Cell Division
Übersicht
You began as one cell. That cell held a full instruction set, and it did something remarkable: it copied every instruction, split in half, and handed a complete set to each of the two cells it became. Those two did it again, and again, and the repetition never really stopped. The skin you shed this morning is being replaced right now by the same process, running quietly in a layer of cells you will never see.
This lesson follows that process in two very different versions. One makes copies so faithful that the new cells are indistinguishable from the old, and it builds and repairs bodies. The other deliberately halves the instruction set, and without it sexual reproduction would double the chromosome number in every generation until nothing worked. You will learn where each one happens, what comes out of it, why a fertilised egg can become a nerve cell or a muscle cell, what makes stem cells so valuable to medicine, and what happens when the controls on cell division fail.
Ziele
- The nucleus of a cell contains chromosomes. Chromosomes carry genes that control the characteristics of the body. Each chromosome carries a large number of genes.
- Many genes have different forms called alleles, which may produce different characteristics.
- In body cells the chromosomes are normally found in pairs.
- Body cells divide by mitosis to produce additional cells during growth or to produce replacement cells.
- When a body cell divides by mitosis: copies of the genetic material are made; the cell then divides once to form two genetically identical body cells.
- Cells in reproductive organs divide to form gametes.
- A cell divides to form gametes by meiosis.
- When a cell divides to form gametes: copies of the genetic information are made; the cell then divides twice to form four gametes, each with a single set of chromosomes.
- Gametes join at fertilisation to form a single body cell with new pairs of chromosomes. This cell repeatedly divides by mitosis to form many cells. As an organism develops, these cells differentiate to form different kinds of cells.
- Most types of animal cell differentiate at an early stage, whereas many plant cells retain the ability to differentiate throughout life. In mature animals, cell division is mainly restricted to repair and replacement.
- Cells from human embryos and adult bone marrow, called stem cells, can be made to differentiate into many different types of human cell.
- In therapeutic cloning an embryo is produced with the same genes as the patient. Stem cells from the embryo will not be rejected by the patient’s body so they may be used for medical treatment.
- Treatment with stem cells may be able to help conditions such as paralysis. Knowledge and understanding of stem cell techniques is not required. Students should be able, when provided with appropriate information, to make informed judgements about the social and ethical issues concerning the use of stem cells from adult bone marrow and embryos in medical research and treatments.
- Tumours result from the abnormal, uncontrolled growth of cells. Benign tumours do not invade other tissues. Cells from malignant tumours invade healthy tissue. Some malignant cells may enter the bloodstream and circulate to other parts of the body, forming secondary tumours.
- Tumours can be caused by chemical carcinogens, eg those found in tobacco smoke and in asbestos, and by ionising radiation, eg UV and X-rays.
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- How many chromosomes are present in a human gamete? A. 23 B. 46 C. 92 D. 12 Answer: A
- Which statement describes what happens when a body cell divides by mitosis? A. The genetic material is copied and the cell divides twice B. The genetic material is copied and the cell divides once C. The cell divides once and the genetic material is then copied D. The genetic material is halved and the cell divides once Answer: B
- How many cells are produced when one cell divides by meiosis? A. One B. Two C. Four D. Eight Answer: C
- What is an allele? A. A length of DNA that carries many genes B. An alternative form of a gene C. A cell that can differentiate into many types D. A pair of matching chromosomes Answer: B
- Which feature is shown by a malignant tumour but not by a benign tumour? A. It is made of cells that divide B. Its cells invade healthy tissue and may spread to other parts of the body C. It grows larger over time D. It results from uncontrolled cell division Answer: B
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