BIOB 160 Principles of Living Systems · Unit 3: Cell division and inheritance · on Exam 3
3.1 The cell cycle and mitosis
Study notes for The cell cycle and mitosis in BIOB 160 Principles of Living Systems at Montana State University: the big ideas, a worked example, common mistakes and an exam tip. The same topic has endless practice questions and flashcards in Mathub.
Big ideas
- A duplicated chromosome is two sister chromatids joined at the centromere. Human body cells are diploid: 2n = 46.
- The cell cycle: interphase (G₁ growth, S DNA replication, G₂ preparation) and the mitotic phase (mitosis + cytokinesis). Prokaryotes divide by binary fission.
- Mitosis: prophase (chromosomes condense, spindle forms), prometaphase (nuclear envelope breaks down, kinetochore microtubules attach), metaphase (chromosomes line up at the metaphase plate), anaphase (sister chromatids separate to opposite poles) and telophase (nuclei re-form).
- Cytokinesis: a cleavage furrow pinches animal cells (an actin ring); plant cells build a cell plate from Golgi vesicles.
- Control: checkpoints at G₁ (the main “restriction point”), G₂ and M (all chromosomes attached to the spindle before anaphase). Cyclins bind Cdks (such as MPF) to drive the cycle; growth factors, density-dependent inhibition and anchorage dependence add signals. Cancer cells ignore these controls.
Worked example
A cell with 2n = 8 goes through mitosis. How many chromosomes and chromatids are in the cell at metaphase, and how many chromosomes are in each daughter cell?
Show the solution
At metaphase: 8 chromosomes, each with 2 sister chromatids, so 16 chromatids. Each daughter cell gets 8 chromosomes (2n = 8): mitosis keeps the chromosome number the same.
Common mistakes
- Thinking chromosome number doubles in S phase. The DNA doubles; the chromosome count stays the same until anaphase.
- Placing DNA replication in mitosis. It happens in S phase of interphase.
- Confusing sister chromatids (identical copies) with homologous chromosomes (one from each parent).
Exam tipReading: Campbell ch. 12 (pp. 234–249). Practise counting chromosomes and chromatids at each phase; it is the most common calculation on this exam.
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