Unit 12: The Cell Cycle, Mitosis, and Meiosis
The checkpoints that decide when a cell is allowed to divide, the choreography that splits its chromosomes, and the extra round that halves them again for sex.
8 topics · 30 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 do G1, S, G2, and M phase each accomplish a different job in preparing a cell to divide?
- Common mistakeWhy isn't S phase the phase where a cell actually splits into two?
- Going furtherWhat happens to a cell that permanently exits the cycle into G0?
Your turn1 of 2
During S phase, a cell's chromosomes each become two sister chromatids. A student assumes S phase must be the point where a cell actually splits into two daughter cells, since something so dramatic is happening to the chromosomes. Why isn't S phase the phase where a cell actually splits into two?Cyclins, CDKs, and Checkpoint Control
- How it worksHow does a rising cyclin level activate its partner cyclin-dependent kinase to trigger the next phase?
- How it worksWalk me through how the G1 checkpoint decides whether a cell is ready to commit to dividing.
- Common mistakeWhy isn't a cyclin-dependent kinase active on its own without a cyclin bound to it?
- Worked exampleFind the fraction of cyclin B remaining after two consecutive 15-minute half-lives once the anaphase-promoting complex begins degrading it.
Your turn1 of 2
Find the fraction of cyclin B remaining after 4 consecutive 16-minute half-lives once the anaphase-promoting complex begins degrading it.Mitosis: Prophase Through Telophase
- How it worksWalk me through how chromosomes condense during prophase and align at the cell's equator by metaphase.
- How it worksHow do sister chromatids separate and get pulled to opposite poles during anaphase?
- Worked exampleFind how long a cell spends in mitosis if the full cell cycle takes 24 hours and mitosis makes up about 5 percent of it.
- Common mistakeWhy isn't prophase the point where sister chromatids actually separate from each other?
- Going furtherWhat happens to a cell's chromosomes when it's treated with a spindle-poisoning drug like colchicine during metaphase?
Your turn1 of 2
Find how long a cell spends in mitosis, in minutes, if its full cell cycle takes 26 hours and mitosis makes up about 8 percent of it.The Mitotic Spindle and Chromosome Segregation
- How it worksHow does a kinetochore on each chromosome capture microtubules growing out from opposite spindle poles?
- How it worksWalk me through how the spindle shortens its microtubules to physically pull sister chromatids apart.
- Common mistakeWhy isn't a chromosome pulled toward just one pole because that pole happens to win a tug of war?
Your turn1 of 2
A chromosome ends up correctly segregated toward one specific spindle pole during anaphase. A student pictures this as the result of a tug of war between the two poles, where one side simply pulled harder and won. Why isn't a chromosome pulled toward just one pole because that pole happens to win a tug of war?The Spindle Assembly Checkpoint
- How it worksHow does an unattached kinetochore send a stop signal that blocks the whole cell from entering anaphase?
- How it worksWalk me through how that checkpoint signal shuts off once every kinetochore is properly attached and under tension.
- Common mistakeWhy isn't the spindle checkpoint just a timer that waits a fixed number of minutes before anaphase?
- Ties togetherHow does the spindle assembly checkpoint's failure connect to the chromosome number errors seen in cancer cells?
Your turn1 of 2
A cell consistently enters anaphase only after every one of its chromosomes has become properly attached to the spindle, regardless of whether that takes a few minutes or considerably longer in a given division. A student assumes the spindle checkpoint must just be a timer counting down a fixed number of minutes before anaphase starts automatically. Why isn't the spindle checkpoint just a timer that waits a fixed number of minutes before anaphase?Cytokinesis in Animal and Plant Cells
- How it worksHow does a contractile ring of actin and myosin pinch an animal cell into two daughter cells?
- How it worksHow does a plant cell build a cell plate from vesicles delivered to the middle of the dividing cell?
- Common mistakeWhy isn't cytokinesis simply the last step of mitosis rather than a separate process running alongside it?
Your turn1 of 2
Cytokinesis typically begins during anaphase of mitosis and finishes shortly after telophase ends, overlapping substantially with the final stages of nuclear division. A student assumes cytokinesis must simply be the last step of mitosis itself. Why isn't cytokinesis simply the last step of mitosis rather than a separate process running alongside it?Meiosis I: Homolog Pairing and Crossing Over
- How it worksHow do homologous chromosomes find each other and pair up during prophase I?
- How it worksWalk me through how crossing over physically swaps segments between homologous chromosomes.
- Common mistakeWhy isn't crossing over the same event as the independent assortment of whole chromosomes?
- Worked exampleFind how many genetically distinct gametes independent assortment alone can produce from 23 pairs of human chromosomes.
Your turn1 of 2
Find how many genetically distinct gametes independent assortment alone can produce from 25 pairs of chromosomes.Meiosis II and Nondisjunction
- How it worksHow does meiosis II separate sister chromatids without another round of DNA replication first?
- How it worksWalk me through how nondisjunction in meiosis I produces gametes with an extra or missing chromosome.
- Common mistakeWhy isn't nondisjunction in meiosis I expected to produce the same outcome as nondisjunction in meiosis II?
- Going furtherWhat happens to an embryo's development when it inherits three copies of chromosome 21 instead of two?
Your turn1 of 2
Nondisjunction (a failure of chromosomes to separate properly) can occur during either meiosis I or meiosis II. A student assumes both cases must produce the exact same outcome in the resulting gametes, since both involve the same basic error. Why isn't nondisjunction in meiosis I expected to produce the same outcome as nondisjunction in meiosis II?