What are the 2 main stages of cell division
- nuclear division: process by which the nucleus divides (mitosis/meiosis)
- cytokinesis: follows nuclear division, the process by which the whole cell divides
What must be done before a nucleus divides and why
- DNA must be replicated
- ensures all daughter cells have the genetic info in order to produce the enzymes and other proteins they’ll need
What model is used to describe the process of DNA replication
Semi-conservative model
What are the requirements are needed for semi-conservative replication to take place
- the 4 types of nucleotide (with their bases of AGCT) much be present
- both strands of DNA act as a template for the attachment of these nucleotides
- the enzyme DNA polymerase
- A source of chemical energy is required to drive this process
What are the 4 steps to the process of semi-conservative replication
1) the enzyme (DNA helicase) breaks the hydrogen bonds between bases on the two polynucleotide DNA strands.
—> this makes the helix unwind to form 2 single strands
2) each original single strand acts as a template for a new strand. Free floating DNA nucleotides are attracted to their complementary exposed bases on each original template strand (complementary base pairing)
—>A with T and C with G
3) DNA polymerase joins nucleotides to each other on the new strand forming phosphodiester bonds to create the phosphodiester backbone
—> hydrogen bonds form between the bases on the original and new strands
4) Each new DNA molecule contains one strand from the original DNA molecule and one new strand
What is the role of DNA polymerase in semi-conservative replication
- catalyses the joining of the nucleotides
If the bases on a portion of the original strand of DNA are ATGCTACG, determine the equivalent sequence of bases on the Newley formed strand
TACGATGC
Explain why the process of DNA replication is described as semi-conservative
Because half the original DNA is built into the new DNA strand
If an inhibitor of DNA polymerase were introduced into cell, explain what the effect would be on DNA replication
- the linking together of the new nucleotides couldn’t take place
- while the nucleotides would match up to their complementary nucleotides on the original DNA strand
- they wouldn’t join together to form a new strand
Who came up with the theory of semi-conservative DNA replication
Watson and Crick
Who validated the theory of semi-conservative replication
- Meselson and Stahl’s experiments
What was the difference between semi-conservative and conservative replication
- conservative: original DNA strands would stay together, and new DNA molecules would obtain two new strands
What was concluded from Meselson and Stahl’s experiments, and what did this lead to
- DNA settled out in the middle = the DNA contained a mixture of heavy and light nitrogen = the bacterial DNA had replicated semi-conservatively in the light nitrogen
- their experiment = confirms DNA replication in bacteria was semi-conservative
- leads to = other scientists carry out experiments to see if t was the case in all living tings
What are isotopes
Different forms of the same element
How did Meselson and Stahl’s experiment work
- 2 samples of bacteria are grown — 1 in a nutrient broth containing light nitrogen, and 1 in heavy nitrogen
- as bacteria reproduced = they took up nitrogen from broth to help make nucleotides for new DNA = so nitrogen gradually became part of the bacteria’s DNA
- sample of DNA taken from each batch of bacteria, then spun in a centrifuge = DNA from heavy nitrogen bacteria settled lower down the centrifuge tube than the DNA from the light nitrogen bacteria – because it’s heavier
- heavy nitrogen bacteria = taken out of broth, put in light nitrogen broth = after 1 round of DNA replication, sample taken and spun in centrifuge again
- if replication conservative = original heavy DNA would still be together = settle at bottom whilst light at the top
- if replication semi-conservative = DNA would settle between where heavy and light nitrogen containing DNA settles (because contains: 1 strand of old DNA containing heavy nitrogen, and 1 strand of new DNA containing light nitrogen)
Name the part of the DNA molecule that contains nitrogen
The organic bases (adenine, guanine, cytosine and thymine)
Explain why, after 1 generation, all the DNA is made up of an equal mixture of 14N (the number on top right if N) and 15N
- Each DNA molecule is made up of 1 strand containing 15N (the original strand) and one strand containing 14N (the new strand)
- therefore the replication was semi-conservative
What were the 3 facts that Meselson and Stahl worked on
- All bases in DNA contain nitrogen
- Nitrogen has 2 forms: the lighter nitrogen (14N) and THE ISOTOPE (15N), which is heavier
- Bacteria will incorporate nitrogen from their growing medium into any new DNA they make
