Cell Division
Identify what is copied, what separates, and what kind of daughter cells result—then apply it to data, heredity, and disease.
Cell Division
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Start with the chromosome—not the stage names
A chromosome is one packaged DNA molecule. Count chromosomes by counting centromeres, the joined region shown in the middle.
During S phase, each chromosome is copied. The two matching copies are called sister chromatids. They remain attached, so the X-shaped structure is still counted as one chromosome, not two.
Homologous chromosomes are a matching pair: one inherited from the mother and one from the father. They carry the same kinds of genes, although they may carry different alleles.
Mitosis is used mainly for growth and repair: one division produces two similar diploid cells. Meiosis produces gametes: two divisions produce four varied haploid cells.
Do not confuse the two pairs
Sister chromatids are copied versions of one chromosome. Homologous chromosomes are the maternal and paternal versions of the same chromosome type.
Learn the actions before the names
Align at the middle, pull apart, and form new nuclei. The phase name matters only after you know which chromosomes are moving.
Ask two questions: what is paired, and what separates?
Situation: A cell image shows two matching X-shaped chromosomes side by side at the middle.
Step 1 — What is paired? Two homologous chromosomes, one maternal and one paternal.
Step 2 — What will separate? The homologs will move to opposite sides while each X remains joined.
Conclusion: This is metaphase I, followed by anaphase I. It cannot be ordinary mitosis because homologous chromosomes do not normally line up as pairs in mitosis.
Why it works
This prevents the most common error: seeing any X-shaped chromosome and guessing a phase name without identifying whether homologs or sister chromatids are involved.
Five forms you should recognize
Before S phase: A cell has 12 chromosomes, each made of one chromatid.
After S phase: It still has 12 chromosomes because there are still 12 centromeres. However, it now has 24 chromatids because every chromosome was copied.
A skin cell needs to produce replacement cells. It copies its DNA, lines up the replicated chromosomes individually, and separates the sister chromatids.
Result: Two similar diploid cells with the same chromosome number as the parent cell.
Maternal and paternal homologous chromosomes pair. They may exchange corresponding DNA segments through crossing over. The homologs then move to opposite sides.
Important: Sister chromatids remain attached during this first division.
There is no second round of DNA replication. In each cell, the sister chromatids finally separate.
Result: Four haploid cells, generally different from one another.
If a homologous pair fails to separate in meiosis I, both homologs may enter the same daughter cell.
Later gametes can therefore receive an extra chromosome or lack that chromosome. The error is about distribution, not failure to copy DNA.
Check before you commit
- Saying interphase is an inactive resting period
- Counting replicated chromatids as separate chromosomes before separation
- Confusing homologous chromosomes with sister chromatids
- Saying DNA replicates between meiosis I and II
- Assuming mitosis always produces haploid cells
- Treating crossing over as a normal event in mitosis
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Cell Division FAQ
Does chromosome number double during S phase?
No. DNA and chromatid number double, but joined sister chromatids are still counted as one chromosome.
Why are meiotic products genetically different?
Crossing over, independent assortment, and later random fertilization generate variation.
What is the difference between mitosis and cytokinesis?
Mitosis divides the nucleus and chromosomes; cytokinesis divides the cytoplasm.
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