Replication and division of nuclei and cells — common mistakes

When describing stages of mitosis in Paper 2, always include what happens to the chromosomes (condensing, aligning, separating), the nuclear envelope (breaking down, reforming), and the spindle fibres (forming, attaching, shortening).

When explaining tumour formation, make sure to link uncontrolled cell division back to mutations in cell cycle regulatory genes (oncogenes, tumour suppressors) and the failure of checkpoints. Don't just state that cells divide uncontrollably; explain why.

Semi-conservative replication is the process where DNA copies itself, resulting in two new DNA molecules, each containing one original DNA strand and one newly synthesised strand. It's crucial in 9700 Biology because it ensures the accurate transmission of genetic information to new daughter cells, maintaining genetic continuity for growth, repair, and reproduction.

The main stages of mitosis are Prophase, Metaphase, Anaphase, and Telophase (PMAT), preceded by Interphase (G1, S, G2). The significance of mitosis is to produce two genetically identical daughter cells from a single parent cell. This is essential for growth, tissue repair, and asexual reproduction in organisms.

Uncontrolled cell division, often triggered by mutations in genes that regulate the cell cycle (like proto-oncogenes becoming oncogenes, or tumour suppressor genes being inactivated), bypasses normal cellular checkpoints. This leads to an excessive proliferation of cells, forming a mass called a tumour. If these cells become invasive and spread (metastasise), it indicates a malignant tumour, or cancer.

The main differences are in cytokinesis and the spindle formation. Animal cells form a cleavage furrow to divide, while plant cells form a cell plate that becomes a new cell wall. Also, animal cells use centrioles to organise the spindle, whereas plant cells do not have centrioles.