Cell division: count nuclei and chromosomes carefully
הוצג| English | משמעות |
|---|---|
| chromatid/ˈkrəʊmətɪd/ | כרומטידה |
| mitotic index/maɪˈtɒtɪk ˈɪndeks/ | מדד מיוזה |
What would explain this observation?
- A root-tip image contains cells at different stages of mitosis. A snapshot can estimate the fraction of cells dividing, but it does not time one observed cell directly.
Build the model
- DNA replication occurs before mitosis.
- Mitosis separates sister chromatids 染色单体, preserving chromosome number in daughter nuclei.
- Meiosis includes homologous chromosome separation and normally halves the chromosome number while generating variation.
- mitotic index 有丝分裂指数: Fraction of scored cells undergoing mitosis.
- chromatid: One copy of a replicated chromosome before separation.
What happens to DNA amount during replication before mitosis?
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
Replication copies every DNA molecule, so the DNA amount doubles before mitosis while each chromosome still counts as one (two sister chromatids joined at a centromere). The DNA never disappears or halves during replication.
Match each technical term to its precise meaning.
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Count chromosomes by centromeres under the stated convention.
- DNA amount and chromosome number are different quantities.
- Crossing over and independent assortment contribute to meiotic variation.
In the lab: Use a prepared root-tip slide or a labelled image. Define a counting rule and field selection before counting. Distinguish dividing cells from damaged or ambiguous cells and report exclusions.
Which two habits make the investigation or model in this case more defensible?
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- Known: 25 dividing cells among 200 scored cells.
- Mitotic index = 25/200 = 0.125, or 12.5%.
- Estimating phase duration from this proportion requires assumptions about a steady, representative cell population.
Your turn: 18 of 120 scored cells are dividing. Calculate mitotic index as a percentage.
18 of 120 scored cells are dividing. Calculate mitotic index as a percentage.
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
The result is 15 %. Known: 18 dividing cells among 120 scored. Use mitotic index = dividing / total x 100. Index = 18/120 x 100 = 15%. Reading phase durations from this assumes a steady, representative population.
Check the conclusion and its limits
- Replication doubles DNA amount without immediately doubling the chromosome count.
- Meiosis does not simply produce two identical diploid cells.
Not true: DNA replication immediately doubles chromosome number under the centromere convention. Under the centromere convention a chromosome with two sister chromatids still counts as one chromosome. Replication doubles DNA content; only sister-chromatid separation later doubles the chromosome count.
- DNA replication occurs before mitosis.
- Count chromosomes by centromeres under the stated convention.
- Watch for: replication doubles DNA amount without immediately doubling the chromosome count.
Write from a mitotic index of 20 percent
- 8 of 40 scored cells are dividing.
- Find the mitotic index as a percentage. Chromosome number is not the same as DNA amount.
DNA replication immediately doubles chromosome number under the centromere convention.
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
Under the centromere convention a chromosome with two sister chromatids still counts as one chromosome. Replication doubles DNA content; only sister-chromatid separation later doubles the chromosome count.
Fraction of scored cells undergoing mitosis: write the technical term.
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
mitotic index means Fraction of scored cells undergoing mitosis.
8 of 40 scored cells are dividing. What is the mitotic index as a percentage, and why is chromosome number different from DNA amount?
8 out of 40 is 0.20, which is a mitotic index of 20 percent.
What is the mitotic index?
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
8 divided by 40 is 20 percent. Other options — 15 percent (the earlier calculation's result for 18 of 120 cells); 12.5 percent (the lesson example's result for 25 of 200 cells).