Chromosome behaviour in mitosis
In the last lesson we met the cell cycle from a distance. Now we zoom right in on mitosis and watch, step by step, exactly what the chromosomes do. We follow a single chromosome through the four named stages every exam expects you to know in order: prophase (the chromosomes condense and become visible as two chromatids, the spindle forms and the nuclear envelope breaks down), metaphase (the chromosomes line up on the equator, or metaphase plate, attached to spindle fibres at their centromeres), anaphase (the centromeres split and the sister chromatids are pulled to opposite poles), and telophase (the chromatids reach the poles, new nuclear envelopes reform and the chromosomes decondense). Alongside the chromosomes we track the spindle, the nuclear envelope and the cell surface membrane. We finish the whole process with cytokinesis — a cleavage furrow pinching in for animal cells, a cell plate building out for plant cells — and then practise the real exam skill of reading photomicrographs, diagrams and microscope slides of a root tip to name each stage and calculate a mitotic index.
By the end you should be able to (NSSCAS Biology (AS) 3.1.2):
- Describe, with the aid of photomicrographs and diagrams, the behaviour of chromosomes in plant and animal cells during the mitotic cell cycle and the associated behaviour of the nuclear envelope, cell surface membrane and the spindle (names of the main stages of mitosis: prophase, metaphase, anaphase and telophase, are expected)
- Interpret photomicrographs, diagrams and microscope slides of mitosis and identify the main stages of mitosis