DNA replication Flashcards
1
Q
Why is semi-conservative replication important?
A
Ensures genetic continuity between generations of cells
2
Q
Describe the process of semi-conservative DNA replication
A
- DNA helicase breaks hydrogen bonds between complementary bases, unwinding the double helix
- Both strands act as templates
- Free DNA nucleotides attracted to exposed bases and join by specific complementary base pairing
- Hydrogen bonds form between adenine-thymine and guanine-cytosine
- DNA polymerase joins adjacent nucleotides on new strand by condensation reactions
- Forming phosphodiester bonds
Semi-conservative - each new DNA molecule consists of one original / template strand and one new strand
3
Q
Use your knowledge of enzyme action to suggest why DNA polymerase
moves in opposite directions along DNA strands
A
● 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
4
Q
Name the two scientists who proposed models of the chemical structure of
DNA and of DNA replication
A
Watson and Crick
5
Q
Describe the work of Meselson and Stahl in validating the Watson-Crick
model of semi-conservative DNA replication
A
- Bacteria grown in medium containing heavy nitrogen (15N) and
nitrogen is incorporated into DNA bases
● DNA extracted & centrifuged → settles near bottom, as all
DNA molecules contain 2 ‘heavy’ strands - 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 - 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
