1.5.2 DNA replication

Question 1

Why is semi-conservative replication important ?

Answer 1

ensures genetic continuity between generations of cells

Question 2

Describe the process of semi-conservative DNA replication

Answer 2

1. DNA helicase breaks hydrogen bonds between complementary bases, unwinding the double helix
2. Both strands act as templates
3. Free DNA nucleotides attracted to exposed bases and join by specific complementary base pairing
4. Hydrogen bonds form between adenine-thymine and guanine-cytosine
5. DNA polymerase joins adjacent nucleotides on new strand by condensation reactions
6. Forming phosphodiester bonds

Question 3

semi conservative ?

Answer 3

each new DNA molecule consists of one original strand and one new strand

Question 4

Use your knowledge of enzyme action to suggest why DNA polymerase
moves in opposite directions along DNA strands

Answer 4

● DNA has antiparallel strands
● So shapes / arrangements of nucleotides on two ends are different
● DNA polymerase is an enzyme with a specific shaped active site
● So can only bind to substrate with complementary shape (phosphate end of developing strand)

Question 5

Name the two scientists who proposed models of the chemical structure of
DNA and of DNA replication

Answer 5

Watson and Crick

Question 6

Describe the work of Meselson and Stahl in validating the Watson-Crick
model of semi-conservative DNA replication

Answer 6

1. Bacteria grown in medium containing heavy nitrogen (15N) so
   nitrogen is incorporated into DNA bases
   ○ DNA extracted & centrifuged → settles near bottom, as all
   DNA molecules contain 2 ‘heavy’ strands
2. Bacteria transferred to medium containing light nitrogen (14N)
   and allowed to divide once
   ○ DNA extracted & centrifuged → settles in middle, as all DNA
   molecules contain 1 original ‘heavy’ and 1 new ‘light’ strand
3. Bacteria in light nitrogen (14N) allowed to divide again
   ○ DNA extracted & centrifuged → half settles in middle, as
   contains 1 original ‘heavy’ and 1 new ‘light’ strand; half settles
   near top, as contains 2 ‘light’ strands