2.2 DNA replication Flashcards
What is nuclear division
Process by which the nucleus divides.
There are two types of nuclear division
What must occur in order for a nucleus to divide its DNA
It must be replicated (copied) to ensure that all the daughter cells have the genetic information to produce the enzymes and other proteins they need
What is cytokinesis
Follows nuclear division and is the process by which the whole cell divides
I’m what way is DNA replication very precise
Because all the new cells are more or less genetically identical to the original way
What model is accepted for DNA
Semi conservative model
what is needed for the semi-conservative model of DNA to happen
- The four types of nucleotide, each with their bases of adenine, guanine, cytosine, or thymine, must be present.
- Both strands of DNA molecule act as a template for the attachment of these nucleotides
- The enzyme DNA polymerase
- A source of chemical energy is required to drive the process
What does the enzyme DNA helicase do for semi conservative replication
- It breaks the hydrogen bonds linking the base pairs of DNA
- As a result the double helix separates its two strands and unwinds
- Each exposed polynucleotide strand then acts as a template to which complementary free nucleotides bind by specific base pairing
What does DNA polymerase do in semi conservative replication
- Free Nucleotides from the nucleus get their bases aligned with their complementary bases on each of the template strands of DNA
- in a condensation reaction by the enzyme DNA polymerase
DNA polymerase catalyses condensation reactions between the deoxyribose sugar and phosphate groups of adjacent nucleotides within the new strands forming the sugar phosphate backbone of the new DNA strands
. Forming the missing polynucleotide strand on each of the two original polynucleotide strands of DNA
Each of the new DNA molecules contains one of the original DNA strands, and one new strand.
So half the original DNA has been saved and built into each of the new DNA molecules
Describe Watson and Cricks conservative model
. The original DNA molecule remained intact and 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 new material whilst the other would be original material.
What does the semi conservative model suggest about DNA replication
. The original DNA molecule split into two separate strands, each of which then replicated its mirror image
. Each of the two new molecules would have one strand of new material and one strand of original
What facts did Meselsohn and stahl use to find out which model about DNA replication is the correct one
Did an experiment basing their work on 3 facts
. All bases in DNA contain nitrogen
. Nitrogen has two forms: The lighter nitrogen N14 and isotope N15 which is heavier
. Bacteria will incorporate nitrogen from their growing medium into any new DNA they make
Describe the experiment did Meselsohn and stahl use to find out about DNA replication
They reasoned that:
. DNA grown on a medium containing N14 would have DNA that was lighter than bacteria growth on a medium containing N15
. They labelled the original DNA of bacteria by growing them on a medium of N15
. Then they transferred the bacteria to a medium of N14 for a single generation to let it replicate once
. The mass of each new DNA molecule would depend on the method of replication had taken place
Then they separated out the different DNA types, and centrifuged the extracted DNA In a special solution- the lighter the DNA, the band was formed nearer the top of the centrifuge tube it collected
The heavier the DNA, the lower down it collected
After spinning the DNA in a centrifuge tube they found this DNA settled between the light and heavy DNA bands produced before
This means it must contain one heavy and one light strand which shows DNA contains half original and half new
They analysed the DNA after two, then three generations and it formed two bands, the original one between light and heavy DNA, but a new one above that.
This new one was formed because after the second generation, two strands of N14N15 are formed and two of only N14 are formed so the two with N14 form a higher band
What did the results from the experiment tell us about DNA
Since the new DNA formed a band between heavy and light DNA, this told the scientists that DNA is formed from one strand of nitrogen 14 and one strand of nitrogen 15
This means DNA replicated semi conservatively, as it contained one strand of new DNA and one strand of old
What would two rounds of replication look like for conservative DNA replication
. After the first round there is one DNA molecule containing nitrogen 14 and one containing Nitrogen 15
. After two rounds of replication we would still have one DNA molecule of only N15, but we’d have 3 strands containing N14
There would be no DNA molecules containing both N15 and N14
Why and when does DNA replication occur
It occurs in preparation for mitosis
In the s phase of the cell cycle (interphase)
