3.1.5.2 DNA Replication Flashcards
When must DNA be replicated in a cell
Before the cell divides
What’s cell division know as
What’s it for
E.g
Mitosis
For growth + repair
E.g replacing skin cells daily, red blood cells after donating blood
Why does a cell have to copy all of the DNA inside the nucleus before it can divide
To become 2 new cells with each s complete copy of DNA
What does DNA replication involve copying
The sequence of organic bases (genome) to reproduce the code again
What’s the genome
Sequence of organic bases
Why are bases difficult to access even though they need to be
As they’re protected by a helix
What must happen to the hydrogen bonds between complimentary base lakes so DNA CNS be replicated
They must be broken
What enzyme breaks hydrogen bonds between. Omplimentsry base pairs
DNA Helicase
What does the DNA helicase cause to happen once hydrogen bonds are broken
The helix to uncoil
What happens to the bases once the helix uncoils
They’re exposed so can be copied
What are the original strands in the double helix known as
Parental strands
What enzyme follows helicase
DNA polymerase
What does DNA polymerase do to the parental strands
Adds free (single) nucleotides to each exposed strand
What does DNA polymerase use the exposed strands as
Templates
Where does DNA polymerase assemble the free nucleotides from
Cytoplasm
Why are the new daughter strands complementary
Due to the base pair rule
What do New strands need to form between their nucleotides
How
What’s produced
Phosphodiester bonds
Condensation reactions
Water
What bonds form between old and new strands to form a double helix again
Hydrogen bonds
What does DNA polymerase work in
Pairs
What direction does DNA polymerase add nucleotides in
Why so specific?
3’ -> 5’direction
Due to substrate
DNA polymerase works 3’->5’ on one template strand, but what does it do on the other
Can work in the 3-5 direction by assembling the free nucleotides in ‘chunks’
Why must DNA polymerase work in the 3-5 direction
As the antiparallel arrangement of nucleotides means they are the right shape to fit in the active site of DNA polymerase
3 adaptations DNA has for replication
Strong phosphodiester bonds - strands remain intact
Hydrogen bonds easily broken - strands can unwind (exposing bases)
Base pair rule - accurate copies made
Who came up with 2 theories for DNA replication in 1953
Watson and crick
What are the 2 DNA replication theories
Conservative replication
Semi-conservative replication
What happens in conservative replication
Original DNA molecules stay intact
New nucleotides assemble to form a separate, new DNA molecule
What happens in semi conservative replication
The original DNA splits - each strand acts as a template for a new molecule
2 reasons why it’s called semi-conservative replication
Each new DNA molecule contains 50% original DNA
Both new molecules are identical - to each other and the original
Who investigated Watson and cricks’ theory
Meselson and stahl
Method of meselson and stahl’s experiment
Grew bacteria in a growth medium containing heavy nitrogen (15N)
Then put the same bacteria into a medium containing light nitrogen (14N) for 1 replication generation
Then extracted and spun the DNA In a centrifuge - separating the DNA based on its mass
What did meselson and stahl use for their experiment
Isotopes of nitrogen (15N heavy nitrogen and 14N light nitrogen)
Why did meselson and stahl use bacteria
As it reproduces quickly
DNA is accessible as it isn’t in the nucleus
What did the original bacteria have in their DNA as they replicated originally
Heavy nitrogen
How does bacteria divide
Binary fission
How does a centrifuge spin
Very fast
