Unit 6: The Cell Cycle, Mitosis, and Meiosis
How one cell becomes two identical cells through mitosis, and how sex cells get half the usual chromosome count through meiosis.
8 topics · 26 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 Cell Cycle and Interphase
- How it worksHow do the G1, S, and G2 phases of interphase each prepare a cell to divide?
- Common mistakeWhy isn't interphase a resting phase where the cell does nothing important?
- Going furtherWhat happens to a cell that skips checkpoint control during interphase?
Your turn1 of 2
Across a cell's entire life cycle, it spends the vast majority of its time in interphase and only a small fraction actively dividing. Why does a cell spend most of its life not dividing at all?Mitosis: Making Identical Copies
- How it worksWalk me through how chromosomes line up and split apart during prophase, metaphase, anaphase, and telophase.
- Common mistakeWhy doesn't mitosis change the number of chromosomes in the resulting cells?
- Going furtherWhat happens to a cell's chromosomes if the spindle fibers fail to attach properly during mitosis?
Your turn1 of 2
A cell entering mitosis has 67 chromosomes at the start of G1, and it replicates its DNA during S phase before dividing. How many chromosomes does each of the two daughter cells have once mitosis is complete?Cytokinesis in Plant and Animal Cells
- How it worksHow does a contractile ring pinch an animal cell in two during cytokinesis?
- How it worksHow does a plant cell build a cell plate down the middle instead of pinching like an animal cell?
- Common mistakeWhy doesn't cytokinesis happen before mitosis finishes dividing the chromosomes?
Your turn1 of 2
Mitosis finishes dividing a cell's chromosomes into two identical sets before cytokinesis begins. Why does cytokinesis need to happen only after mitosis, rather than before mitosis finishes dividing the chromosomes?Why Sex Cells Need Meiosis
- How it worksHow does meiosis cut the chromosome number in half before egg and sperm form?
- Common mistakeWhy isn't meiosis just mitosis that happens to make four cells instead of two?
- Ties togetherHow does meiosis producing haploid cells connect to fertilization restoring the full chromosome number?
Your turn1 of 2
A species produces gametes with a haploid chromosome number of 30. What is the diploid chromosome number found in its body cells before meiosis begins?Meiosis I and Meiosis II
- How it worksWalk me through how homologous chromosomes separate during meiosis I.
- How it worksWalk me through how sister chromatids finally separate during meiosis II.
- Worked exampleFind the number of chromosomes in each resulting cell after meiosis I and after meiosis II, starting from 46.
- Common mistakeWhy doesn't meiosis II separate homologous chromosomes the way meiosis I does?
Your turn1 of 2
A cell's chromosome number right after meiosis I finishes is 34. Since meiosis II only separates sister chromatids without changing the chromosome count any further, how many chromosomes does a resulting cell have once meiosis II is also complete?Comparing Mitosis and Meiosis Side by Side
- How it worksWalk me through mitosis and meiosis running side by side from the same starting cell to see where the chromosome counts diverge.
- Going furtherWhat happens to the final chromosome number if a cell mistakenly runs meiosis's steps where mitosis should happen?
- Common mistakeWhy doesn't mitosis produce four genetically different cells the way meiosis does?
- Ties togetherHow does the difference between mitosis's two identical cells and meiosis's four unique cells explain why siblings aren't clones?
Your turn1 of 2
The body uses mitosis for ordinary growth and tissue repair, and a completely separate process, meiosis, only for producing sex cells. Why does the body use two entirely different kinds of cell division for these two jobs?Crossing Over and Genetic Variation
- How it worksHow do homologous chromosomes swap matching segments during crossing over in meiosis I?
- Going furtherWhat happens to the combination of alleles on a chromosome after crossing over compared to before?
- Common mistakeWhy doesn't crossing over change how many genes are on a chromosome, only which alleles travel together?
Your turn1 of 2
You inherit half your DNA from each parent, yet you aren't a straightforward blend or a simple copy of either one. Why are you genetically different from both of your parents even though half your DNA comes from each?Nondisjunction and Chromosomal Disorders
- How it worksHow does a pair of chromosomes failing to separate during meiosis leave one gamete with an extra copy?
- Worked exampleFind the number of chromosomes in a gamete when chromosome 21 fails to separate during meiosis I in a human.
- Going furtherWhat happens to a fertilized egg's chromosome count when a nondisjunction gamete meets a normal one?
Your turn1 of 2
In a species with a haploid chromosome number of 34, nondisjunction during meiosis I causes one homologous pair to fail to separate, leaving one resulting gamete with an extra copy of that chromosome. How many chromosomes does that nondisjunction gamete have?