Everything from the 2 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
A chromosome is one long DNA molecule wound round histone proteins. In interphase the cell grows (G1), replicates every DNA molecule (S) and grows and checks again (G2); then mitosis divides the nucleus and cytokinesis divides the cytoplasm. After S phase each chromosome is two identical sister chromatids joined at the centromere, so mitosis gives two genetically identical daughter cells, used for growth, replacement, repair and asexual reproduction. Telomeres protect the chromosome ends, stem cells supply new cells, and division that escapes control forms a tumour.
A chromosome is not always X-shaped. Before S phase it is a single chromatid (one DNA molecule); the X shape only appears after replication, once the chromosome has condensed for division. Two sister chromatids are identical copies, not a homologous pair from two parents.
Interphase is not a resting stage. The cell is very active: it makes proteins, replicates its DNA and checks it. And DNA replication happens in S phase of interphase, not during mitosis; mitosis only shares out DNA that is already copied.
A tumour forms by mitosis, which is why the tumour cells are genetically identical to the first mutated cell. Do not say carcinogens make cells divide; they cause mutations in the genes that control division. Usually several mutations in one cell line are needed, which is why risk rises with age.
In prophase the chromosomes condense and become visible as two sister chromatids, the nucleolus disappears, the spindle forms (from centrosomes in animal cells) and the nuclear envelope breaks down. In metaphase spindle fibres attach at the centromeres and line the chromosomes up on the equator. In anaphase the centromeres divide and the chromatids are pulled to opposite poles, centromere first. In telophase the chromatids reach the poles and uncoil, nuclear envelopes re-form and the spindle breaks down. Cytokinesis then divides the cell: a cleavage furrow in animal cells, a cell plate in plant cells.
In anaphase it is the sister chromatids that separate, not homologous chromosomes (that is meiosis). And the chromosome number does not halve in mitosis: a cell with 46 chromosomes gives two cells with 46 each. Count centromeres to count chromosomes.
The mitotic index counts cells in prophase, metaphase, anaphase and telophase only, never interphase, and divides by the total number of cells, not by the number in interphase. Count a large sample (at least 30, ideally more) from several fields of view.