Division cellulaire et mitose
Baccalauréat international · IB Diploma · Biology · HL Sujet 32 3:51 Narration en anglais · Sous-titres anglais + 中文 incrustés
Chapitres
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A root-tip image contains cells at different stages of mitosis.
根尖图像包含处于有丝分裂不同阶段的细胞。
A snapshot can estimate the fraction of cells dividing, but it does not time one observed cell directly.
快照可以估算正在分裂的细胞比例，但无法直接测量单个被观察细胞的分裂时间。
DNA replication occurs before mitosis.
DNA复制发生在有丝分裂之前。
Mitosis separates sister chromatids, preserving chromosome number in daughter nuclei.
有丝分裂分离姐妹染色单体，保持子细胞核中的染色体数目不变。
Meiosis includes homologous chromosome separation and normally halves the chromosome number while generating variation.
减数分裂包括同源染色体分离，通常使染色体数目减半并产生变异。
Mitotic index: Fraction of scored cells undergoing mitosis.
有丝分裂指数：进行有丝分裂的细胞所占的比例。
Chromatid: One copy of a replicated chromosome before separation.
染色单体：复制后的染色体在分离前的一个拷贝。
Count chromosomes by centromeres under the stated convention.
根据既定规则，通过着丝粒计数染色体。
DNA amount and chromosome number are different quantities.
DNA含量与染色体数目是两个不同的量。
Crossing over and independent assortment contribute to meiotic variation.
交叉互换和自由组合共同导致减数分裂中的遗传变异。
In the lab: Use a prepared root-tip slide or a labelled image.
在实验室中，使用预先制备的根尖装片或标注图像。
Define a counting rule and field selection before counting.
在计数前，先确定计数规则和视野选择标准。
Distinguish dividing cells from damaged or ambiguous cells and report exclusions.
区分分裂细胞与受损或模糊不清的细胞，并报告排除项。
Known: twenty-five dividing cells among two hundred scored cells.
已知：在200个被计数的细胞中，有25个处于分裂期。
Mitotic index equals twenty-five divided by two hundred, which is zero point one two five, or twelve point five percent.
有丝分裂指数等于25除以200，即0.125，或12.5%。
Estimating phase duration from this proportion requires assumptions about a steady, representative cell population.
根据该比例估算各时期持续时间，需要假设细胞群体处于稳定且具有代表性的状态。
Your turn: 18 of 120 scored cells are dividing.
轮到你了：120个被计数的细胞中有18个正在分裂。
Calculate mitotic index as a percentage.
计算有丝分裂指数，以百分比表示。
Pause the video and work it out.
请暂停视频，自己算一算。
Check: 18 divided by 120 gives a mitotic index of 15 percent.
核对：18 ÷ 120，有丝分裂指数为 15%。
Replication doubles DNA amount without immediately doubling the chromosome count.
复制使DNA含量加倍，但不会立即使染色体数目加倍。
Meiosis does not simply produce two identical diploid cells.
减数分裂并非简单地产生两个相同的二倍体细胞。
Not true: DNA replication immediately doubles chromosome number under the centromere convention.
不正确。 在着丝粒命名法下，DNA复制并不会立即使染色体数目加倍。
Under the centromere convention a chromosome with two sister chromatids still counts as one chromosome.
根据着丝粒命名法，含有两条姐妹染色单体的染色体仍计为一条染色体。
Replication doubles DNA content; only sister-chromatid separation later doubles the chromosome count.
复制会使DNA含量加倍；只有随后发生的姐妹染色单体分离才会使染色体数目加倍。
DNA replication occurs before mitosis.
DNA复制发生在有丝分裂之前。
Count chromosomes by centromeres under the stated convention.
按照既定规则，通过着丝粒计数染色体。
Watch for: replication doubles DNA amount without immediately doubling the chromosome count.
注意：复制会使DNA含量加倍，但不会立即使染色体数目加倍。
Eight out of forty scored cells are dividing.
40个被计数的细胞中有8个正在分裂。
Find the mitotic index as a percentage.
请计算有丝分裂指数，并以百分比表示。
Remember that chromosome number is not the same as DNA amount.
请注意，染色体数目不等于DNA含量。
The result is a mitotic index of 20 percent.
结果是有丝分裂指数为20%。
Follow the two chromosomes.
跟踪这两条染色体。
They condense, line up on the plate, and their chromatids separate, so each daughter nucleus ends with the same two chromosomes as the parent.
它们凝缩，排列在赤道板上，姐妹染色单体分开，所以每个子细胞核得到与亲代相同的两条染色体。
In fictional snapshots, 30 of 200 cells and 18 of 120 cells are dividing.
在虚构的快照中，200 个细胞中有 30 个、120 个细胞中有 18 个正在分裂。
Does the smaller dividing count imply slower division?
较少的分裂细胞数是否意味着分裂较慢？
Could these observations establish phase duration?
这些观察能否确定各时期的持续时间？
Both indices are 15 percent.
两个有丝分裂指数都是 15%。
The counts have different denominators.
两组计数的分母不同。
Inferring duration also needs a representative, asynchronous, steady population and cycle information; a snapshot alone is insufficient.
要推断时期长短，还需要一个有代表性、非同步、处于稳态的细胞群体以及细胞周期信息；单凭一次快照是不够的。
Pause and check yourself. Explain each point on screen in your own words, then change one number in the last example and say which assumption must still hold.
暂停一下，自我检查：用自己的话解释屏幕上的每个要点，再改动最后一个例子中的一个数字，说明哪个假设仍然必须成立。