Safety and evidence boundary
Use prepared images, paper models, supplied datasets, or teacher-approved classroom microscopy. Do not culture unknown cells or microbes, collect human tissue, handle stains or preservatives without qualified supervision, or use personal medical records. This course does not diagnose growth disorders or disease.
Explicit science instruction
The cell cycle is an ordered sequence of growth, DNA replication, chromosome segregation, and cell division. In many dividing eukaryotic cells, interphase includes G1, S, and G2. During G1 the cell grows and performs its specialized functions. During S phase DNA is replicated. During G2 the cell continues preparation and checks whether conditions support division. Mitosis separates duplicated chromosomes into two nuclei, and cytokinesis partitions the cell.
Replication changes DNA amount, but it does not automatically change chromosome number. After S phase, each replicated chromosome contains two sister chromatids joined at a centromeric region. Counting rules depend on what structure is being counted.
Vocabulary: cell cycle; interphase; G1; S phase; G2; mitosis; cytokinesis; chromosome; sister chromatid; centromere.
Worked example with reasoning
A fictional diploid cell begins G1 with 6 chromosomes. After S phase, it still has 6 chromosomes, but each chromosome has been replicated into two sister chromatids. Just before mitosis there are 12 chromatids organized as 6 replicated chromosomes. After sister chromatids separate during mitosis, each daughter nucleus receives 6 chromosomes.
- G1: 6 chromosomes.
- After S: 6 replicated chromosomes, 12 chromatids.
- After segregation: each daughter nucleus has 6 chromosomes.
Guided question
Why does DNA replication double the amount of DNA without necessarily doubling the chromosome count?
Independent work
- Draw a four-panel cycle model for G1, S, G2, and M/cytokinesis.
- Track one chromosome through all four panels.
- For a cell beginning with 8 chromosomes, state chromosome and chromatid counts after S phase.
- Add one checkpoint where division could pause.
Misconception check
Interphase is not a resting stage. Cells can be metabolically active, grow, duplicate organelles, synthesize molecules, and replicate DNA during interphase.
Formative understanding check
A cell has 10 chromosomes in G1. Immediately after S phase, how many chromosomes and sister chromatids does it have?
Show the worked response
It still has 10 chromosomes, now replicated into 20 sister chromatids. Chromosome number is defined by centromeric units, not simply by DNA molecule count.
Open-book formative practice only; not an official mastery attempt or grade.
Mentor evidence conference
Ask Archaemenes to test your chromosome-counting logic, cell-cycle model, evidence classification, and stated limitations. The mentor may challenge reasoning but does not provide protected assessment answers.
Open Archaemenes for this lessonTeacher guidance
Foundation: use a labeled cell-cycle wheel and chromosome cards. Core: track DNA amount and chromosome structure across G1, S, G2, mitosis, and cytokinesis. Extended: add G0, discuss why cycle lengths differ among cell types, and distinguish DNA content from chromosome number quantitatively.
Preserve learning evidence
Keep the model, calculation or image analysis, correction note, and explanation in the matching Week 15 evidence field.
Return to monday Evidence Workspace