Track DNA and chromosome number through a cell cycle

Mechanism before a label

A chromosome is counted by its centromere organization, while DNA content measures the amount of DNA. After S phase each replicated chromosome has sister chromatids. DNA amount doubles, but the chromosome count of the undivided cell does not double until sister centromeres separate.

For a stipulated diploid species with 2n=6, a G1 nucleus has six chromosomes and six chromatids. At G2 it still has six chromosomes but twelve chromatids. During anaphase, separated sisters are chromosomes; after division each normal daughter nucleus again contains six.

Mitosis segregates replicated chromosomes into daughter nuclei; cytokinesis divides the cytoplasm and may not occur by identical mechanisms in plants and animals. A DNA-content histogram distinguishes some cell-cycle states but cannot separate G2 from mitosis solely because both can have replicated DNA.

Place replication and division on a cycle

The usual proliferating eukaryotic sequence is G1 → S → G2 → M. G1 includes growth and cellular work; S replicates DNA; G2 prepares for division; M includes mitosis and usually cytokinesis. Cells can enter G0, a nondividing state that may be reversible or associated with differentiation. G0 is not simply a second name for dead cells.

During prophase chromosomes condense and the division apparatus forms. Nuclear-envelope breakdown and spindle attachment occur during the transition often distinguished as prometaphase. Metaphase places duplicated chromosomes at the spindle midregion; anaphase separates sister chromatids; telophase reestablishes daughter nuclei. Cytokinesis partitions cytoplasm. Plant cell plates and animal cleavage furrows accomplish that partition differently.

Construct: label a four-stage chromosome drawing for a 2n=6 cell and put replication before, not inside, mitosis. A metaphase drawing has six duplicated chromosomes, an anaphase cell has twelve separated chromosomes across the entire undivided cell, and each normal daughter nucleus receives six. Count the indicated boundary before answering. Cell-cycle reference.

Link preparation and division to cellular functions

G1 includes organelle and cytoplasmic growth; S duplicates nuclear DNA; preparation through S/G2 includes protein production, energy supply and organization of the division machinery. In animal cells, centrosome duplication begins during S and maturation continues toward mitosis, so it should not be confused with nuclear chromosome separation. Mitosis contributes to organismal growth, tissue replacement/repair and some asexual reproduction. A successful division transfers a complete genome under the normal model; mutations or segregation errors can violate that expectation. OpenStax cell-cycle account, checked 31 October 2026.

Changed case: A fictional cell has completed S phase but cannot assemble a functioning spindle. Does doubled DNA prove it has already produced two viable daughter cells?

Check preparation versus outcome

No. DNA copying supplies duplicated chromosomes, not their segregation or cytoplasmic partition. The spindle and checkpoint evidence determine whether division proceeds normally. Likewise, a tissue-growth observation does not alone establish the chromosome count of each cell or exclude cell enlargement as a contributor.

Predict, compare and calculate

The following record is an original synthetic teaching dataset. Its assigned conditions are not a claim that an actual organism was measured. Write your prediction before revealing the changed condition.

Per undivided cell	Value (counts or relative DNA as labeled)
G1 chromosomes	6
G1 DNA units	2
G2 chromosomes	6
G2 DNA units	4

The baseline C value is one haploid unreplicated genome; units are explicitly different.

Your prediction before comparison

This scratch prediction is not saved automatically. Copy it to the lesson notebook to retain it.

Evidence conditionReference comparisonChanged condition / follow-up

Track DNA and chromosome number through a cell cycle

Supplied synthetic evidence: The baseline C value is one haploid unreplicated genome; units are explicitly different.

Per undivided cell: reference condition

Quantity	Value	Unit
G1 chromosomes	6	counts or relative DNA as labeled
G1 DNA units	2	counts or relative DNA as labeled
G2 chromosomes	6	counts or relative DNA as labeled
G2 DNA units	4	counts or relative DNA as labeled

Task: A species has 2n=10. Give chromosome and chromatid counts in G2 and in each normal daughter after mitosis.

A mixed table is for accounting, not a common physical axis.

Compare with the conditional interpretation

S phase doubles DNA content from 2C to 4C without changing the pre-separation chromosome count.

After replication there are twelve chromatids but six paired centromere units. A drawing with six X-shaped chromosomes must not be counted as twelve chromosomes before the sisters separate.

Case index for this investigation

0. Explain signaling distance and receptor specificity
1. Trace reception, relay and cellular response
2. Locate a signaling defect with rescue controls
3. Distinguish stabilizing feedback from reinforcement
4. Track DNA and chromosome number through a cell cycle
5. Infer stage durations cautiously from cell samples
6. Explain cell-cycle checkpoints and loss of regulation

No numerical confidence, laboratory credit or independent mastery is inferred from opening a supplied case. Save your first prediction and comparison in the notebook.

Reset this unsaved comparison

Worked reasoning

After replication there are twelve chromatids but six paired centromere units. A drawing with six X-shaped chromosomes must not be counted as twelve chromosomes before the sisters separate.

S phase doubles DNA content from 2C to 4C without changing the pre-separation chromosome count.

Apply to a new case

A species has 2n=10. Give chromosome and chromatid counts in G2 and in each normal daughter after mitosis.

Compare your reasoning with the worked response

G2 has ten chromosomes and twenty chromatids. Each daughter has ten chromosomes with one chromatid each, assuming normal segregation. Distinguish one nucleus from totals across both daughters.

This public response supplies feedback, not a score for independent reasoning. Record which part of your prediction changed and why.

Evidence and investigation boundary

A mixed table is for accounting, not a common physical axis. Mutation, loss and unequal segregation are omitted.

For a supervised investigation, predeclare the question, biological unit of replication, treatment/comparison assignment, quantities and units, exclusion rules, graph and uncertainty method. Keep the raw records and the first prediction separate from later revisions. A simulated case does not supply laboratory hours or establish AP authorization. No physical manipulation is required by this lesson.

Source and continuation

Framework topics: 4.5. Current AP Biology framework; supporting scientific reference. Sources checked 30 October 2026; all teaching wording, questions and numbers here are original. Return to AP Biology to compare this mechanism with another scale of organization.