Unit 12: The Cell Cycle, Cancer, and Experimental Methods
How a cell decides to divide, what happens when that decision breaks, and the lab tools that let us watch any of it directly.
12 topics · 39 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
The Phases of the Cell Cycle
- How it worksHow does a cell move through G1, S, G2, and M phases in order, each with a distinct job to do?
- Worked exampleFind how much of a 24-hour cell cycle is spent in mitosis if M phase lasts just 1 hour.
- Going furtherWhat happens to a cell that gets stuck in G1 and never proceeds into S phase?
Your turn1 of 2
Find what percent of a 26 hour cell cycle is spent in mitosis if M phase lasts just 1.47 hour.Checkpoints and the Decision to Divide
- How it worksHow does the G1 checkpoint check for DNA damage and adequate cell size before allowing entry into S phase?
- Common mistakeWhy isn't the cell cycle simply running on a fixed timer that ignores what's actually happening inside the cell?
- Going furtherWhat happens to a cell's division when it detects unrepaired DNA damage at the G2 checkpoint?
Your turn1 of 2
A cell pauses at several checkpoints before committing to divide, rather than simply dividing on a fixed schedule. A student assumes the cell cycle must still be running on a fixed internal timer, with the checkpoints just adding small delays here and there. Why isn't the cell cycle simply running on a fixed timer that ignores what's happening inside the cell?Cyclins and Cyclin-Dependent Kinases
- How it worksHow does a cyclin protein binding a cyclin-dependent kinase switch that kinase on?
- How it worksWalk me through cyclin levels rising and falling in a wave that drives the cell through each phase of the cycle.
- Worked exampleFind how many distinct cyclin-CDK complexes are needed to drive a cell cycle with 4 phase transitions, one dedicated complex per transition.
- Going furtherWhat happens to the cell cycle when a cyclin gets degraded too early, before its job is finished?
Your turn1 of 2
Find how many distinct cyclin-CDK complexes are needed to drive a cell cycle with 6 phase transitions, one dedicated complex per transition.p53 and the Guardian-of-the-Genome Checkpoint
- How it worksHow does p53 detect DNA damage and halt the cell cycle to allow time for repair?
- Going furtherWhat happens to a cell when its p53 gene is mutated and can no longer respond to DNA damage?
- Common mistakeWhy isn't p53's job to repair the DNA damage itself?
Your turn1 of 2
p53 is often called the guardian of the genome because it halts the cell cycle when DNA damage is detected, giving repair machinery time to work. A student assumes p53 must directly sense DNA damage the same way a smoke detector directly senses smoke particles. How does p53 actually detect DNA damage and halt the cycle?Apoptosis: Programmed Cell Death
- How it worksHow does a caspase cascade dismantle a cell's proteins and DNA during apoptosis without spilling its contents?
- Common mistakeWhy isn't apoptosis the same process as a cell bursting open from damage?
- Going furtherWhat happens to a tissue's cell number when the genes controlling apoptosis get damaged?
Your turn1 of 2
A cell undergoing apoptosis dismantles its own proteins and DNA in an orderly sequence, without spilling its contents into the surrounding tissue the way a damaged cell might. A student assumes this must just be a slower, gentler version of a cell simply falling apart from damage. How does a caspase cascade actually accomplish this controlled dismantling?Cancer as Accumulated Mutations in Growth-Control Genes
- How it worksHow does a mutation that activates an oncogene push a cell toward uncontrolled division?
- How it worksWalk me through how losing both copies of a tumor suppressor gene removes a cell's brakes on division.
- Common mistakeWhy isn't a single mutation in a single gene usually enough to turn a normal cell into a cancer cell?
- Going furtherWhat happens to a cell's behavior as it accumulates mutations in growth-control genes, DNA-repair genes, and apoptosis genes together?
Your turn1 of 2
A single mutation occurs in a single growth-control gene inside a cell, and a student assumes this alone should already be enough to turn that cell fully cancerous, since the mutation directly affects growth control. Why isn't a single mutation in a single gene usually enough to turn a normal cell into a cancer cell?Angiogenesis and Metastasis
- How it worksHow does a tumor secrete signals that make nearby blood vessels sprout new branches toward it?
- Going furtherWhat happens once cancer cells break away from a tumor and travel through the bloodstream to a distant organ?
- Ties togetherHow does a metastasizing cancer cell breaking free of its neighbors reuse the same cell-adhesion chemistry that normally holds a tissue together?
Your turn1 of 2
A growing tumor recruits its own private blood supply by causing nearby blood vessels to sprout new branches directly toward it. A student assumes this must happen passively, simply because the tumor is physically present nearby. What does a tumor actually do to make this happen?Spectroscopy: How Absorbance Measures Concentration
- How it worksHow does a molecule absorbing specific wavelengths of light let a spectrophotometer measure its concentration?
- Worked exampleFind the concentration of a solution with an absorbance of 0.6, a molar extinction coefficient of 6000 per molar per centimeter, and a 1 centimeter path length.
- Going furtherWhat happens to a solution's absorbance reading when its concentration is diluted by half?
Your turn1 of 2
Find the concentration of a solution with an absorbance of 1.07, a molar extinction coefficient of 8490 per molar per centimeter, and a 1.9 centimeter path length.Chromatography: Separating Molecules by Their Chemistry
- How it worksHow does each molecule's different affinity for a stationary phase versus a mobile phase separate a mixture as it flows through a column?
- Going furtherWhat happens to the order molecules elute from a column when you swap in a stationary phase of the opposite polarity?
- Common mistakeWhy isn't chromatography separating molecules mainly by their color?
Your turn1 of 2
A mixture of similar-looking molecules gets pulled apart into distinct, separated bands just by flowing it through a column. A student assumes this separation must depend on the molecules physically differing in size alone, the way a sieve would separate objects. What actually drives separation in chromatography?Gel Electrophoresis: Separating Molecules by Size and Charge
- How it worksHow does an electric field pull negatively charged DNA fragments through a gel's mesh of pores at different speeds?
- Worked exampleFind which of two DNA fragments, 500 base pairs and 2000 base pairs, migrates farther through a gel during the same run.
- Common mistakeWhy isn't a protein's charge alone what determines how far it travels on an SDS-PAGE gel?
Your turn1 of 2
In a gel run for a fixed time, migration distance is roughly proportional to 1 over the logarithm of fragment size. A fragment of 2150 base pairs travels 8 centimeters. Find how far a fragment of 2017 base pairs would travel, assuming the same proportionality.Enzyme Kinetics Assays in the Lab
- How it worksHow does a kinetics assay track the appearance of product or the disappearance of substrate across a series of time points?
- Worked exampleFind the initial reaction velocity from a kinetics assay that produces 40 micromoles of product in the first 2 minutes.
- Going furtherWhat happens to a kinetics assay's measured velocity if you wait too long and let the substrate run out?
Your turn1 of 2
Find the initial reaction velocity from a kinetics assay that produces 87 micromoles of product in the first 6 minutes.Structural Biology: X-Ray Crystallography and Cryo-EM
- How it worksHow does X-ray crystallography use the diffraction pattern from a protein crystal to reconstruct its atomic structure?
- How it worksWalk me through how cryo-electron microscopy freezes a protein in place and images it directly without needing a crystal at all.
- Common mistakeWhy isn't a crystallized protein necessarily sitting in a different shape than it takes inside a living cell?
- Ties togetherHow does knowing hemoglobin's exact atomic structure from crystallography connect back to explaining its cooperative binding curve?
Your turn1 of 2
X-ray crystallography reveals a protein's atomic structure not by directly photographing the protein, but by analyzing the pattern of X-rays scattered off a crystal of it. A student assumes such an indirect method must be far less reliable than simply taking a direct picture. How does X-ray crystallography actually reconstruct atomic structure from a diffraction pattern?