Write the stated diploid or haploid number and whether the cell has replicated.
02Count
Separate chromosomes, chromatids, DNA molecules, ploidy, and cell number.
03Divide
Name whether homologs or sister chromatids separate in the division.
04Pattern
Use all four products to locate a segregation error.
Lessons use retrieval with corrective feedback and mixed application, consistent with the evidence summarized by the Institute of Education Sciences practice guide ↗. Source links support review; they do not convert these drafts into reviewed content.
Three linked objectives
Make the chromosome state visible.
Before answering, write the starting 2n or n value, whether DNA has replicated, the relationship that separates next, and the number of resulting cells.
01
BIO-CMB-DIV-01 · 14 MIN
draft
Cell cycle and mitotic chromosome accounting
Track chromosome number, chromatid number, DNA amount, and chromosome movement through S phase and mitosis.
ESSENTIAL QUESTIONAre we counting centromeres, DNA copies, chromosome sets, or cells?
ORIGINAL DATTRAIN SCHEMATIC · TEXT EQUIVALENT INCLUDED
01
Count chromosomes by centromeres
Before sister chromatids separate, a replicated chromosome still counts as one chromosome because its two chromatids share one centromeric unit. Replication doubles DNA molecules, not ploidy.
One replicated chromosome = two sister chromatids
S phase doubles DNA, not chromosome sets
02
Mitosis separates sister chromatids
Chromosomes align individually at metaphase. At anaphase, sister centromeres separate and each former chromatid is then counted as an independent chromosome moving to a pole.
Metaphase: sisters still joined
Anaphase: separated chromatids are chromosomes
03
Nuclear division and cytoplasmic division differ
Mitosis divides the nuclear chromosome set; cytokinesis divides the cytoplasm. Animal cells usually form a cleavage furrow, whereas plant cells build a cell plate.
Mitosis is not cytokinesis
Animal furrow; plant cell plate
Worked example
A diploid cell has 2n = 6. How do its chromosome and DNA-molecule counts change from G1 to G2 and then to each daughter after mitosis?
1
In G1, the cell has 6 chromosomes and 6 nuclear DNA molecules: one DNA double helix per unreplicated chromosome.
2
After S phase, G2 still has 6 chromosomes but 12 DNA molecules because each chromosome now has two sister chromatids.
3
Mitosis separates the sisters, and cytokinesis gives each daughter 6 chromosomes and 6 DNA molecules.
ConclusionS phase doubles DNA content without changing ploidy; mitosis distributes one complete diploid set to each daughter.
Close the notes first
Retrieve the chromosome ledger.
01Why does chromosome number not double immediately after S phase?
Each pair of sister chromatids remains joined as one replicated chromosome.
Chromosomes are counted by centromeric units, not by DNA molecules.
02What separates during mitotic anaphase?
Sister chromatids after their centromeric connection is released.
Each separated chromatid is then an independent chromosome.
03How does plant cytokinesis differ from animal cytokinesis?
Plants build a cell plate; animals typically form a cleavage furrow.
A rigid plant cell wall prevents the membrane from pinching inward in the same way.
02
BIO-CMB-DIV-02 · 15 MIN
draft
Meiosis I, meiosis II, and ploidy
Compare mitosis, meiosis I, and meiosis II while correctly tracking homologs, sister chromatids, and chromosome sets.
ESSENTIAL QUESTIONWhich relationship is separating: homologs or sisters?
ORIGINAL DATTRAIN SCHEMATIC · TEXT EQUIVALENT INCLUDED
01
Meiosis uses one replication and two divisions
DNA replicates once before meiosis I. There is no second S phase between meiosis I and meiosis II, so the two divisions partition the replicated material into four products.
One S phase
Two nuclear divisions
02
Meiosis I is the reduction division
Homologous chromosomes pair and then separate in meiosis I. Each resulting nucleus has one chromosome from each homologous pair and is haploid, although each chromosome still has two sister chromatids.
Meiosis I: homologs apart
Ploidy falls before sisters separate
03
Meiosis II separates sisters
Chromosomes align individually and sister chromatids separate in meiosis II, much as they do in mitosis. Meiosis II changes DNA copy number per chromosome but does not reduce ploidy again.
Meiosis II: sisters apart
Haploid stays haploid
Worked example
A diploid meiotic cell has 2n = 4. What does each product contain after meiosis I and after meiosis II?
1
After premeiotic S phase, the starting cell has 4 replicated chromosomes and 8 chromatids.
2
After meiosis I, each of two cells is haploid with 2 replicated chromosomes and 4 chromatids.
3
After meiosis II, each of four cells remains haploid with 2 unreplicated chromosomes and 2 chromatids.
ConclusionHomolog separation in meiosis I reduces ploidy; sister separation in meiosis II reduces DNA copies per chromosome.
Close the notes first
Retrieve the chromosome ledger.
01What separates in meiosis I?
Homologous chromosomes.
This leaves one homolog from each pair in each haploid product.
02What separates in meiosis II?
Sister chromatids.
The centromeric connection is released during anaphase II.
03When and between which chromatids does meiotic crossing over occur?
During prophase I between nonsister chromatids of homologous chromosomes.
Synapsis aligns homologs so corresponding segments can be exchanged.
03
BIO-CMB-DIV-03 · 15 MIN
draft
Variation and nondisjunction signatures
Predict variation from assortment and fertilization and distinguish meiosis-I from meiosis-II nondisjunction products.
ESSENTIAL QUESTIONWhich chromosomes failed to separate, and how many products must carry the error?
ORIGINAL DATTRAIN SCHEMATIC · TEXT EQUIVALENT INCLUDED
01
Independent assortment counts homolog choices
For n homologous pairs, independent assortment alone can produce 2ⁿ chromosome combinations in gametes, before considering crossing over. Each pair contributes two orientation choices at metaphase I.
n pairs → 2ⁿ combinations
Exclude crossing over unless included
02
Crossing over reshuffles linked alleles
Exchange between nonsister chromatids in prophase I creates recombinant chromatids. Random fertilization then combines one gamete from each parent, multiplying the available gamete possibilities.
Crossing over changes a chromosome
Fertilization multiplies gamete combinations
03
The gamete pattern locates nondisjunction
For one homologous pair, meiosis-I nondisjunction sends both homologs together, so all four products are abnormal: two n+1 and two n−1. Meiosis-II nondisjunction in one cell yields two normal products, one n+1, and one n−1.
MI error: zero normal products
MII error: two normal products
Worked example
Four gametes from one meiosis are two normal n gametes, one n+1 gamete, and one n−1 gamete. Where did nondisjunction occur?
1
The two normal gametes show that one meiosis-I product completed meiosis II normally.
2
The n+1 and n−1 pair came from failure of sister chromatids to separate in the other meiosis-I product.
3
A meiosis-I nondisjunction would instead make all four gametes abnormal for that chromosome.
ConclusionThe signature identifies nondisjunction in meiosis II of one of the two cells.
Close the notes first
Retrieve the chromosome ledger.
01How many assortment-only gamete combinations can an organism with n = 5 produce?
2⁵ = 32.
Each of five homologous pairs has two independent metaphase-I orientations.
02What four-product pattern follows meiosis-I nondisjunction of one pair?
Two n+1 and two n−1 products.
Both homologs move together, so neither meiosis-I product has the normal count for that pair.
03What four-product pattern follows meiosis-II nondisjunction in one cell?
Two n, one n+1, and one n−1 products.
The other meiosis-I product divides normally and supplies the two normal gametes.
Randomized retrieval set
Now remove the stage label.
Mitotic accounting, meiotic comparison, variation, and segregation errors are interleaved. Every rationale identifies the exact counting or separation mistake.
12 ORIGINAL DRAFT ITEMS
Retrieve before you review.
Question order and all five answer options are shuffled when you begin. Correctness follows a stable option identity, never a letter position.
Transparent limits
Mechanism practice, not a score prediction.
The ADA lists Mitosis / Meiosis within Cell and Molecular Biology but does not publish a subtopic item quota. DATTRAIN does not invent one.
All counts use the definitions stated in the lesson and prompt. Species-specific gametogenesis timing and syndrome memorization are outside this draft route unless a question supplies the relevant context. Every item remains uncalibrated pending qualified review and pilot evidence.