Lesson 5 of 9 / Mutation, cell division and cancer
Mitosis preserves chromosome number
What doubles in S phase, and what separates in anaphase?
In this lesson: Describe the mitotic cycle, named stages and its normal significance.
About 8 min
The key ideaDNA replication makes sister chromatids; mitosis separates them so daughter nuclei retain the parental chromosome complement.
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.
Explore the idea
Track DNA amount through the cycle
Growth and ordinary activity. In the diploid model, chromosome number is 2n and DNA amount is 2C.
C is a relative DNA-content unit. This diagram distinguishes DNA amount from chromosome number; it does not depict the brief anaphase count within an undivided cell.
Explanation
Interphase includes G1 growth, S-phase DNA replication and G2 preparation. After S phase each chromosome consists of two sister chromatids joined at a centromeric region. DNA amount has doubled, but chromosome number counted by centromeres has not yet doubled.
In prophase chromosomes condense and the spindle develops; in the usual animal-cell model centrosomes containing centrioles move apart and the nuclear envelope breaks down. At metaphase chromosomes align individually at the equator, with sister chromatids attached towards opposite poles.
During anaphase sister chromatids separate and move to opposite poles, each now counted as a daughter chromosome. In telophase nuclear envelopes re-form and chromosomes decondense. Cytokinesis divides the cytoplasm; animal cells constrict their membrane, whereas plant cells build a cell plate.
Mitosis supports growth, tissue repair and asexual reproduction by preserving genetic information, apart from mutations. Regulation checks whether events such as DNA copying and chromosome attachment have been completed appropriately. Failure of regulation can permit damaged or incorrectly segregated genomes to propagate.
Step by step
- 1
Separate replication from division
S phase precedes mitosis.
- 2
Track the unit moving
Anaphase mitosis separates sisters, not homologous pairs.
- 3
State the daughter outcome
Give chromosome number and genetic relationship.
Worked example
Work through the evidence
A diploid cell has 6 chromosomes before S phase. How many chromosomes and chromatids does it have at metaphase?
One way to explain it
Six chromosomes and twelve chromatids. After separation, each normal daughter nucleus receives six chromosomes.
Why this answer works
- Count chromosomes by joined centromeric units before anaphase.
- DNA replication makes a sister copy of each chromosome.
Is this true? "DNA replication in S phase immediately doubles ploidy."
It doubles DNA content while the number of homologous chromosome sets remains the same.