what are the 7 stages of semi-conservative replication?
- DNA helicase breaks the weak hydrogen bonds joining the base pairs
- double helix separates into its two strands and unwinds
- each exposed polynucleotide strand acts as a template which attracts free-floating nucleotides
- energy from ATP hydrolysis activates the free nucleotides
- free nucleotides form complementary base pairs with the exposed bases
- new nucleotides are joined into a single strand by DNA polymerase, which forms phosphodiester bonds
- each new DNA molecule now contains one ‘old’ strand from the original DNA molecule, and one new strand
what is the role of the enzyme DNA helicase?
it breaks the weak hydrogen bonds joining the base pairs
where is energy provided from and what does it do?
energy from ATP hydrolysis activates the free nucleotides
what is the role of the enzyme DNA polymerase?
it joins the new nucleotides into a single strand, which forms phosphodiester bonds
why is this called a semi-conservative model?
since each new DNA molecule now contains one ‘old’ strand from the original DNA molecule, and one new strand
in which direction is new DNA always being made?
from the 5’ to the 3’ direction (‘ = prime)
how is replication different in the leading and lagging strands?
- in the leading strand, the replication can be continuous
- in the lagging strand, it has to be done in fragments
> DNA polymerase can only bind in one direction
why are new strands formed in opposite directions?
because DNA strands are antiparallel
why does enzyme action mean DNA polymerase can only go from 5’ to 3’? [4]
- DNA polymerase has an active site with a specific shape
- the active site of DNA polymerase is only complementary with the 3’ end of the strand
- 5’ end is phosphate group, 3’ is hydroxyl group so the ends are different shapes
- since nucleotides are added at the 3’ end, the new strand extends from 5’ to 3’
description of conservative replication for 2 replications
- first replication produces entirely new strand of DNA
- by second replication, 1/4 has entire parental DNA and none of the others do
(where ‘a’ is parental DNA and ‘b’ is new DNA)
- first replication: aa bb
- second replication: aa bb bb bb
what is a description of semi-conservative replication? (proven)
- in first replication, 1 strand of parental DNA is present in both copies
- in second replication, 1 strand of parental DNA is present in 2 of the 4 copies
> (where ‘a’ is parental DNA, ‘b’ is new DNA)
first replication: ab ab
second replication: ab bb ab bb
describe dispersive replication for 2 replications
- first replication: both strands produced have a jumble of parental and new DNA
- second replication: all 4 strands also have a jumble of parental and new DNA
what are the 4 stages of the meselson stahl experiment?
- isolate bacteria with DNA containing heavier nitrogen (¹⁵N)
- when centrifuged, DNA settles at bottom of centrifuge tube
- bacteria with DNA containing only heavier nitrogen allowed to replicate in broth with only light nitrogen (¹⁴N)
- semi-conservative replication was proven to be correct
what would have been observed if conservative replication was correct?
- new DNA containing only lighter nitrogen settles at top
- original DNA containing only heavier nitrogen settles at bottom
what was observed which proved that semi-conservative replication was correct?
DNA, now containing mixture of lighter and heavier DNA, settles in middle
