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Questions and Answers
During binary fission, what initiates the division process in a bacterial cell?
What is the role of centromeres in the eukaryotic cell cycle?
What happens during the G0 phase of the cell cycle?
Which phase of mitosis involves breakdown of the nuclear envelope?
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What characterizes metaphase in mitosis?
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Which proteins regulate the transition from G1 to S phase in the cell cycle?
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What occurs at the DNA damage checkpoint during the cell cycle?
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How do unattached kinetochores affect the spindle assembly checkpoint during mitosis?
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"What is the primary role of CDK inhibitor proteins in controlling the cell cycle progression?"
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What is the impact of lack of tension in centromeres during mitosis?
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What is the primary function of microtubules during cell division?
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In the Johnson & Rao's Cell Fusion Experiments, what was the purpose of fusing cells in different phases of the cell cycle?
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In the Markert & Masui's Microinjection Experiments, what was the observed effect when mitotic phase cytoplasm was injected into an interphase cell?
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What is the key difference between normal cells and cancer cells in terms of cell division?
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What is the purpose of using natural extracts like vincristine and taxol as chemotherapy drugs?
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What is the purpose of the Pim 1 biomarker for pancreatic cancer?
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What is the mechanism by which alcohol consumption can lead to cancer?
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What is the purpose of CAR-T cell therapy in cancer treatment?
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What is the key difference between Stage 1 and Stage 4 cancer?
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What was the significance of Steve Jobs having his whole genome sequenced?
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In the experiment with $^{15}$N and $^{14}$N isotopes of nitrogen, which model of DNA replication was rejected based on the results?
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During DNA replication in prokaryotes, which of the following enzymes is responsible for removing the RNA primers and filling the gaps between Okazaki fragments?
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Which of the following enzymes is responsible for catalyzing the lengthening of telomeres in eukaryotic cells?
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In the laser beam experiment, what was observed when comparing the length of the microtubules on either side of the photobleached reference point during anaphase?
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Which of the following is responsible for minimizing torsional strain from unwinding during DNA replication in eukaryotes?
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What is the primary function of single-stranded binding proteins during DNA replication?
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Which of the following is responsible for joining the $3'$ end of an Okazaki fragment to an adjacent nucleotide during DNA replication?
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In eukaryotic cells, what is the primary reason for the shortening of telomeres over time?
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Which of the following enzymes is responsible for adding telomere repeats to the ends of chromosomes in eukaryotic cells?
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During DNA replication in prokaryotes, which of the following statements is true about the leading and lagging strands?
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Study Notes
DNA Replication
- The semiconservative model of DNA replication was proposed by Watson and Crick
- Evidence from Meselson and Stahl's experiment:
- Cultured E. coli in 15N and transferred it to 14N
- Extracted DNA samples and centrifuged to separate DNA based on densities
- Bacteria in 15N isotope had one band after transfer and one round of replication in 14N
- DNA was a single band with lower density (higher than 15N band)
- Band must be a hybrid of 14N and 15N DNA
- Rejected conservative model because no individual band for each isotope
- After multiple generations in 14N, extracted DNA separated into 2 bands: 1 only 14N, 1 hybrid
- Rejected dispersive model
Chromosome Replication
- Replication of eukaryotic DNA in media containing fluorescent nucleotides
- Chromosomes of eukaryotic cells contain hybrid and fully labeled nucleotides
- Supports semiconservative replication
Cellular Processes
- S-phase of the cell cycle starts at origins of replication
- Prokaryotes have a single origin of replication
- Ori sites: specific points where DNA replication begins
- Unwinding creates replication forks
- Template strand is copied from the 3’ end to the 5’ end
- Daughter strand elongates in a 5’ to 3’ direction
- Incoming nucleotides hydrogen bond to the template strand and make phosphodiester bond
- Produce pyrophosphate
- RNA primase makes primer
- DNA polymerase moves in a 5’ to 3’ direction
- After short RNA primer (5-10 nucleotides)
- DNA polymerase can only add nucleotides to the 3’ end
- Polymerase III: leading strand: continuous replication from one primer
- Lagging strand: discontinuous (Okazaki fragmented) replication
- Replicate the DNA away from the replication fork
- Polymerase I: removes RNA primers and fills the gaps between Okazaki fragments
- Eukaryotes use a different set of DNA polymerases
- Replication complex: helicase, break hydrogen bonds to unwind DNA
- Single-stranded binding proteins: bind to and stabilize each strand
- Topoisomerase: binds ahead of replication fork to minimize torsional strain from unwinding
- Triggers the process of unwinding
Eukaryotic Cell Cycle
- Many origins of replication
- DNA replication machinery can't complete 5’ ends of daughter strands when the DNA is linear
- Lagging strand: RNA primer at the end cannot be replaced with DNA nucleotides
- Shorter DNA molecules are produced
- Telomeres: nucleotide sequences made of TTAGGG, non-coding buffer zone to protect coding genes
- Become shorter over time, does not occur in gametes or stem cells
- Telomerase enzyme: catalyzes lengthening of telomeres
- Reverse transcriptases: DNA from an RNA template, contains the RNA template in its ribonucleoprotein complex
Mitosis
- Laser beam experiment: researchers used fluorescent dyes to highlight DNA (chromosomes)
- Blue kinetochore microtubules are yellow
- How are chromosomes moved to the opposite poles?
- Top: metaphase, bottom: anaphase
- Laser beams photobleached the dye
- Compare the length of the microtubules on either side of the photobleached reference point
- Change on the side closer to the chromosome and kinetochores compared to microtubules closer to the poles
- Predict that the photobleached sections will still be visible as chromosomes begin to move
- It gets shorter on the side nearest to the chromosomes and kinetochores
- Conclude that the microtubules shorten at one end
- Chromosomes during mitosis: chromosomes are blue, kinetochores are black, microtubules are yellow
- Microtubules are broken down into tubulin subunits
- Tubulin subunits are not destroyed, recycled, and used again
Cell Fusion Experiments
- Johnson & Rao's cell fusion experiments: how do microtubules move?
- Used the formation of heterokaryons
- Membranes of cells are dynamic and fluid
- Use chemicals or electrical charge to get cells in different phases of the cell cycle to fuse together
- Where are the cell cycle control enzymes located and what are they doing?
- M (mitotic) phase cell fused to an interphase cell
- Mitotic cell influences the interphase cells
- Interphase cell chromosomes are starting to thicken
Cancer
- Normal cells vs cancer cells
- Cancer cells: large in size, variably shaped nuclei, many dividing cells, very disorganized in arrangement
- More variation in size and shape in comparison to normal cells
- Lost normal features, excessive cell division
- Divide faster than normal cells, uncontrolled cell division
- Cell cycle is not regulated, and checkpoints are disrupted
- Normal cells: exhibit limited growth, anchorage dependence, density-dependent inhibition
- Stages of cancer: Stage 1-4
- Chemotherapy drugs: Vincristine, Taxol, anti-microtubule agents
- Steve Jobs had his whole genome sequenced
- Pim 1 Biomarker for pancreatic cancer
- Afinitor is used in the treatment of this form of pancreatic cancer
- Cancer immunotherapy: influences an individual's immune system to seek out and destroy cancer cells
- CAR-T cell therapy: harnesses the potential of the individual's own immune system to target & destroy cancer cells
Prevention
- Alcohol: Acetaldehyde is toxic and can lead to irreversible DNA damage
- Liver converts ethanol in alcoholic beverages into acetaldehyde
- Small amounts of ethanol are also broken down in the mouth and stomach
- If too much alcohol is consumed, the body has difficulty processing the acetaldehyde fast enough
- Builds up in the body, leading to cancer
- Breast cancer: cause circulating oestrogen levels to increase
- Lead to breast cancer in women through: abnormal development of breast tissue, increased cell production and rearrangement, increased DNA damage
- Pharynx & larynx: alter the cells in the mouth and throat
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Description
Test your knowledge on the asexual reproduction process in bacteria through binary fission. Learn about how division is initiated, DNA replication, cell elongation, and synthesis of a new cell membrane and wall.
