Meiosis & Sexual Reproduction — Print · Grade 10 Science
Chapter: Cell Division, Reproduction & DevelopmentMeiosis & Sexual Reproduction
Learning goals
- I can compare meiosis and mitosis and explain how meiosis produces haploid gametes.
- I can explain how crossing over, independent assortment, and fertilization create genetic variation.
Readings for this unit
Before this unit
Distinguish homologous chromosomes from sister chromatids.
Use three flexible study sessions: core explanation and Parts 1–2; Parts 3–4 with their reasoning checks and models; then the whole-unit application, worked examples, independent practice, and exit check. Allow about 50–60 minutes per session, including diagrams, data work, and notebook explanations. Attempt each check before opening its guidance. Schedule chapter investigations, review, and exams separately; add time where prerequisites are uncertain. Trace the explained visual parts and write a first model, then attempt the illustrated and changed independent cases. In the third session explain a correction and revise the opening account using evidence.
Start with something to explain
How can a haploid nucleus still contain duplicated chromosomes?
Illustrative evidence case. Set count falls in meiosis I before sisters separate in meiosis II.
Notice and wonder
Record two observations using the stated quantities or relationships. Keep observations separate from possible causes. Write one question that a new measurement could help answer.
How can a haploid nucleus still contain duplicated chromosomes?
Your first model
Label homologs, sisters and per-nucleus counts rather than count every colored arm.
Build the language from the idea
- meiosis
- Two linked nuclear divisions after one DNA replication, normally producing four haploid nuclear products from a diploid precursor. Subsequent gamete differentiation varies among organisms.
- gamete
- A sex cell — a sperm or an egg — that carries one set of chromosomes.
- diploid
- Having two full sets of chromosomes, one from each parent (46 in humans).
- haploid
- Having one single set of chromosomes (23 in humans).
- homologous chromosomes
- A matching pair of chromosomes, one inherited from each parent, that carry the same genes.
- crossing over
- The exchange of matching segments between homologous chromosomes during meiosis, creating new combinations of alleles.
- fertilization
- The joining of two haploid gametes to form a diploid zygote.
- ploidy
- Number of chromosome sets, distinct from DNA-copy amount.
- nondisjunction
- Failure of the relevant chromosomes or chromatids to separate normally.
Mitosis normally preserves corresponding chromosome sets — but sexual reproduction needs the opposite: gametes carrying varied allele combinations, and that carry only half a genome. Here is the problem it has to solve. Your body cells are diploid, meaning they carry two full sets of chromosomes (46 in humans), one set inherited from each parent. If a sperm with 46 chromosomes fertilized an egg with 46, the child would have 92 — and the number would double every single generation. Something has to cut the count in half before fertilization.
That something is meiosis, a special kind of cell division that produces gametes — sperm and egg cells. A standard meiosis model produces four haploid nuclear products after two divisions. These do not always become four equivalent mature gametes: egg formation and plant life cycles include additional differentiation. Human gametes normally carry one set of 23 chromosomes. When a haploid sperm (23) later fertilizes a haploid egg (23) at fertilization, the full diploid number of 46 is restored — half from each parent — and the cycle can begin again in the next generation.
It helps to line meiosis up against mitosis. Mitosis is one division that normally preserves ploidy in two daughter cells; meiosis is two divisions that yield four haploid cells, all genetically different. The key move happens in the first division, when homologous chromosomes — the matching pairs, one from each parent — are separated from each other. (The second division then separates sister chromatids, much as mitosis does.) Starting from a duplicated diploid cell and dividing twice is exactly what gets you from 46 down to 23.
Meiosis does not just halve the chromosome number — it actively scrambles the genes, and it does so in two clever ways. First, in crossing over, homologous chromosomes line up and swap matching segments, so a single chromosome can end up carrying a brand-new mix of a parent's alleles. Second, in independent assortment, each pair of homologous chromosomes lines up and separates randomly, independently of every other pair — and for humans, that alone allows over 8 million (2^23) different gamete combinations.
Add in the fact that fertilization randomly pairs one of millions of possible eggs with one of millions of possible sperm, and the number of genetically distinct children two parents could produce becomes staggering — which is exactly why siblings can share features without identical inherited combinations; even identical twins can differ through development and later changes. This variation is not a flaw; it is the raw material of evolution. A population full of genetically varied individuals is far more likely to contain someone who can survive a new disease, a hotter climate, or a scarcer food supply — so meiosis, quietly, is one of the engines that keeps life adaptable.
Meiosis consists of two divisions after one DNA replication. Homologous chromosomes separate in the first division, and sister chromatids generally separate in the second. The resulting cells are haploid in a typical diploid sexual-life-cycle model.
Crossing over and independent assortment create combinations of alleles. Fertilization combines gametes and restores the diploid chromosome number in the model. These mechanisms produce variation but do not guarantee that every possible outcome is equally likely at linked genes.
Compare chromosome number, DNA amount and allele combination separately. A gamete is not simply half of each chromosome; it receives one member of each homologous pair, potentially with recombined segments. Use diagrams with labels rather than only counting colored shapes.
Build your understanding
Use three flexible study sessions: core explanation and Parts 1–2; Parts 3–4 with their reasoning checks and models; then the whole-unit application, worked examples, independent practice, and exit check. Allow about 50–60 minutes per session, including diagrams, data work, and notebook explanations. Attempt each check before opening its guidance. Schedule chapter investigations, review, and exams separately; add time where prerequisites are uncertain.
Part 1 · Homologous pairs, and what halving means
Homologous chromosomes carry corresponding gene locations, potentially with different alleles. Diploid means two chromosome sets; haploid means one. In typical meiosis I, homologs separate into different nuclei while sisters remain associated, reducing set number. Meiosis II later separates sisters. The four nuclear products of a standard model are not always four equivalent mature gametes: egg formation and plant life cycles add different developmental steps. Track chromosome sets and DNA copies separately, and avoid interpreting haploid as half of every chromosome.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Is a nucleus after normal meiosis I still diploid because its chromosomes have two sisters?
Visual reasoning — after your attempt
No. It has one homolog of each pair, hence one set, even though each chromosome remains replicated.
A body cell carries chromosomes in homologous pairs — two versions of each chromosome, one inherited from each parent, carrying the same genes in the same order but not necessarily the same alleles. A human body cell has 23 such pairs, 46 chromosomes in total, and is described as diploid. A gamete carries one member of each pair, 23 chromosomes, and is haploid.
Halving is not an arbitrary feature. If gametes carried full sets, fertilization would double the chromosome number every generation. Meiosis halves before fertilization restores, which is what keeps the number constant across generations. Reading "haploid" as "half the DNA" is close enough numerically but misses the structural point: what a gamete has is one complete set, not half of each chromosome.
Pause and explain
Why must gametes be haploid rather than simply smaller diploid cells?
Check your reasoning after answering
Fertilization combines two gametes, so each must carry one complete set for the zygote to have the normal two. Diploid gametes would produce a zygote with four sets, and the number would double every generation.
Part 2 · Two divisions, and where variation enters
Crossing over exchanges corresponding regions between nonsister chromatids of paired homologs. Independent assortment describes the varied orientations and segregation of different homologous pairs. These mechanisms change allele combinations; they do not themselves require producing every allele by a new mutation. Linked genes can be inherited together, with recombination affecting the relationship. A simple 2ⁿ assortment count assumes independent orientations of n chromosome pairs and does not include all variation or guarantee equal outcomes for every linked gene combination.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
How does exchange differ from merely sending whole homologs to opposite cells?
Visual reasoning — after your attempt
Exchange recombines segments before segregation; whole-homolog separation changes which chromosome copy a cell receives.
Meiosis has two divisions after a single replication. In meiosis I, homologous chromosomes pair, exchange segments by crossing over, then separate — this is the division that halves the number. In meiosis II, the two copies of each remaining chromosome separate, as in mitosis. Four haploid cells result, each genetically distinct.
Two mechanisms generate that distinctness. Independent assortment means each homologous pair separates without regard to the others, so with n pairs there are 2ⁿ possible chromosome combinations — 8 for three pairs, and over 8 million for the human 23. Crossing over then produces chromosomes carrying combinations of alleles that neither parent had, so the actual variety greatly exceeds 2ⁿ.
Pause and explain
An organism has 3 pairs of chromosomes. Why is the number of genetically distinct gametes greater than 8?
Check your reasoning after answering
Independent assortment alone gives 2³ = 8 chromosome combinations, but crossing over exchanges segments between paired chromosomes, so individual chromosomes themselves differ from either parental version. The number of distinct gametes is therefore much larger than 8.
Part 3 · Comparing meiosis with mitosis
In the supplied 2n=4 model, DNA replication gives eight chromatids within four replicated chromosomes. After meiosis I each nucleus has two replicated chromosomes and four chromatids; after meiosis II each normally has two unreplicated chromosomes. Typical mitosis instead preserves the starting ploidy in daughter nuclei. This distinguishes division number, chromosome number and DNA content. The standard model predicts variation but does not guarantee that every meiotic product must be different at every locus; sequence, pairing and chance constrain the actual combinations.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Why would “meiosis halves each chromosome” be inaccurate?
Visual reasoning — after your attempt
It reduces chromosome-set number by segregation; chromosomes remain whole structures, with possible exchanged regions.
The two processes are easy to confuse because both separate chromosomes. The decisive difference is what separates. Mitosis separates the two copies of each duplicated chromosome, so the number is maintained and the daughters are genetically identical. Meiosis I separates homologous pairs, which halves the number, and only meiosis II separates copies.
Their purposes follow from that difference. Mitosis supports growth, repair, and replacement, where identical cells are what is wanted. Meiosis produces gametes, where halving and variation are both required. Asexual reproduction uses mitosis and yields offspring that differ from the parent only by mutation; sexual reproduction uses meiosis and yields offspring that differ substantially.
Pause and explain
Both mitosis and meiosis II separate the two copies of a chromosome. Why does only meiosis halve the chromosome number?
Check your reasoning after answering
The halving happens in meiosis I, where homologous pairs separate so each cell receives one member of each pair. Meiosis II then divides copies within those already-halved cells, so the number stays halved rather than being reduced again.
Part 4 · When separation fails
Nondisjunction is a failure of chromosomes to separate normally. A homolog-separation failure in meiosis I and a sister-separation failure in meiosis II produce different sets of potential outcomes. Aneuploidy means an atypical number of particular chromosomes rather than necessarily an entire extra genome. A chromosome-distribution model predicts possible gamete counts but does not establish the viability or phenotype of every resulting individual. Locate the failed step and trace its products rather than say all errors have one identical consequence.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Does nondisjunction require a new chromosome to be manufactured during division?
Visual reasoning — after your attempt
No. An existing set can be distributed unequally when separation fails.
Separation is not infallible. Nondisjunction occurs when a pair fails to separate in meiosis I, or two copies fail to separate in meiosis II, so one gamete receives an extra chromosome and another receives none. A zygote formed from such a gamete has three copies of that chromosome or only one, and the consequences depend on which chromosome is involved.
A karyotype — an ordered display of an individual's chromosomes — shows these differences directly, which makes them a good illustration of how a cellular process produces an observable outcome. The reasoning runs in one direction only: an unusual chromosome count indicates that separation failed, but it does not identify which division failed unless further information is available.
Pause and explain
A karyotype shows three copies of one chromosome. What does this establish, and what does it leave open?
Check your reasoning after answering
It establishes that a nondisjunction event occurred, so one gamete carried an extra copy of that chromosome. It leaves open whether the failure happened in meiosis I or meiosis II, and in which parent's gamete, since the karyotype alone does not distinguish these.
Apply the whole unit · Count the possibilities and diagnose a failure
Illustrative case. A plant species has 5 pairs of homologous chromosomes, so 2n = 10. Pollen analysis finds the following chromosome counts among 1,000 grains: 968 grains with 5 chromosomes, 17 with 6, 15 with 4.
Your task
Calculate how many chromosome combinations independent assortment alone can produce in this species, explain why the observed variety of gametes exceeds that figure, calculate the percentage of grains with an abnormal count, and explain what single process accounts for both the 6-chromosome and 4-chromosome grains. Then state what the data cannot tell you about when the failure occurred.
What your response should show
- Choose the appropriate biological model or relationship and state the conditions it assumes.
- Show the counts, rates, ratios, or intermediate reasoning the question requires.
- Use the supplied evidence to justify the conclusion and separate observation from inference.
- Identify a meaningful limitation, uncertainty, or next measurement.
Response guidance — after your attempt
Independent assortment: with 5 pairs, 2⁵ = 32 chromosome combinations are possible.
Explain the excess variety: crossing over exchanges segments between paired chromosomes, so individual chromosomes differ from either parental version, and the number of genetically distinct gametes far exceeds 32.
Abnormal grains: 17 + 15 = 32 of 1,000, which is 3.2 percent.
Account for both classes: nondisjunction produces reciprocal products — one cell gains a chromosome while its partner loses one — so 6-chromosome and 4-chromosome grains arise from the same events and should occur in roughly equal numbers, which 17 against 15 is consistent with.
Note the limit: counts alone do not reveal whether the failure occurred in meiosis I or meiosis II, nor which chromosome was involved.
With 5 homologous pairs, independent assortment alone allows 2⁵ = 32 chromosome combinations, but the genetic variety among pollen grains is far greater because crossing over produces chromosomes that differ from both parental versions before assortment distributes them. Abnormal counts appear in 32 of 1,000 grains, or 3.2 percent, and the near-equality of the 6-chromosome and 4-chromosome classes is the expected signature of nondisjunction: each failure produces one cell with an extra chromosome and one lacking it, so the two classes should be roughly equal. The counts cannot show whether the failures occurred in meiosis I, where homologous pairs separate, or meiosis II, where copies do, nor which of the five chromosomes was involved; identifying the chromosome would require a karyotype or a molecular marker, and distinguishing the division would require examining the two chromosomes' parental origin.
Scientific references and adapted source figures
Parent / teacher teaching notes
The sentence to get right is "meiosis I separates homologous pairs; mitosis and meiosis II separate copies" — almost every confusion in this unit dissolves once that is secure. Use the 2ⁿ calculation in small cases before mentioning the human figure, because the point is the mechanism rather than the size of the number. The reciprocal nature of nondisjunction is a good test of understanding: students who see why the extra-chromosome and missing-chromosome classes should be about equal have grasped what separation actually does. Ask for an explanation of each label, arrow and axis before calculation. Keep first and revised models; attempt the changed case before consulting guidance. Distinguish invented instructional records from named historical sources. Source chromosome symbols are allele-category models, not DNA sequences. These independently authored high-school supplements are not the full OpenSciEd teacher editions or an endorsement.
The key idea
Meiosis separates homologs and then sisters after one replication, normally reducing chromosome-set count in four nuclear products. Gamete maturation and life cycles vary. Recombination, assortment and fertilization contribute to varied inherited combinations.
Gamete Chromosome Number
haploid (gamete) number = diploid number / 2
meiosis halves the chromosome number to make gametes.
Gamete Variety from Independent Assortment
different gametes = 2^n
n = number of homologous chromosome pairs (the haploid number); this ignores the extra variety from crossing over.
Worked examples
Example 1
- Meiosis halves the chromosome number, so each gamete receives one set: 46 / 2 = 23.
- Meiosis involves two rounds of division, turning one cell into four.
- So one cell produces four gametes, each with 23 chromosomes.
Example 2 · Worked application
- Meiosis halves the chromosome complement to 5 per gamete.
- Fusion of two gametes restores 5+5=10.
- Recombination and independent assortment also produce variation.
Halving refers to homologous sets, not arbitrary removal of any chromosomes.
Example 3
- Each homologous pair can line up two ways, contributing either the mother's or the father's chromosome to a gamete — independently of the other pairs.
- With 3 pairs acting independently, multiply the possibilities: 2 x 2 x 2.
- 2 x 2 x 2 = 2^3 = 8.
Example 4
- The gamete has one set, half of 16: 8.
- Two normal gametes combine: 8+8=16.
Example 5 · Find and fix the mistake
- Test the claim against the stated conditions: Halving refers to homologous sets, not arbitrary removal of any chromosomes.
- Normal meiosis involves two divisions and produces haploid products, with variation from recombination and assortment.
Halving refers to homologous sets, not arbitrary removal of any chromosomes.
Example 6 · Reason from the visual evidence
- In the supplied 2n=4 model, DNA replication gives eight chromatids within four replicated chromosomes. After meiosis I each nucleus has two replicated chromosomes and four chromatids; after meiosis II each normally has two unreplicated chromosomes. Typical mitosis instead preserves the starting ploidy in daughter nuclei. This distinguishes division number, chromosome number and DNA content. The standard model predicts variation but does not guarantee that every meiotic product must be different at every locus; sequence, pairing and chance constrain the actual combinations.
- It reduces chromosome-set number by segregation; chromosomes remain whole structures, with possible exchanged regions.
Original chromosome-count account for a two-pair model, not human numbers or all possible errors.
Avoid these mistakes
- Meiosis involves one replication followed by two divisions.
Check your understanding
A species has diploid number 16. What is the normal gamete number and zygote number after fertilization?
Discuss the answer
Gametes have 8 chromosomes; the zygote has 16.
How does crossing over differ from the separation of homologous chromosomes?
Discuss the answer
Crossing over exchanges corresponding regions between nonsister chromatids. Homolog segregation distributes chromosome copies, potentially after their regions have recombined.
Return to the phenomenon
Return to the initial case. How can a haploid nucleus still contain duplicated chromosomes? Revise your drawing or explanation using two named ideas and one stated limitation.
Compare explanations after your attempt
Haploidy counts sets, not DNA-copy amount; recombination and segregation change combinations through distinct mechanisms.
What should we investigate next?
Which comparison would test a remaining uncertainty? State what you would change, keep comparable and measure.
Investigate, explain and revise
Complete these tasks after the teaching parts. Preserve your first model and a supported revision; use guidance after a real attempt.
Activity 1 · Return to the opening case
Use the opening question, defined terms, Parts 1–4 and your notebook. The supplied-data route needs no equipment or external account.
Your task
Return to the initial case. How can a haploid nucleus still contain duplicated chromosomes? Revise your drawing or explanation using two named ideas and one stated limitation.
Success criteria
- Use relevant stated observations.
- Explain the mechanism with a labeled model.
- Distinguish quantities and assumptions.
- Describe a revision and a useful next test.
Hint
Follow objects, materials and information separately. Check what each measured quantity means.
Response guidance — discuss after your attempt
Haploidy counts sets, not DNA-copy amount; recombination and segregation change combinations through distinct mechanisms.
Activity 2 · Transfer to a changed case
Cover the worked guidance and use the unit explanations/figures to solve a changed problem.
Your task
A diploid model has 2n=6. Give chromosome and chromatid counts after S phase, after meiosis I per nucleus and after meiosis II per nucleus.
Success criteria
- Use the stated measurements or conditions, with correct units.
- Connect the result to the biological mechanism or evidence relationship.
- Identify the assumption, model limit or further evidence requested.
Hint
Name the stated conditions, select the relevant mechanism and show intermediate reasoning.
Response guidance — discuss after your attempt
After S: 6 chromosomes and 12 chromatids. After I: 3 replicated chromosomes and 6 chromatids per nucleus. After II: 3 unreplicated chromosomes per nucleus. The standard model includes one replication before two divisions.
Words to know
- meiosis
- Two linked nuclear divisions after one DNA replication, normally producing four haploid nuclear products from a diploid precursor. Subsequent gamete differentiation varies among organisms.
- gamete
- A sex cell — a sperm or an egg — that carries one set of chromosomes.
- diploid
- Having two full sets of chromosomes, one from each parent (46 in humans).
- haploid
- Having one single set of chromosomes (23 in humans).
- homologous chromosomes
- A matching pair of chromosomes, one inherited from each parent, that carry the same genes.
- crossing over
- The exchange of matching segments between homologous chromosomes during meiosis, creating new combinations of alleles.
- fertilization
- The joining of two haploid gametes to form a diploid zygote.
- ploidy
- Number of chromosome sets, distinct from DNA-copy amount.
- nondisjunction
- Failure of the relevant chromosomes or chromatids to separate normally.
When meiosis miscounts
Meiosis usually separates chromosomes flawlessly, but sometimes a pair fails to split — an error called nondisjunction — and a gamete ends up with an extra or a missing chromosome. If a gamete with an extra copy of chromosome 21 is fertilized, the child has three copies (trisomy 21), the cause of Down syndrome. This shows how tightly a body's development depends on getting exactly the right chromosome number.
Crossing over made gene mapping possible
Genes that sit close together on the same chromosome tend to be inherited together, because crossing over rarely separates them. Alfred Sturtevant realized in 1911 that the frequency of crossing over between two genes reveals how far apart they are — the farther apart, the more often they're separated. Counting recombinant offspring let scientists build the first genetic maps long before anyone could read DNA directly.
Why sex is worth the trouble
Sexual reproduction is oddly 'expensive' — it requires two parents and passes on only half of each one's genes. So why is it everywhere? The leading answer is variation: shuffling genes every generation helps a population stay ahead of ever-evolving parasites and diseases (an idea called the Red Queen hypothesis) and cope with changing environments. Meiosis is the engine that generates that continual reshuffling.
The staggering math of human variation
Independent assortment alone lets a single human produce about 8.4 million (2^23) genetically different gametes. Multiply the possibilities from one parent by those from the other, and a single couple could in principle produce over 70 trillion genetically distinct children — and that's before crossing over adds still more combinations. It's a numerical way of seeing why siblings differ so much yet still resemble one another.
Meiosis vs. mitosis at a glance
The two divisions are easy to confuse, so it helps to contrast them directly. Mitosis: one division, two daughter cells, diploid, genetically identical to the parent, used for growth and repair. Meiosis: two divisions, four daughter cells, haploid, all genetically different, used to make gametes. The single biggest difference is meiosis's first division, where homologous pairs — not just sister chromatids — are separated.
In the real world
The genetic variation meiosis creates is why one family can include siblings with very different heights, coloring, and health risks — and, over long timescales, why populations carry the diversity that natural selection acts on.
Exit check
Explain how meiosis contributes to variation without intentionally creating needed traits.
Check your explanation after answering
Crossing over and independent assortment reshuffle genetic variants. Mutations can add variation independently of need; gamete combinations add further variation at fertilization.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.
Problem 1
- two identical body cells for growth
- new proteins from mRNA
- an exact copy of a single chromosome
- four haploid gametes (sex cells) with half the chromosome number
Answer and explanation
four haploid gametes (sex cells) with half the chromosome number
Meiosis makes four haploid gametes, each with half the parent cell's chromosome number.
Problem 2
Answer and explanation
23 chromosomes
Gametes are haploid, carrying half of 46, which is 23.
Problem 3
- exchanges matching segments between homologous chromosomes, creating new allele combinations
- copies a chromosome exactly
- destroys half of a cell's genes
- turns a body cell into a gamete without any division
Answer and explanation
exchanges matching segments between homologous chromosomes, creating new allele combinations
In crossing over, homologous chromosomes swap segments, producing chromosomes with new mixes of alleles.
Problem 4
Answer and explanation
Meiosis halves the chromosome number when it makes gametes, so each gamete carries only one set. When two gametes join at fertilization, the two halves add back up to the full number, keeping it constant instead of doubling each generation.
Problem 5
Answer and explanation
32 gametes
Each of the 5 pairs assorts independently in 2 ways: 2^5 = 32.
Problem 6
Answer and explanation
Gametes have 8 chromosomes; the zygote has 16.
The gamete has one set, half of 16: 8.
Two normal gametes combine: 8+8=16.
Response criteria
- Use the stated conditions.
- Show the evidence or calculation.
- Explain the conclusion and any stated limitation.
Problem 7
Hints
Halving refers to homologous sets, not arbitrary removal of any chromosomes.
Answer and explanation
n=7; fertilization restores 14.
Problem 8
Hints
State what the supplied observations or conditions establish, and separate that from any assumption.
Answer and explanation
It exchanges segments between homologous chromosomes, rearranging existing variants.
Problem 9
Answer and explanation
It reduces chromosome-set number by segregation; chromosomes remain whole structures, with possible exchanged regions.
In the supplied 2n=4 model, DNA replication gives eight chromatids within four replicated chromosomes. After meiosis I each nucleus has two replicated chromosomes and four chromatids; after meiosis II each normally has two unreplicated chromosomes. Typical mitosis instead preserves the starting ploidy in daughter nuclei. This distinguishes division number, chromosome number and DNA content. The standard model predicts variation but does not guarantee that every meiotic product must be different at every locus; sequence, pairing and chance constrain the actual combinations.
It reduces chromosome-set number by segregation; chromosomes remain whole structures, with possible exchanged regions.
Response criteria
- Read the structures, axes, directions and units correctly.
- Connect the evidence with the biological mechanism.
- Respect stated model assumptions and distinguish an observation from a further inference.
Problem 10
Answer and explanation
After S: 6 chromosomes and 12 chromatids. After I: 3 replicated chromosomes and 6 chromatids per nucleus. After II: 3 unreplicated chromosomes per nucleus. The standard model includes one replication before two divisions.
After S: 6 chromosomes and 12 chromatids. After I: 3 replicated chromosomes and 6 chromatids per nucleus. After II: 3 unreplicated chromosomes per nucleus. The standard model includes one replication before two divisions.
Response criteria
- Use the stated measurements or conditions, with correct units.
- Connect the result to the biological mechanism or evidence relationship.
- Identify the assumption, model limit or further evidence requested.