new molecules would have one strand of new material and one original strand

Question 6

What facts did Watson and Crick base their work on? (3)

Answer 6

1) All the bases in DNA contain nitrogen

2) Nitrogen has two forms > 14N (lighter) and 15N (the isotope which is heavier)

3) Bacteria will incorporate nitrogen from their growing medium to any new DNA that they make

Question 7

Describe the method that acts as evidence for semi-conservitive replication - bacteria replication stage (4)

Answer 7

1) Bacteria grown in 14N medium

2) Bacteria grown in 15N medium for many generations

3) Bacteria grown in 15N transported to 14N medium > samples move at intervals

Question 8

How is DNA extracted and prepared from bacteria for - test tubes stage? (3)

Answer 8

1) DNA extracted from bacteria

2) DNA extract suspended in special solution

3) DNA suspension centrifuged

Question 9

Describe the test tube stage of the semi-conservative replication proof (4)

Answer 9

1) 1st test tube > acts as a control > both strands light 14N > high up ok test tube after centrifuge

2) 2nd test tube >both strands heavy 15N as bacteria grown in 15N medium multiple generations> low down on tear tube after centrifuge

3) 3rd test tube > 1st generation > one heavy and one light molecule > mixed DNA molecule > middle of test tune after centrifuge

Basically, DNA is at intervals after centrifugation depending on whether it has 14N or 15N ( the heavies hang lower)