Cell and Nuclear Division
Senior 5 · Biology
Cell and Nuclear Division
Lesson 2 of 4
25 min
The Madagascar periwinkle, Catharanthus roseus, is a small pink or white flower grown in gardens and along roadsides across the tropics. In the 1950s researchers testing its leaves found two chemicals, vinblastine and vincristine, that stop cells dividing. They block the assembly of the spindle, the structure that pulls chromosomes apart. Because cancer cells divide far more often than most body cells, these drugs became some of the first successful treatments for childhood leukaemia. To see why a drug that attacks the spindle can treat cancer, you need to follow mitosis in detail.
Definition
Mitosis: division of the nucleus that produces two nuclei with the same number and type of chromosomes as the parent nucleus, genetically identical to it.
Spindle: a structure of protein fibres (microtubules, made of tubulin) that forms between opposite poles of the cell and moves the chromosomes.
Centrosome: the region near the nucleus of an animal cell, containing a pair of centrioles, from which the spindle microtubules grow. Kinetochore: a protein structure on the centromere of each chromatid, to which spindle fibres attach.
Cytokinesis: division of the cytoplasm to form two cells, which usually follows mitosis.
| Phase | What happens |
|---|---|
| Prophase | Chromatin condenses into chromosomes, each with two sister chromatids joined at the centromere. The nucleolus disappears. In animal cells the two centrosomes move to opposite poles, and microtubules grow from them to form the spindle. At the end of prophase the nuclear envelope breaks into small vesicles. |
| Metaphase | Spindle fibres from opposite poles attach to the two kinetochores of each chromosome. The chromosomes are pulled into line on the equator, the metaphase plate. The spindle checkpoint waits until every chromosome is attached to both poles. |
| Anaphase | The proteins (cohesins) that hold sister chromatids together are cut by an enzyme. The kinetochore fibres shorten and pull the separated chromatids, centromere first, to opposite poles; other fibres lengthen and push the poles further apart. Each chromatid is now called a chromosome. |
| Telophase | The chromosomes reach the poles and uncoil. A nuclear envelope re-forms around each set, the nucleoli reappear, and the spindle breaks down. |
| Cytokinesis | Animal cells: a ring of actin and myosin filaments pinches the membrane inwards, forming a cleavage furrow, until the cell splits. Plant cells: vesicles from the Golgi body gather at the equator and fuse into a cell plate, which becomes the new cell wall. Plant cells have no centrioles but still form a spindle. |
Mitosis keeps the chromosome number constant and the genes identical in every daughter cell. Living things use it for:
Cancer is a group of diseases in which cells divide by mitosis without control. It begins with mutations in two kinds of gene that regulate the cell cycle:
Usually several mutations must build up in one cell, which is one reason most cancers are more common in older people. The cells form a tumour. A benign tumour grows slowly, stays in one place inside a capsule and does not spread. A malignant tumour invades nearby tissue, and its cells can break away and travel in the blood or lymph to form secondary tumours elsewhere: metastasis.
Causes and prevention: agents that cause mutations (carcinogens) include tobacco smoke, ultraviolet radiation, aflatoxin from mouldy maize and groundnuts, and some viruses. Human papillomavirus (HPV) causes most cases of cervical cancer, and hepatitis B virus can cause liver cancer; both have vaccines. In 2011 Rwanda became one of the first African countries to vaccinate girls against HPV through a national programme.
Treatment: surgery removes a tumour; radiotherapy damages the DNA of cancer cells; and chemotherapy drugs target dividing cells. Vinblastine stops the spindle forming, and taxol stops it breaking down, so in both cases cells are trapped in mitosis. Because the drugs also hit normal fast-dividing cells, in hair roots, the gut lining and bone marrow, patients may lose hair, feel sick and become prone to infection.
Worked example 1: how long does a tumour take to grow?
A tumour of about cells (roughly 1 g) is often the smallest that can be felt or seen on a scan. Assume it grew from one cell, every cell divided, and none died. (a) How many rounds of division were needed? (b) If the cells doubled every 60 days, how long did this take?
Interpretation: a tumour may grow silently for years before it is detected, and only about 10 more doublings would take it to cells, about 1 kg. This is why screening, such as cervical cancer screening, aims to find changes early.
Worked example 2: mitotic index and cycle time
The fraction of cells in mitosis equals the fraction of the cell cycle spent in mitosis. In a sample of normal epithelium the mitotic index was 1.5 %; in a tumour biopsy it was 9.0 % (illustrative data). Mitosis lasts about 1 hour in both. Estimate the length of the cell cycle in each.
CrashCourse: "Mitosis: Splitting Up is Complicated" (Crash Course Biology #12). Watch for the jobs of the centrosomes, spindle and kinetochores in each phase, and the link between uncontrolled division and cancer.
Warning
Common mistakes
Tip
Memory hook for the phases: "Please Make A Tea": Prophase, Metaphase, Anaphase, Telophase. In Metaphase they line up in the Middle; in Anaphase they move Away.
Practice: mitosis and cancer
Key points
Sources and credits
Figure 1, the table, the worked examples and the practice questions are written by Ganzaa. The mitotic indices in Worked example 2 are illustrative.
