2.2 DNA replication Flashcards
When does DNA replication start and what is it?
In interphase more specifically S phase
It is the DNA replicating to provide a copy for the new cells
What is semi conservative replication
In the daughter DNA one strand is from the parental DNA and one strand is newly synthesised
(daughter DNA is the dna created and parental DNA is the original DNA )
Why is it important for the nitrogenous bases to be complimentary?
Enables identical copes of DNA to be created and reduce copying errors which could lead to mutations
Name the two enzymes affiliated in DNA replication
DNA helicase and DNA polymerase
Describe the process of semi-conseravtive DNA replication. (7)
- DNA helicase breaks the hydrogen bonds between the two DNA strands and the DNA helix unwinds
- Each strand acts as a template for a new strand
- Individual free DNA nucleotides join up along the template strand by complementary base pairings
- DNA polymerase joins the individual nucleotides together, so that the sugar-phosphate backbone forms
- Hydrogen bonds them form between the bases on each strand and the strand twist to form a double-helix
- Two identical DNA molecules are produced
- Each of the new molecules contains a single strand from the original DNA molecule and a single new strand
Name the two scientists who proposed models of the chemical structure of
DNA and of DNA replication
Watson and Crick
They hypothesied that DNA must replicate either conservatively or semi conservatively
Along with the help of Rosalind Franklin
What was their first hypothesis
That DNA was replicated semi conservatively-Each replicated DNA molecule contains one of the original DNA strands strand and one newly synthesied DNA strand
What was the second hypothesis
That DNA was replicated conservatively-The original DNA remains intact following DNA replication and the two newly synthesised strands of DNA join together
What are the three experimental background information
- DNA bases are nitrogenous
- Nitrogen has two isotopes N14 and N15
- Bacteria take in the nitrogen isotopes to make new DNA nucelotides
How would you determine which DNA sample has Isotope N15 or N14
Spin the sample in a centrifuge to determine thier density
N15 will settle lower down as it is more dense
N14 will settle high as it is more less dense
C-ATOM ARRANGEMENT IN PENTOSE SUGAR MOLECULES
C-atoms numbered after oxygen clockwise. Carbon-5 is outside pentose molecule.
Carbon 3 has hydroxyl (OH) group attached.
Carbon 5 has phosphate group attached.
As the strands are antiparallel, One strand runs 3’to 5’
(ends with phosphate group) and one goes
5’ to 3’ (ends with hydroxyl group)
DNA REPLICATION ENZYMES
DNA HELICASE - unzipping, breaks hydrogen bonds to separate the polynucleotide strands.
DNA POLYMERASE - join together adjacent nucleotide. forming phosphodiester bonds
DNA PRIMASE - catalyze the synthesis of short RNA molecules used as primers for DNA polymerases.
DNA LIGASE - joins DNA fragments together.
ROLE OF DNA LIGASE
DNA polymerase can only work in one direction (5’ to 3’) because the active site of the enzyme is only complementary to the 3’ (phosphate) end so joins nucleotides at 3’ end.
The 5’ to 3’ strand is the leading strand as the DNA polymerase follows the DNA helicase.
As the strands are antiparallel, the other strand runs 3’ to 5’.
DNA polymerase wants to go 5’ to 3’
So does a section of strand then jumps to next bit leaving Okazaki fragments. This is the lagging strand.
DNA ligase joins together the shorter polynucleotide strands/fragments together.
EVIDENCE FOR SEMI-CONSERVATIVE REPLICATION
They bases their experiments on 3 facts:
All bases in DNA contain nitrogen
Nitrogen has two forms: the lighter nitrogen 14N and the heavier isotope 15N.
Bacteria will incorporate nitrogen from their growing medium into any new DNA that they make.
They reasoned that bacteria grown on a medium containing 14N would have DNA that was lighter than bacteria grown on a medium containing 15N. Bacteria divide quickly so that is why bacteria were used.
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
2. 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
3. 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
WHAT THE RESULTS WOULD SHOW IF CONSERVATIVE REPLICATION TOOK PLACE
No banding ever in the middle
Bottom band
Top band will increase in thickness after each replication
Entirely new material for each new double helix
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)
