BM: DNA Replication COPY Flashcards
How does DNA replicate?
What does this mean?
Semi-conservative replication.
- Half the strands in each new DNA molecule are from the original DNA molecule.*
- Means there is genetic continuity between generations of cells.*
What are the four requirements needed for semi-conservative replication to take place?
- The four types of (free) nucleotides, with bases A, T, G & C, must be present.
- Both strands of DNA must act as a template for these nucleotides to attach onto.
- Enzyme DNA polymerase must be present.
- Chemical source of energy is required to drive the process.
Explain the process of semi-conservative replication:
- Enzyme helicase breaks hydrogen bonds between base pairs.
- Helix unwinds to form two single strands.
- Exposed polynucleotide strands act as templates.
- Free floating nucleotides are attracted to their complementary exposed bases on each original strand - due to complementary base pairing.
- Condensation reactions join the nucleotides of the new strands together - catalysed by the enzyme DNA polymerase.
- Hydrogen bonds form between the bases on the orignal and the new strand.
- Each new DNA molecule contains one strand from original DNA molecule and one new strand.
What two enzymes are involved in semi-conservative replication?
What is the function of each?
Helicase = breaks hydrogen bonds between base pairs.
Polymerase = catalyses condensation reactions between nucleotides of new strand.
How does DNA polymerase move across the DNA molecule?
It moves in opposite ways along antiparallel DNA strands.
Describe the arrangement of the DNA strands in a DNA molecule:
The strands are antiparallel.
What does ‘antiparallel’ mean in terms of DNA strands?
They run in opposite directions.
How are the two ends of a DNA polynucleotide strand different?
One end is 5’ and one end is 3’
What is the active site of DNA polymerase complementary to?
What does this mean the enzyme can do?
The 3’ end of the newly forming DNA strand.
This means the enzyme can only add nucleotides to the new strand at the 3’ end.
If DNA polymerase can only add nucleotides to the new strand at the 3’ end, how does this affect the direction in which the new strand is made?
How does the enzyme move across the template strand?
It means the new strand is made in a 5’ to 3’ direction.
The enzyme moves down the template strand in a 3’ to 5’ direction.
So you end up with antiparallel strands.
Who came up with the theory of semi-conservative replication?
Watson and Crick
Who validated the theory of semi-conservative replication through experiments?
Meselson and Stahl
Before any theory was validated, what did scientists think about DNA replication?
They were unsure whether it was conservative or semi-conservative.
If DNA replication was conservative, what would this mean?
The original DNA strand would stay together and the new DNA molecules would contain the two new strands.
What did Meselson and Stahl experiment with?
Two isotopes of nitrogen (as DNA contains nitrogen) - heavy nitrogen (15N) and light nitrogen (14N).