Cell and nuclear division — IB Diploma Biology HL

Revision notes

6 short notes, in the order of the specification. Each one in short:

1. Cell division and cytokinesis

   Cytokinesis is the splitting of the cytoplasm of a parent cell between two daughter cells. In animal cells a contractile ring of actin and myosin pinches the membrane inwards; in plant cells vesicles build a new membrane and cell wall across the cell. It is usually equal, but unequal in oogenesis and yeast budding.

2. Mitosis, meiosis and chromosome movement

   Mitosis is nuclear division that keeps the chromosome number and genome the same, while meiosis halves the chromosome number and generates genetic diversity. Both need DNA replication first, so each chromosome is two chromatids. In both, histones supercoil DNA to condense chromosomes, and microtubules with motor proteins move them.

3. Phases of mitosis

   Practical

   Mitosis has four phases: prophase, metaphase, anaphase and telophase. Chromosomes condense, line up on the equator, have their sister chromatids pulled to opposite poles, and are enclosed in two new nuclei. Because each nucleus receives one copy of every chromatid, mitosis produces two genetically identical daughter nuclei.

4. Meiosis, non-disjunction and variation

   Meiosis is a reduction division: two divisions turn one diploid nucleus into four haploid nuclei. Homologous chromosomes separate in meiosis I and sister chromatids in meiosis II. Errors such as non-disjunction cause conditions like Down syndrome. Crossing over and random orientation of bivalents make every haploid nucleus genetically different.

5. Cell proliferation and the cell cycle

   HL only

   Cell proliferation is the repeated division of cells for growth, cell replacement and tissue repair, and it is achieved using the cell cycle. Interphase (G1, S and G2) is a metabolically active growth period, followed by mitosis and cytokinesis. Rising and falling cyclin concentrations control the cycle: a threshold level of a specific cyclin is needed to pass each checkpoint.

6. Mutations, tumours and the mitotic index

   PracticalHL only

   A tumour is a mass of cells formed by uncontrolled cell division, caused by mutations in genes that control the cell cycle. Proto-oncogenes can mutate into oncogenes and tumour suppressor genes can lose their function. Benign tumours grow slowly and stay in place; malignant tumours invade tissue and metastasise, causing cancer.