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
What does DNA helicase do
- Breaks the hydrogen bonds between the nitrogenous bases to unwind the double helix
What is the role of DNA polymerase
To join adjacent nucleotides together to make the new strand via condensation reactions
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
Who discovered the structure of DNA
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.
DNA PRIMASE - catalyze the synthesis of short RNA molecules used as primers for DNA polymerases.
DNA LIGASE - joins DNA fragments together.
STAGES OF SEMI-CONSERVATIVE DNA REPLICATION
DNA helicase attaches to molecule, breaks hydrogen bonds between bases on parental DNA . 2 strands unwind from each other and separate. Each strand is a template for new strand.
Free activated DNA nucleotides (3 phosphate groups) attach to complementary exposed base pairs from original strand. Only held by hydrogen bonds between nucleotides.
DNA polymerase catalyses condensation reactions (loss of water) so that a phosphodiester bond can form between the activated adjacent nucleotides to form a new polynucleotide chain/sugar-phosphate backbone. Activated nucleotides lose 2 of their phosphate groups which leave and provide energy for the reaction.
Now have 2 sets of daughter DNA each with one strand of original DNA and one newly synthesised strand.
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.
MODELS OF DNA REPLICATION
Watson and Crick worked out structure of DNA which led them to suggest ways of coping the DNA. Their hypothesis was that DNA replicates by semi-conservative replication but both models needed to be scientifically tested before any conclusion could be drawn.
The conservative model suggested that the original DNA molecule remained intact and that a separate daughter DNA copy was built up from new molecules of deoxyribose, phosphate and organic bases. Of the two molecules produced, one would be made of entirely new material while the other would be entirely original material.
The semi-conservative model proposed that the original DNA molecule split into two separate strands, each of which then replicated its mirror image (i.e. the
missing half). Each of the two new molecules would therefore have one strand of new material and one strand of original material.
EVIDENCE FOR SEMI-CONSERVATIVE REPLICATION
Meselson and Stahl proved that DNA replicated by semi-conservative replication (Watson and Crick’s theory) and not by 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.
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
why is semi conservative replication importsnt
Ensures genetic continuity between generations of cells
Descrvibe the work of meselson and stahl in validating the watson-crick model of semi conservative DNA replication
Bacteria grown in medium containing heavy nitrogen (N15) and nitrogen is incorparated into DNA bases
-DNA extracted and centrifuged —> settles near bottom as all DNA molecules contain 2 heavy strands
Bacteria transferred to medium containing light nitrogen (N14) and allowed to divide once
-DNA extracted and centrifuged–> settles in middle as all DNA molecules contain 1 original heavy and 1 new light strand
Bacteria in light nitrogen (N14) allowed to divide again
-DNA extracted and centrifuged–> half settles in middle as contains 1 original heavy and 1 new light starnd half settles near top as contains 2 light strands
