Lesson 6 of 9 / Mutation, cell division and cancer
Follow chromosomes through mitosis
What separates, and where does it go?
I can describe main mitotic stages, including chromosome, envelope, membrane and centriole behaviour.
I can describe the mitotic cycle and principal mitotic stages.Syllabus
Syllabus 8876, Core 2(g). Describe the mitotic cycle and principal mitotic stages.
About 6 min
Make a guess. You are not marked.
A cell with 46 chromosomes divides by mitosis. How many chromosomes are in each daughter nucleus?
Show the answer
46, the same as the parent.
Sister chromatids separate in anaphase, so each nucleus receives a full set.
The key idea
Replicated chromosomes condense, align and separate as sister chromatids, then new nuclei form around corresponding sets.
Explore the idea
Follow the chromosome copies
Chromosomes condense, centrosomes containing centrioles move apart and a spindle develops. The nuclear envelope breaks down; a dashed envelope marks that transition.
Original simplified animal-cell model with two chromosomes for clarity. The full human complement is not shown. Plant cytokinesis uses a cell plate rather than this cleavage furrow.
Explanation
During prophase, chromosomes condense and become visible as duplicated structures. In the animal-cell model, duplicated centrosomes containing centrioles move apart and the spindle develops. The nuclear envelope breaks down, allowing spindle attachment to chromosomes.
At metaphase, chromosomes align at the equator with sister chromatids connected to opposite spindle poles. The arrangement positions the copies for equal distribution. It is not pairing of maternal and paternal homologues for a reduction division.
During anaphase, sister chromatids separate at their centromeres and move to opposite poles. Once separated, each is an individual chromosome. During telophase, chromosomes reach the poles and decondense, and nuclear envelopes form around the two sets.
Cytokinesis accompanies or follows nuclear division. An animal cell's surface membrane forms a cleavage furrow as the cytoplasm divides. Plant cells form a cell plate that develops into new separating membranes and wall material; do not draw an animal-style pinching membrane through a rigid plant wall.
A common graph plots two distances against time during mitosis: the distance between the centromeres of a pair of sister chromatids, and the distance from each centromere to the pole of the spindle. Both stay level through metaphase, when the chromosomes are held at the equator. At the start of anaphase the centromeres split. The distance between sister centromeres then rises steeply, while the distance from each centromere to its pole falls as the spindle fibres shorten and pull the chromatids apart.
The mitotic index measures how many cells in a tissue are dividing. Mitotic index = number of cells in mitosis / total number of cells counted x 100. In a stained root-tip squash where 24 of 300 counted cells show prophase, metaphase, anaphase or telophase, the mitotic index is 24 / 300 x 100 = 8.0%. A higher index means a larger proportion of cells is dividing, as in a growing root tip or a tumour.
Step by step
- 1
Condense and attach
Chromosomes become organised for spindle-mediated movement as the nuclear envelope breaks down.
- 2
Align and separate copies
Equatorial alignment is followed by sister-chromatid separation to opposite poles.
- 3
Re-form and divide
New envelopes enclose sets and cytokinesis separates the cells.
Worked example
Identify a stage by evidence
An animal cell has no intact nuclear envelope, and sister chromatids are moving towards opposite poles. Identify the stage and explain the evidence.
One way to explain it
Anaphase. The decisive evidence is sister-chromatid separation and movement to opposite poles, rather than merely the absence of the nuclear envelope.
Why this answer works
- Use the chromosome behaviour as the strongest clue.
- A missing envelope occurs during more than one stage.
Watch out for this
A student says: "Mitosis halves the number of chromosome sets." What is wrong with this?
Show the answer
Mitosis normally preserves the chromosome complement in daughter nuclei. Reduction of sets is a role of meiosis.
Check your understanding
What moves apart during mitotic anaphase?
Show the answer
Sister chromatids, which become individual daughter chromosomes.
Corresponding copied information is distributed to opposite poles.
Quick recall
What separates, and where does it go?