Biology & life sciences · 04 / 12
Mitosis and meiosis: maintain or reduce
High school foundations · Worked example and 4 exercises with solutions, at your pace.
The principle, simply
DNA replication creates two chromatids per chromosome. Mitosis separates chromatids and normally preserves chromosome number. Meiosis has two divisions after one replication, reducing ploidy and producing haploid cells through recombination and chromosome segregation in the usual model.
- 01Replication
- 02Chromosomes / chromatids
- 03Division type
- 04Product ploidy
WORKED EXAMPLE
From principle to reasoning
A diploid 2n=4 cell yields mitotic daughters with 4 chromosomes; meiotic products have n=2. Duplication changes DNA quantity, not chromosome count by centromeres.
Solve a problem in this subject
- Frame a question
State the relationship to explain and its scale: molecule, cell, organism, population or geological time.
- Describe evidence
Give a measurable observation, unit, comparison and conditions before explaining it. Observation and inference do different jobs.
- Connect a mechanism
Use a precise concept and label arrows as production, transfer, action or transformation. Sequence alone does not prove causation.
- Answer with a limit
Connect evidence and knowledge to the question. Distinguish the simplified model from living systems and identify a missing condition.
YOUR TURN
01 · Try before checking
2n=6: how many chromosomes in a normal gamete?
One hint
Meiosis reduces to n.
Reveal and understand the solution
n=3: one chromosome from each pair, with different possible combinations.
Compare method, conditions and answer; identify the exact part to revisit.
IMPROVE THROUGH PRACTICE
Three additional challenges with solutions
Start with the first challenge. Write your method before opening a hint; use the solution to find the first step that needs work.
A cell has 4 chromosomes before replication. How many after replication, before separation?
A hint to get started
Count centromeres rather than chromatids alone.
Step-by-step solution
- It still has 4 chromosomes.
- Each consists of two sister chromatids.
- DNA amount doubled without doubling chromosome count at this stage.
Which step did you miss? Retry without help, then explain why the method works.
What distinguishes mitosis from meiosis in the school model?
A hint to get started
Compare division count and ploidy.
Step-by-step solution
- Mitosis generally preserves ploidy and produces two cells.
- Meiosis has two divisions after one replication and reduces ploidy.
- The canonical model produces four genetically varied haploid cells.
Which step did you miss? Retry without help, then explain why the method works.
Why are meiotic products not all identical?
A hint to get started
Identify two sources of recombination.
Step-by-step solution
- Exchange between homologous chromosomes creates new allele associations.
- Homologous pairs also segregate in different combinations.
- These processes vary combinations without necessarily creating new alleles.
Which step did you miss? Retry without help, then explain why the method works.
The trap to avoid
Chromatids are not automatically separate chromosomes before segregation.
Without notes, can you…
- Explain the principle in your own words?
- Rebuild the diagram and its connections?
- Solve the exercise and justify the method?
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