1.5.2 DNA Replication Flashcards
Why is semi-conservative replication important?
Ensures genetic continuity between generations of cells
Describe the process of semi-conservative DNA replication
- 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
What does semi-conservative mean?
Semi-conservative - each new DNA molecule consists of one original / template strand and one new strand
Use your knowledge of enzyme action to suggest why DNA polymerase moves in opposite directions along DNA strands
● 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)
Name the two scientists who proposed models of the chemical structure of DNA and of DNA replication
Watson and Crick
Describe the work of Meselson and Stahl in validating the Watson-Crick model of semi-conservative DNA replication
- 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 - 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
